IM: U: Trap Manager
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1. 8 7 JeBeuey deil ig 8 8 CHAPTER 8 Trap Manager Daisy Chain of Patches It is possible to patch a system software routine with more than just one patch this is called a daisy chain of patches Typically you extract from the trap dispatch table the address of the routine you wish to patch save this address and then install your own patch routine When your patch has completed its tasks it should jump to the address you previously extracted from the trap dispatch table In this way the patches take the general form of a daisy chain Each patch will execute in turn and jump to the next patch until the last link in the chain which returns control to the trap dispatcher IMPORTANT Although this chapter describes patching in some depth you should rarely if ever find a need to use patches in an application The primary purposes of patches as their name suggests are to fix problems and augment routines in ROM code A A patch can be implemented as either a head patch tail patch or come from patch These are described in the next sections Head Patch Normal Patch A head patch also referred to as a normal patch is a routine that gets executed before the original system software routine A head patch performs its primary action and then uses a jump instruction JMP to jump to the system software routine Thus the head patch does not regain control after the execution of the system software routine After the e2. Constants newOS EQU 200 access Operating System trap dispatch table newTool EQU 600 access Toolbox trap dispatch table 8 36 Summary of the Trap Manager CHAPTER 8 Trap Manager Trap Macros Trap Macros Requiring Register Setup Trap macro name Registers on entry _GetTrapAddress DO trap number _SetTrapAddress DO trap number AO address of patch _Unimplemented Summary of the Trap Manager Registers on exit AO address of patch 8 37 Jobeueyy des e
3. DESCRIPTION 8 30 You can use the NSet TrapAddress procedure to install a patch address into either an Operating System trap dispatch table or a Toolbox trap dispatch table PROCEDURE NSetTrapAddress trapAddr LongInt trapNum Integer tTyp TrapType trapAddr The patch address trapNum A line instruction or a trap number If you specify a A line instruction the function extracts the trap number you tTyp The trap type If you supply the t Typ parameter with the constant OSTrap the NSet TrapAddress procedure installs the address into the Operating System trap dispatch table If you supply the t Typ parameter with the constant ToolTrap the NGet TrapAddress function installs the address into the Toolbox trap dispatch table The NSet TrapAddress procedure places the patch address specified by the trapAddr parameter into a trap dispatch table Use the t Typ parameter to specify whether the patch address belongs in the Operating System trap dispatch table or the Toolbox trap dispatch table If tTyp is OSTrap the NSet TrapAddress procedure installs the address into the Operating System trap dispatch table If tTyp is ToolTrap the NGet TrapAddress function installs the address into the Toolbox trap dispatch table Use the t rapNum parameter to specify the location of the patch address in the dispatch Trap Manager Reference CHAPTER 8 Trap Manager table The trap number may be any word value its irrelevant high order
4. e Application calls a Toolbox routine e Gets the address from pe the trap dispatch table FillRect r pat e Transfers control to the routine at the retrieved address System software routine PROCEDURE FillRect r Rect pat Pattern Note Not all A line instructions are defined When the trap dispatcher receives an undefined A line instruction the trap dispatcher returns the address of the Toolbox procedure Unimplemented When called the Unimplemented procedure triggers a system error Patches and System Software Routines You can modify the trap dispatch table so that the address that gets returned to the trap dispatcher points to a different routine instead of the intended system software routine this is useful if you want to augment or override an existing system software routine The routine that augment an existing system software routine is called a patch The method of augmenting or overriding a system software routine is called patching a trap For example you can augment the FillRect procedure with your own procedure MyPatchFillRect Figure 8 4 illustrates another call to the Toolbox procedure FillRect When the application calls the Fil1lRect procedure the application defined patch MyPatchFillRect is executed first After the application defined patch MyPatchFillRect completes its primary action it transfers control through a JMP instruction to the original
5. ARRAY RECORD string lt 4 bytes Value word or long word ARRAY RECORD string gt 4 bytes Value at address given by pointer SET one byte Byte value SET one word 16 bits value SET two words 32 bits value SET gt two words Value at address given by pointer Note A 1 byte size return value occupies the high order byte of the word allocated for it The Auto Pop Bit The auto pop bit is bit 10 in an A line instruction for a Toolbox routine Some language systems prefer to generate jump subroutine calls JSR to intermediate routines called glue routines which then call Toolbox routines instead of executing the Toolbox routine directly This glue method would normally interfere with Toolbox traps because the return address of the glue subroutine is placed on the stack between the Toolbox routine s parameters and the address of the place where the glue routine was called from where control returns once the Toolbox routine has completed execution The auto pop bit forces the trap dispatcher to remove the top 4 bytes from the stack before dispatching to the Toolbox routine After the Toolbox routine completes execution control is transferred back to the place where the glue routine was called from not back to the glue routine Most development environments including MPW do not use this feature About Trap Macros A trap macro is an assembly language macro that assembles into an A line instruction used for calling a
6. Fi11Rect procedure About the Trap Manager CHAPTER 8 Trap Manager IMPORTANT Although this chapter describes patching in some detail you should avoid any unnecessary patching of the system software One very good reason to avoid patching is that is causes a performance reduction The performance reduction is especially substantial when your patch is executed on a PowerPC processor based Macintosh computer where it is necessary to switch execution environments when entering and exiting your patch code For more information about patching PowerPC system software see Inside Macintosh PowerPC System Software amp Figure 8 4 Augmenting the FillRect procedure with a single patch e Application calls a Toolbox routine FillRect r pat Toolbox trap dispatch table Address SYYYYYYYY 1024 Trap dispatcher e Gets the address from the trap dispatch table t e Transfers control to the routine at the retrieved address System software routine PROC EDURE GES Note MyPatchFillRect Rect e Calls orginal routine XXXXXXXX PROCEDURE FillRect r Rect pat pat Pattern To prevent dangling patch addresses you must ensure that your patch routine is in a locked memory block while its address is in the trap dispatch table About the Trap Manager Pattern
7. here only for the sake of completeness PROCEDURE SetTrapAddress trapAddr LongInt trapNum Integer trapAddr The address of the system software routine trapNum A line instruction or a trap number If you specify an A line instruction the function extracts the trap number you The Set TrapAddress procedure was used when both the Operating System routine addresses and Toolbox routine adddresses were located in the same trap dispatch table Today any routine address with the trap number 00 to 4F 54 or 57 is installed into the Operating System dispatch table any other system software routine is installed into the Toolbox dispatch table WARNING The Set TrapAddress procedure is not supported under Power PC A WARNING The Set TrapAddress procedure ignores the high order bits in the t rapNum parameter the procedure is not able to differentiate between Operating System routines and Toolbox routines The Set TrapAddress procedure is not reliable on any computer today A Trap Manager Reference 8 33 Jobeueyy des lea CHAPTER 8 Trap Manager Summary of the Trap Manager Pascal Summary Constants CONST Gestalt selectors gestaltOSTable ostt base of Operating System dispatch table gestaltToolboxTable tbtt base of Toolbox dispatch table gestaltExtToolboxTable xttt 0 unless Toolbox dispatch table is disjoint in which case base of upper half system error
8. its features But if you expect your application to run on a system older than System 6 0 4 the Gestalt Manager may not be available The example in this section shows how you can use the Trap Manager to check whether a particular system software routine is available on the installed system At startup time system software places the address of the Unimplemented procedure into all entries of each trap dispatch table that do not contain an address of a Toolbox or Operating System routine or the address of a come from patch Listing 8 1 illustrates how you can use these Unimp1lemented addresses to determine whether a particular system software routine is available on the user s system If a system software routine is available its address differs from the address of the Unimplemented procedure Listing 8 1 Determining if a system software routine is available FUNCTION MySWRoutineAvailable trapWord Integer Boolean VAR trType TrapType BEGIN first determine whether it is an Operating System or Toolbox routine IF ORD BAND trapWord 0800 0 THEN trType OSTrap ELSE trType ToolTrap filter cases where older systems mask with S1FF rather than S 3FF IF trType ToolTrap AND ORD BAND trapWord SO3FF gt 200 AND Get ToolboxTrapAddress SA86E GetToolboxTrapAddress SAA6E THEN ySWRoutineAvailable FALSE ELSE ySWRoutineAvailable NGetTrapAddress tra
9. microprocessors recognizes a defined set of instructions When the microprocessor encounters an instruction that it doesn t recognize an exception is generated An exception refers to bus errors interrupts and unimplemented instructions When an exception occurs the microprocessor suspends normal execution and transfers control to an appropriate exception handler In the MC680x0 family of microprocessors all instructions starting with the hexadecimal digit A are unimplemented instructions These unimplemented instructions are also called A line instructions System software uses these unimplemented A line instructions to execute system software routines When you call a system software routine the call to the system software routine is translated into an A line instruction The MC680x0 microprocessor doesn t recognize this A line instruction and transfers control to an exception handler System software provides an exception handler called a trap dispatcher to handle exceptions generated by A line instructions Whenever a MC680x0 microprocessor encounters an A line instruction an exception is generated and the microprocessor transfers control to the trap dispatcher An exception generated by an A line instruction is called a trap About the Trap Manager 8 3 Jobeueyy des e CHAPTER 8 Trap Manager When the trap dispatcher receives the A line instruction it looks into a table called a trap dispatch table to find the addre
10. the section A Line Instructions for Toolbox Routines beginning on page 8 14 Note The exception handler is located at address 28 on computers with an MC68000 microprocessor and at address 28 offset from the address in the microprocessor s Vector Base Register VBR on computers with other MC680x0 microprocessors Consult the relevant microprocessor handbook for the precise details of exception handling on the MC680x0 microprocessor of interest to you A Line Instructions for Operating System Routines An Operating System trap is an exception that is caused by an A line instruction that executes an Operating System routine When dispatching an Operating System trap the trap dispatcher extracts the trap number from the A line instruction and uses it as an index into the Operating System trap dispatch table The entry in the Operating System trap dispatch table contains the address of the desired Operating System routine Figure 8 7 illustrates an A line instruction for an Operating System routine Figure 8 7 An A line instruction for an Operating System routine 15 14 13 12 1110 9 8 7 0 Trap number Return save AO bit Flags A 0 Operating System trap Bit 11 tells the trap dispatcher that this A line instruction invokes an Operating System routine Two flag bits bit 10 and bit 9 are reserved for use by the Operating System routine itself and are discussed in detail in Flag Bits on pag
11. word is pushed last In the case of a byte size SET the called procedure accesses only the low order half of the word that is pushed About the Trap Manager CHAPTER 8 Trap Manager Note A byte pushed on the stack occupies the high order byte of the word allocated for it according to conventions for the MC680x0 microprocessors WARNING A value of type Char is passed as a word value The value occupies the low order half of the word a Function Results Function results are returned by value or by address on the stack Space for the function result is allocated by the caller before the parameters are pushed The caller is responsible for removing the result from the stack after the call For types Boolean Char and Integer and for enumerated and subrange types the caller allocates a word on the stack to make space for the function result Values of type Boolean enumerated types with fewer than 128 elements and subranges within the range 128 to 127 are returned as signed byte values The value is placed in the high order byte of the word Values of type Integer and Char and all enumerated and subrange types not covered above are returned as signed word values Pointers and values of type LongInt are returned as signed 32 bit values Values of type Real are returned as 32 bit real values For types whose values are greater than 4 bytes in size the caller pushes a pointer to a temporary location into which the functio
12. xttt 0 unless Toolbox dispatch table is disjoint in which case base of upper half values of TrapType enum OSTrap ToolTrap system errors triggered by Trap Manager enum dsCoreErr 12 unimplemented trap error dsBadPatchHeader 83 attempt to install come from patch Data Types typedef unsigned char TrapType Summary of the Trap Manager 8 35 JeBeuey deil e CHAPTER 8 Trap Manager Routines Accessing Addresses From the Trap Dispatch Tables pascal long NGetTrapAddress short trapNum TrapType tTyp pascal long GetOSTrapAddress short trapNum GetToolboxTrapAddress is also spelled as GetToolTrapAddress pascal long GetToolboxTrapAddress short trapNum Installing Patch Addresses Into the Trap Dispatch Tables pascal void NSetTrapAddress a long trapAddr short trapNum TrapType tTyp pascal void SetOSTrapAddress ane long trapAddr short trapNum SetToolboxTrapAddress is also spelled as SetToolTrapAddress pascal void SetToolboxTrapAddress long trapAddr short trapNum pascal void SetToolTrapAddress long trapAddr short trapNum ans Detecting Unimplemented System Software Routines pascal void Unimplemented void Manipulating One Trap Dispatch Table Obsolete Routines pascal long GetTrapAddress short trapNum pascal void SetTrapAddress long trapAddr short trapNum Assembly Language Summary
13. 7 the addresses of come from patches are permanently placed in the trap dispatch table at system startup time The addresses of come from patches are hidden and cannot be manipulated by any of the Trap Manger routines For example if a system software routine has a come from patch and if you use the Trap Manger function NGet TrapAddress to retrieve the address of the system software routine you will not get the address in the trap dispatch table which is the address of the come from patch NGet TrapAddress instead returns the address of the routine that is executed immediately after the come from patch This address could be the address of another patch or the system software routine If a system software routine has a come from patch and if you use the Trap Manager procedure NSet TrapAddress to install a patch to the system software routine the address of the patch is not written into the trap dispatch table Instead the NSet TrapAddress procedure installs the address of the patch into the last come from patch The patch is executed after the completion of the come from patch WARNING In system software before System 7 if a come from patch is invoked by a tail patch the come from patch does not work correctly The come from patch never sees the ROM address on the stack only the return address of the tail patch a Patch for One Application If you install a patch into your application heap the patch applies only to your appl
14. Beuey des e For example a call to the Toolbox function WaitNextEvent is translated to the A line instruction A860 This A line instruction causes the microprocessor to transfer control to the trap dispatcher which deals with any instruction of the form Axxx The trap dispatcher examines the other bits in the A line instruction The value 0 of bit 11 indicates that WaitNextEvent is a Toolbox routine and that the value in bits 9 0 is the index into the Toolbox trap dispatch table The trap dispatcher uses the index which is 60 in this example to find the address of the WaitNextEvent function in the Toolbox trap dispatch table When the address is found the trap dispatcher transfers control to the WaitNextEvent function Figure 8 10 illustrates the stack after the trap dispatcher has transferred control to a Toolbox routine Figure 8 10 Stack when entering a Toolbox routine Stack on entry to Toolbox routine A L Z Additional parameters 7 and extra information Program Counter Stack Pointer V Z The value of the Program Counter that is left on the stack before entry to the Toolbox routine points to the instruction that is executed after the completion of the Toolbox routine About the Trap Manager 8 15 8 16 CHAPTER 8 Trap Manager After the trap dispatcher completes execution the internal status of the stack is restored and normal execution resumes from the point at which proce
15. CHAPTER 8 Trap Manager This chapter describes how your application can use the Trap Manager to augment or override an existing system software routine Although this chapter describes patching in some depth you should rarely if ever find a need to use patches in an application The primary purposes of patches as their name suggests are to fix problems and augment routines in ROM code To use this chapter you should have some knowledge of assembly language For information about the instruction sets of microprocessors in the Motorola MC680x0 family see the appropriate user s manual for example the MC68020 32 Bit Microprocessor User s Manual This chapter describes how the Trap Manager works and then shows how you can use the Trap Manger to m check for the availability of a system software routine m alter the behavior of a system software routine About the Trap Manager The Trap Manager is a collection of routines that lets you add extra capabilities to system software routines In order to execute system software routines system software takes advantage of the unimplemented instruction feature of the MC680x0 family of microprocessors which are the central processing units CPUs used in the Macintosh family of computers The MC680x0 like other microprocessors executes a stream of instructions Information encoded in an instruction indicates the operation to be performed by the microprocessor The MC680x0 family of
16. Toolbox or Operating System routine from assembly language The names of all trap macros begin with the underscore character _ followed by the name About the Trap Manager CHAPTER 8 Trap Manager of the corresponding routine As a rule the macro name is the same as the name used to call the routine from Pascal For example to call the Window Manager function NewWindow you should use an instruction with the macro name _NewWindow There are some exceptions however in which the spelling of the macro differs from the name of the Pascal routine itself these are noted in the documentation for the individual routines Trap macros for Toolbox routines take no arguments any parameters must be pushed on the stack before invoking the routine See Calling Conventions for Stack Based Routines on page 8 16 for more information Trap macros for Operating System routines may have as many as three optional arguments The first argument if present is used to load a register with a parameter value for the routine you re calling The remaining arguments control the settings of the various flag bits in the A line instruction Jobeueyy des e About Routine Selectors A routine selector is a value that is pushed on the stack to select a particular routine from a group of routines to be executed Many trap macros take routine selectors For example the trap macro _HFSDispatch has the possibility of calling 42 different system software routi
17. atch address into the Operating System trap dispatch table or into the Toolbox trap dispatch table Trap Manager Reference CHAPTER 8 Trap Manager SetOSTrapAddress You can use the SetOSTrapAddress procedure to install an Operating System patch address into an Operating System trap dispatch table PROCEDURE SetOSTrapAddress trapAddr LongInt trapNum Integer trapAddr The Operating System patch address trapNum Operating System A line instruction or a trap number If you specify an Operating System A line instruction the function extracts the trap number located in bits 0 7 for you DESCRIPTION The SetOSTrapAddress procedure places the Operating System patch address specified by the t rapAddr parameter into the Operating System trap dispatch table The t rapNum parameter specifies the location of the Operating System patch address in the Operating System trap dispatch table The procedure call SetOSTrapAddress trapAddr trapNum performs the same operation as aNSetTrapAddress trapAddr trapNum OSTrap procedure call Note If the system software routine that is being patched has any come from patches the SetOSTrapAddress procedure installs the address of the patch into the exit JMP instruction of the last come from patch in the chain rather than into the trap dispatch table SEE ALSO For information about the Unimplemented procedure see page 8 29 For more information about the NSet TrapAddress fun
18. bits are masked according to the value of the t Typ parameter Note If the system software routine that is being patched has a come from patch the NSet TrapAddress procedure installs the address of the patch into the exit JMP instruction of the come from patch rather than into the trap dispatch table A WARNING If the first 4 bytes of the trapAddr parameter is 60064EF9 indicating a come from patch NSet TrapAddress triggers a system error A ASSEMBLY LANGUAGE INFORMATION The registers on entry for the __Set TrapAddress macro are Registers on entry DO An A line trap word A0 Address of next routine in the daisy chain a system software routine or a patch When calling the __Set TrapAddress macro you set bit 9 of the A line trap word to indicate a new system that is any version since the Macintosh Plus or Macintosh 512K You use bit 10 to indicate whether the system software routine that is being patched is a Toolbox routine by setting bit 10 to 1 or an Operating System routine by setting bit 10 to 0 Macintosh development environments provide the modifier words newToo1 and newOS to be used as arguments in the __Set TrapAddress macro Given an A line instruction in register DO and a system software address in register AO you set the Toolbox routine with the trap number in register DO to have the address in AO you use the macro _SetTrapAddress newTool This is equivalent to calling NSetTrapAddress trapAddr trapN
19. ction see page 8 30 SetToolboxTrapAddress You can use the Set ToolboxTrapAddress procedure to install a Toolbox patch address into the Toolbox trap dispatch table The Set ToolboxTrapAddress procedure is also available as the Set ToolTrapAddress procedure PROCEDURE SetToolboxTrapAddress trapAddr LongInt trapNum Integer trapAddr The Toolbox patch address trapNum Toolbox A line instruction or a trap number If you specify a Toolbox A line instruction the function extracts the trap number located in bits 0 9 for you Trap Manager Reference 8 29 JoBeuey des e DESCRIPTION SEE ALSO CHAPTER 8 Trap Manager The Set ToolboxTrapAddress procedure places the Toolbox patch address specified by the trapAddr parameter into the Toolbox trap dispatch table The t rapNum parameter specifies the location of the Toolbox patch address in the Toolbox trap dispatch table The Set ToolboxTrapAddress trapAddr trapNum procedure performs the same operation as aNSet TrapAddress trapAddr trapNum ToolTrap procedure call Note If the system software routine that is being patched has any come from patches the Set ToolboxTrapAddress procedure installs the address of the patch into the exit JMP instruction of the last come from patch in the chain rather than into the trap dispatch table For information about the Unimplemented procedure see page 8 29 The NSetTrapAddress function is described next NSetTrapAddress
20. e 8 14 Bit 8 indicates whether the value in register AO is returned from the Operating System routine If bit 8 is 0 the value in register AO is returned from the Operating System routine If bit 8 is 1 the value in register AO is not returned by the Operating System routine As previously described the trap number is in bits 7 0 and is used to determine which of the 256 possible Operating System routines is executed For example a call to the Operating System function Get Pt rSize is translated to the A line instruction A021 This A line instruction causes the microprocessor to transfer About the Trap Manager 8 11 Jobeueyy des e CHAPTER 8 Trap Manager control to the trap dispatcher which deals with any instruction of the form Axxx The trap dispatcher first saves registers DO D1 D2 A1 and if bit 8 is 0 AO The trap dispatcher places the A line instruction itself into the low order word of register D1 so that the Operating System routine can inspect the flag bits Next the trap dispatcher examines the other bits in the A line instruction The value 0 of bit 11 indicates that GetPtrSize is an Operating System routine and that the value in bits 7 0 is the index into the Operating System trap dispatch table The trap dispatcher uses the index which is 33 in this example to find the address of the Get Pt rSize function in the Operating System trap dispatch table When the address is found the trap dispatcher transfers contro
21. e NGet TrapAddress function returns the address of the system software routine specified by the tTyp and t rapNum parameters If t Typ is OSTrap the NGet TrapAddress function retrieves the address from the Operating System trap dispatch table If tTyp is ToolTrap the NGet TrapAddress function retrieves the address from the Toolbox trap dispatch table If the desired system software routine is not supported on the installed system software NGet TrapAddress returns the address of the Unimplemented procedure The t rapNum parameter should contain a trap number in bits 0 7 if tTyp is OSTrap and in bits 0 9 if tTyp is ToolTrap The t rapNum parameter may have any word value its irrelevant high order bits are masked according to the value of the t Typ parameter Note If the system software routine has a come from patch the NGet TrapAddress function returns the address of the routine immediately following the come from patch Trap Manager Reference 8 27 JeBeuey deil RE CHAPTER 8 Trap Manager ASSEMBLY LANGUAGE INFORMATION SEE ALSO The registers on entry and exit for the __Get TrapAddress macro are Registers on entry DO An A line trap word Registers on exit A0 Address of next routine in the daisy chain a system software routine or a patch When calling the _Get TrapAddress macro you set bit 9 of the A line instruction to indicate a new system that is any version since the Macintosh Plus or Macintosh 512K Yo
22. eed 124 bytes _NewPtr SYS CLEAR allocate requested memory in system heap and initialize to zeroes The SYS modifier specifies allocation from the system heap regardless of the value of the global variable TheZone and the CLEAR modifier specifies that the Memory Manager should initialize the block contents to zero For further details consult Inside Macintosh Memory A Line Instructions for Toolbox Routines A Toolbox trap is an exception that is caused by an A line instruction that executes a Toolbox routine When dispatching a Toolbox trap the trap dispatcher extracts the trap number from the A line instruction and uses it as an index into the Toolbox trap dispatch table The index points to the entry in the Toolbox trap dispatch table that contains the address of the desired Toolbox routine Figure 8 9 illustrates an A line instruction that is used to access a Toolbox routine About the Trap Manager CHAPTER 8 Trap Manager Figure 8 9 An A line instruction for a Toolbox routine 15 14 13 12 11 10 9 0 Trap number Auto pop bit A 1 Toolbox trap Bit 11 tells the trap dispatcher that this A line instruction is used to access a Toolbox routine Bit 10 is the auto pop bit Bits 9 0 contain the trap number which as previously described determine which of the 1024 possible Toolbox routines is executed The auto pop bit is described in detail in The Auto Pop Bit on page 8 20 Jo
23. entry in the Operating System trap dispatch table contains a 32 bit address of an Operating System routine and each entry in the Toolbox trap dispatch table contains a 32 bit address of a Toolbox routine The system software routines can be located in either ROM or RAM Process for Accessing System Software Routines As previously described when your application calls a system software routine an A line instruction is sent to the microprocessor The microprocessor does not recognize this instruction and an exception is generated This exception is then handled by the trap dispatcher When the trap dispatcher receives the A line instruction it looks into one of the two trap dispatch tables to find the address of the called system software routine When the trap dispatcher retrieves the address it transfers control to the specified system software routine For example Figure 8 3 illustrates a call to the Toolbox procedure Fil1Rect When the application calls the FillRect procedure an exception is generated The trap dispatcher looks into the Toolbox trap dispatch table to find the address of the Fi11Rect procedure When the address is found the trap dispatcher transfers control to the Fil1lRect procedure About the Trap Manager 8 5 Jobeueyy des RE CHAPTER 8 Trap Manager Figure 8 3 Accessing the Fil1Rect procedure Toolbox trap dispatch table Address SXXXXXXXX 1024 Trap dispatcher 8 6
24. er 4 bits of an A line instruction have the hexadecimal value A hence the name A line instruction Bit 11 of the A line instruction indicates the type of system software routine to be invoked a value of 0 in bit 11 indicates an Operating System routine a value of 1 in bit 11 indicates a Toolbox routine The trap number in an A line instruction is used as an index into the appropriate dispatch table The meaning of the flags vary accordingly to the type of A line instruction When your application calls a system software routine thereby generating an exception the microprocessor pushes an exception stack frame onto the stack Figure 8 6 shows a typical exception stack frame After pushing the exception stack frame on the stack the microprocessor transfers control to the trap dispatcher Figure 8 6 Exception stack frame on Macintosh computers with a MC68020 microprocessor 8 10 or greater Stack on eniry to trap dispatcher Vector offset Program Counter Status register Stack Pointer Ves x About the Trap Manager CHAPTER 8 Trap Manager The trap dispatcher discards the status register and vector offset Depending on whether the A line instruction is used to invoke an Operating System routine or a Toolbox routine the trap dispatcher deals with the stack and registers in two very different ways as described in the next section A line Instructions for Operating System Routines and in
25. g conventions In this example a routine calls the application defined function MyPascalFn When the application calls the function MyPascalFn the application must first make room on the stack for the function result then push the parameters on the stack in left to right order About the Trap Manager CHAPTER 8 Trap Manager Figure 8 11 Pascal calling convention Stack on entry to function Stack on return from function a Z a Reserved space for 2 Function result function result Stack Pointer n Return address Stack Pointer D xe D 5 D oO A FUNCTION MyPascalFn x Integer y Integer z LongInt LongInt Figure 8 12 illustrates C calling conventions In this example a routine calls the application defined function MyCFn When the application calls the function MyCFn the application pushes the parameters on the stack in right to left order The function result is returned in register DO and not on the stack Figure 8 12 C calling convention Stack on entry to function Return address Stack Pointer A long MyCFn short a short b long c About the Trap Manager 8 17 8 18 CHAPTER 8 Trap Manager Parameter Passing Conventions for Toolbox Routines All variable parameters parameters of type VAR are passed as pointers to the actual storage location In the case of byte sized types pa
26. he address of the Toolbox routine specified by the t rapNum parameter If the desired Toolbox routine is not supported on the installed system software the Get ToolboxTrapAddress function returns the address of the Unimplemented procedure The t rapNum parameter should contain a trap number in bits 0 9 Get ToolboxTrapAddress masks the irrelevant high order Trap Manager Reference CHAPTER 8 Trap Manager bits A Get ToolboxTrapAddress trapNum function call performs the same operation as a NGet TrapAddress trapNum ToolTrap function call SEE ALSO For more information about the Unimplemented procedure see page 8 29 The NGet TrapAddress function is described next For an example of how to use the Get ToolboxTrapAddress function see Listing 8 1 on page 8 22 NGetTrapAddress You can use the NGet TrapAddress function to retrieve the address of either an Operating System routine or a Toolbox routine FUNCTION NGetTrapAddress trapNum Integer tTyp TrapType LongInt trapNum A line instruction or a trap number If you specify an A line instruction the function extracts the trap number for you tTyp The trap type If you supply the t Typ parameter with the constant OSTrap the NGet TrapAddress function retrieves the address from the Operating System trap dispatch table If you supply t Typ parameter with the constant ToolTrap the NGet TrapAddress function retrieves the address from the Toolbox trap dispatch table DESCRIPTION Th
27. ication When your application is switched out your application s heap and patch is swapped out For example if you patch FillRect with the patch MyPatchFillRect the MyPatchFillRect patch is executed only when the FillRect procedure is called from your application Note When running in System 7 or under MultiFinder in System 6 each application has its own copy of the trap dispatch tables This ensures that an application s patches apply only when it is running and that they re discarded when the application quits Patch for All Applications If you install a patch from a system extension during system startup your patch is placed in the system heap and applies to all applications For example if you patch the FillRect procedure with the patch MyPatchFillRect from a system extension the MyPatchFillRect patch is executed every time the FillRect procedure is called no matter which application calls it About the Trap Manager 8 9 Jobeueyy des e CHAPTER 8 Trap Manager A Line Instructions When your application calls a Toolbox or an Operating System routine an A line instruction is sent to the microprocessor Each A line instruction contains information about the called system software routine Figure 8 5 shows the layout of an A line instruction Figure 8 5 A line instruction format 15 14 13 12 11 10 0 Flags and trap number A 0 Operating System trap 1 Toolbox trap The high ord
28. in this example was designed to install both Operating System and Toolbox patches it uses the NGet TrapAddress and NSetTrapAddress routines The NGetTrapAddress and NSetTrapAddress routines both need a parameter that indicates which type of routine is being patched an Operating System or Toolbox routine Trap Manager Reference This section describes the routines provided by the Trap Manager You can use these routines to m access an address in a trap dispatch table m install a patch address into a trap dispatch table This section also documents the Unimplemented procedure Routines This section describes the routines provided by the Trap Manager Accessing Addresses From the Trap Dispatch Tables You can access the address of a system software routine by using the Get OSTrapAddress Get ToolboxTrapAddress or NGetTrapAddress function The Get OSTrapAddress function retrieves only an Operating System routine address and the Get ToolboxTrapAddress retrieves only a Toolbox routine address The NGet TrapAddress function is the most general of these functions you can use the function to retrieve the address of an Operating System routine or a Toolbox routine Trap Manager Reference 8 25 Jobeueyy des fi CHAPTER 8 Trap Manager GetOSTrapAddress DESCRIPTION SEE ALSO You can use the GetOSTrapAddress function to access the address of an Operating System routine that is located in the Operating Sy
29. ing 8 3 illustrates a patch for the SysBeep Operating System procedure When SysBeep is called this application defined patch MySysBeep is executed before transferring control to the original SysBeep procedure Listing 8 3 Patching the SysBeep Operating System procedure PROCEDURE MySysBeep duration Integer VAR oldPort GrafPtr wMgrPort GrafPtr aise Integer BEGIN GetPort oldPort GetWMgrPort wMgrPort SetPort wMgrPort FOR 3 DOWNTO 0 DO BEGIN InvertRect wMgrPort portBits bounds ND SetPort oldPort END of MySysBeep ea To transfer control to the next routine in the daisy chain in this example the original SysBeep procedure the application defined MyInstal1APatch procedure Listing 8 5 uses the application defined procedure MyFollowDaisyChain shown in Listing 8 4 The MyFollowDaisyChain duplicates the parameter for the SysBeep procedure and then pushes the address of the SysBeep procedure on the stack Listing 8 4 shows the application defined procedure MyFollowDaisyChain Using the Trap Manager 8 23 JoBeuey des e CHAPTER 8 Trap Manager Listing 8 4 Jumping to the next routine in the daisy chain MyFollowDaisyChain PROC EXPORT IMPORT MYSYSBEEP Ct BRA S 2 1 DC L 50FFC001 2 MOVE W 4 A7 A7 duplicate the parameters OVE L 1 A7 and push the chain link BRA S MYSYSBEEP NOP ENDPROC The applica
30. l to the Get Pt rSize function Figure 8 8 illustrates the stack after the trap dispatcher has transferred control to an Operating System routine Figure 8 8 The stack on entry to an Operating System routine 8 12 Stack on entry to Operating System routine Program Counter Unspecified long word Saved registers A0 A1 D0 D2 Program Counter Stack in trap dispatcher Pointer The Operating System routine may alter any of the registers DO D2 and AO A2 but it must preserve registers D3 D7 and A3 A6 The Operating System routine may return information in register DO and A0 if bit 8 is set To return to the trap dispatcher the Operating System routine executes the RTS return from subroutine instruction When the trap dispatcher resumes control first it restores the value of registers D1 D2 A1 A2 and if bit 8 is 0 AO The values in registers DO and if bit 8 is 1 in AO are not restored Calling Conventions for Register Based Routines Register based routines receive their parameters from microprocessor registers and they pass their results in microprocessor registers Virtually all Operating System routines are register based routines About the Trap Manager CHAPTER 8 Trap Manager An Operating System routine returns information only in registers DO and if bit 8 is 1 AO The stack and all other registers are unchanged Many Operating System routines return a result code i
31. n places the result these types include Double 8 bytes Comp 8 bytes and Ext ended 10 or 12 bytes types SET ARRAY RECORD and strings greater than 4 bytes in size For a 1 byte SET for types SET ARRAY and RECORD and for strings whose size is one word the caller allocates a word on the stack For types SET ARRAY and RECORD and strings whose size is two words the caller allocates a long word on the stack The conventions for returning results of functions are summarized in Table 8 2 Table 8 2 Conventions for returning results from Toolbox functions Data object left on stack or returned Function result type through pointer on stack Boolean Byte range 0 to 1 Char 16 bits range 0 to 255 Integer 16 bits range 32768 to 32767 LongInt 32 bits Pointer 32 bits Enumeration range 0 to 127 Byte range 0 to 127 Enumeration range 0 to 32767 16 bits range 0 to 32767 continued About the Trap Manager 8 19 Jobeueyy des RE 8 20 CHAPTER 8 Trap Manager Table 8 2 Conventions for returning results from Toolbox functions continued Data object left on stack or returned Function result type through pointer on stack Subrange range 128 to 127 Byte range 128 to 127 Subrange range 32768 to 32767 16 bits range 32768 to 32767 Real Real Double Double at address given by pointer Comp Comp at address given by pointer Extended Extended at address given by pointer
32. n the low memory word of register DO to report whether the requested operation was performed successfully A result code of 0 indicates that the routine completed successfully any other value typically indicates an error Just before the trap dispatcher finishes execution it tests the low order word of register DO with a TST W instruction to set the condition codes of the microprocessor Note Calling conventions for PowerPC microprocessor based Macintosh computers are different from the calling conventions described for in this section For information about calling conventions for PowerPC processor based Macintosh computers see Inside Macintosh PowerPC System Software Parameter Passing Conventions for Operating System Routines Jobeueyy des e By convention register based routines normally use register AO for passing addresses such as pointers to data objects and register DO for other data values such as integers For routines that take more than two parameters the parameters are normally collected in a parameter block in memory and a pointer to the parameter block is passed in register AO See the description of an individual routine in the appropriate Inside Macintosh book for exact details Function Results Most Operating System functions return their function result or result code in register DO Parameters are returned through register AO usually as a pointer to a parameter block Whether the trap dispatche
33. nes Hence the trap macro has 42 different routine selectors The routine selector that is passed on the stack for _HFSDispacth to access selects which of the 42 software routines _HFSDispatch executes Most system software routines that are accessed through a trap macro and a routine selector also have a corresponding macro that expands to call the original trap macro and automatically puts the correct routine selector on the stack For example the trap macro _GetCatInfo expands to call _HFSDispatch and places the selector 0009 on the stack after the parameters Using the Trap Manager You can use the Trap Manger to read from and write to a trap dispatch table To read an address from a trap dispatch table you can call the NGet TrapAddress Get OSTrapAddress or Get ToolboxTrapAddress functions To write an address to a trap dispatch table you can use the NGet TrapAddress GetOSTrapAddress or Get ToolboxTrapAddress procedures This section shows how you can use the Trap Manager to m determine if a system software routine is available m patch a system software routine Determining If a System Software Routine is Available You can use the Trap Manager to determine the availability of system software routines Using the Trap Manager 8 21 CHAPTER 8 Trap Manager The Gestalt Manager introduced in System 6 0 4 and discussed in the chapter Gestalt Manager in this book is the primary tool for querying the system about
34. pWord trType lt gt Get ToolboxTrapAddress _Unimplemented Note Macintosh Plus and Macintosh SE computers with system software prior to System 7 masked their trap numbers with 1FF in the Get ToolboxTrapAddress function so that the address of A line instruction AA6E whether implemented or not would be the same as A line instruction A86E which invokes the InitGraf routine You can use the application defined procedure MySWRout ineAvailable to check for system software routines not supported by the Gestalt Manager A notable example is the WaitNextEvent function which has never had Gestalt selectors Listing 8 2 shows two common uses of the application defined MySWRout ineAvailable procedure 8 22 Using the Trap Manager CHAPTER 8 Trap Manager Listing 8 2 Determining whether WaitNextEvent and Gestalt are available VAR gHasWNE gHasGestalt Boolean check for the availability of WaitNextEvent gHasWNE MySWRoutineAvailable _WaitNextEvent check for the availability of Getstalt gHasGestalt MySWRoutineAvailable _Gestalt Patching a System Software Routine Although this chapter describes patching in some depth you should rarely if ever find a need to use patches in an application The primary purposes of patches as their name suggests are to fix problems and augment routines in ROM code The examples in this section are only included for the sake of completeness List
35. r preserves register AO depends on the setting of bit 8 in the A line instruction If bit 8 is 0 the trap dispatcher saves and restores register AO if it s 1 the routine passes back register AO unchanged Thus bit 8 of the A line instruction should be set to 1 only for those routines that use register AO to return information The trap macros automatically set this bit correctly for each routine To see in which register the function passes the function result see the description of the individual function in the appropriate Inside Macintosh book About the Trap Manager 8 13 8 14 CHAPTER 8 Trap Manager Flag Bits Many Operating System routines use the flag bits in an A line instruction to encode additional information used by the routine For example the A line instructions that invoke Memory Manager routines define the two flag bits like this Bit Explanation 9 If 1 initialize all bytes in the allocated memory to 0 If 0 do not initialize all bytes in the allocated memory to 0 8 If 1 allocate memory from the system heap If 0 allocate memory from the application heap These two bits are defined in assembly language as CLEAR EQU 200 j initialize block to zero SYS EQU 400 use the system heap When used with a Memory Manager A line instruction these modifiers cause flag bits 9 and 10 respectively to be set They could be used in an assembly language instruction sequence like MOVEQ 124 D0 n
36. rameters of type VAR may have odd values Nonvariable parameters are passed in different ways depending on the type of the parameter Values of type Boolean elements of an enumerated type with fewer than 128 elements and subranges within the range 128 to 127 are passed as signed byte values Values of type Integer and Char and all other enumerations and subranges are passed as signed word values Pointers and values of type LongInt are passed as signed 32 bit values Table 8 1 summarizes the parameter passing conventions Table 8 1 Toolbox parameter passing conventions Parameter type Boolean Char Integer LongInt Pointer Enumeration range 0 to 127 Enumeration range 0 to 32767 Subrange range 128 to 127 Subrange range 32768 to 32767 Real Double Comp Extended ARRAY RECORD string lt 4 bytes ARRAY RECORD string gt 4 bytes SET Data object pushed on stack Byte range 0 to 1 16 bits range 0 to 255 16 bits range 32768 to 32767 32 bits 32 bits Byte range 0 to 127 16 bits range 0 to 32767 16 bits range 128 to 127 Word range 32768 to 32767 Address of Ext ended copy Address of Ext ended copy Address of Ext ended copy Address of argument Value word or long word Address of value SET value rounded to whole number of words A parameter of type SET is passed by rounding its size up to the next whole word if necessary then pushing its value so that the lowest order
37. s triggered by the Trap Manager dsCoreErr 12 unimplemented trap error dsBadPatchHeader 83 attempt to install a come from patch Data Types TYPE TrapType OSTrap ToolTrap Routines Accessing Addresses From the Trap Dispatch Tables FUNCTION GetOSTrapAddress trapNum Integer LongInt GetToolboxTrapAddress is also spelled as GetToolTrapAddress FUNCTION GetToolboxTrapAddress trapNum Integer LongInt FUNCTION NGetTrapAddress trapNum Integer tTyp TrapType LongInt Installing Patch Addresses Into the Trap Dispatch Tables PROCEDURE SetOSTrapAddress trapAddr LongInt trapNum Integer SetToolboxTrapAddress is also spelled as SetToolTrapAddress 8 34 Summary of the Trap Manager CHAPTER 8 Trap Manager PROCEDURE SetToolboxTrapAddress trapAddr LongInt trapNum Integer PROCEDURE NSetTrapAddress trapAddr LongInt trapNum Integer tTyp TrapType Detecting Unimplemented System Software Routines PROCEDURE Unimplemented Manipulating One Trap Dispatch Table Obsolete Routines FUNCTION GetTrapAddress trapNum Integer LongInt PROCEDURE SetTrapAddress trapAddr LongInt trapNum Integer C Summary Constants Gestalt selectors define gestaltOSTable SOSTE base of Operating System dispatch table define gestaltToolboxTable EbEt base of Toolbox dispatch table define gestaltExtToolboxTable
38. ss of the called system software routine After the trap dispatcher retrieves the address it transfers control to the specified system software routine Figure 8 1 illustrates the processing of instructions that include the A line instructions that the microprocessor does not recognize Figure 8 1 How the CPU processes A line instructions Machine instruction Does CPU CPU executes recognize Ys instruction instruction No Exception trap Retrieves address Trap dispatch table Executes System software routine Trap dispatcher You can use the Trap Manager routines to read from and write to the two trap dispatch tables maintained by system software About the Trap Manager CHAPTER 8 Trap Manager Trap Dispatch Tables System software uses trap dispatch tables to locate the address of system software routines System software maintains two trap dispatch tables an Operating System trap dispatch table and a Toolbox trap dispatch table Figure 8 2 illustrates the two trap dispatch tables Figure 8 2 Trap dispatch tables Toolbox trap dispatch table Operating System trap dispatch table Address Address At system startup time system software builds the trap dispatch tables and places them in RAM The Operating System trap dispatch table contains 256 entries and the Toolbox trap dispatch table contains 1024 entries Each
39. ssing was suspended A Toolbox routine changes the Stack Pointer in register A7 and pops the return address and any input parameters A routine might also alter registers DO D2 AO and A1 WARNING Some Toolbox routines for example the LongMu1 procedure described in the chapter Mathematical and Logical Utilities in this book preserve more than the required set of registers However you should assume all of registers DO D2 AO and A1 are altered by Toolbox routines A Calling Conventions for Stack Based Routines Stack based routines receive their parameters on the stack and return their results on the stack Virtually all Toolbox routines are stack based routines Most Toolbox routines follow Pascal calling conventions that is Toolbox routine parameters are evaluated from left to right and are pushed onto the stack in the order in which they are evaluated Function results are returned by value or by address on the stack Space for the function result is allocated by the caller before the parameters are pushed on the stack The caller is responsible for removing the result from the stack after the call Note Calling conventions for PowerPC microprocessor based Macintosh computers are different from the calling conventions described in this section For information about calling conventions for PowerPC processor based Macintosh computers see Inside Macintosh PowerPC System Software Figure 8 11 illustrates Pascal callin
40. stem trap dispatch table FUNCTION GetOSTrapAddress trapNum Integer LongInt trapNum Operating System A line instruction or a trap number If you specify an Operating System A line instruction the function extracts the trap number for you The Get OSTrapAddress function returns the address of the Operating System routine specified by the t rapNum parameter If the desired Operating System routine is not supported on the installed system software the GetOSTrapAddress function returns the address of the Unimplemented procedure The t rapNum parameter should contain a trap number in bits 0 7 GetOSTrapAddress masks the irrelevant high order bits A Get OSTrapAddress trapNum function call performs the same operation as a NGet TrapAddress trapNum OSTrap function call For more information about the Unimplemented procedure see page 8 29 For information about the NGet TrapAddress function see page 8 27 GetToolboxTrapAddress DESCRIPTION 8 26 You an use the Get ToolboxTrapAddress function to access the address of a Toolbox routine which is located in the Toolbox trap dispatch table The Get ToolboxTrapAddress function is also available as the Get Tool TrapAddress function FUNCTION GetToolboxTrapAddress trapNum Integer LongInt trapNum Toolbox A line instruction or a trap number If you specify a Toolbox A line instruction the function extracts the trap number for you The Get ToolboxTrapAddress function returns t
41. tion defined procedure My InstallAPatch in Listing 8 5 installs a patch into the daisy chain in this example the MySysBeep patch First the procedure calls the NGet TrapAddress function to get the address of the next routine in the daisy chain This address could be the address of another patch or the system software routine Next MyInstallAPatcnh calls the NSetTrapAddress procedure to put the address of the new patch in this example the address of MySysBeep patch into the trap dispatch table Listing 8 5 Installing a patch 8 24 PROGRAM MyPatchInstaller USES Memory TooliIntf OSIntf OSUtils Windows ToolUtils Traps Resources SamplePatch TYP PatchCodeHandle PatchCodePtr PatchCodePtr PatchCodeHeader PatchCodeHeader rs RECORD branch Integer oldTrapAddress LongInt END T PROCEDURE MyFollowDaisyChain duration Integer EXTERNAL PROCEDURE MyInstallAPatch trapNumber Integer tType TrapType pPatchCode PatchCodePtr T BEGIN pPatchCode oldTrapAddress NGetTrapAddress trapNumber tType NSetTrapAddress ORD4 pPatchCode trapNumber tType END of MyInstallAPAtch Using the Trap Manager CHAPTER 8 Trap Manager BEGIN InitGraf qd thePort InitFonts InitWindows MyInstallAPatch _SysBeep ToolTrap PatchCodePtr MyFollowDaisyChain SysBeep 1 END of MyPatchInstaller Note The MyInstallAPatch procedure used
42. u use bit 10 to indicate whether the trap in question is a Toolbox routine by setting bit 10 to 1 or an Operating System routine by setting bit 10 to 0 Macintosh development environments provide the modifier words newTool and newOS to be used as arguments in the __Get TrapAddress macro To obtain the address of a Toolbox trap whose number is in register DO you use the macro _GetTrapAddress newTool This is equivalent to calling Ncet TrapAddress trapNum newTool The trapNum parameter is the A line trap word placed in register DO for the assembly language call Similarly to obtain the address of an Operating System routine whose A line trap word is in register DO you use the macro _GetTrapAddress newOS This is equivalent to calling NcetTrapAddress trapNum new0OS For information about the Unimplemented procedure see page 8 29 For information about the NSet TrapAddress function see page 8 30 Installing Patch Addresses Into the Trap Dispatch Tables 8 28 You can install the address of a patch into a trap dispatch table by using the SetOSTrapAddress Set ToolboxTrapAddress or NSetTrapAddress procedure The SetOSTrapAddress procedure installs a patch address into the Operating System trap dispatch table and the Set ToolboxTrapAddress installs a patch address into the Toolbox trap dispatch table The NSet TrapAddress procedure is the most general of these procedures You can use the NSet TrapAddress procedure to install a p
43. um newTool The t rapAddr parameter is the address placed in register AO The t rapNum parameter is the A line instruction placed in D0 for the assembly language call Similarly to set the address of an Operating System trap whose A line instruction is in register DO to the address in register AO you use the macro _SetTrapAddress newOS This is equivalent to calling NSetTrapAddress trapAddr trapNum new0OS Trap Manager Reference 8 31 Jobeueyy des e SEE ALSO CHAPTER 8 Trap Manager The Unimplemented procedure is described next For information about the NGet TrapAddress function see page 8 27 For an example of how to use the NSet TrapAddress function see Listing 8 5 on page 8 24 Detecting Unimplemented System Software Routines This section describes the Unimplemented procedure The address of this procedure is placed in all undefined entries of a trap dispatch table When invoked the Unimplemented procedure triggers a system error Unimplemented DESCRIPTION The Unimplemented procedure triggers a system error when called PROCEDURE Unimplemented The address of the Unimplemented procedure is at system startup time placed into all entries of each trap dispatch table that do not contain an address of a system software routine When called the Unimplemented procedure triggers the system error 12 dsCoreErr which crashes the currently running application WARNING Your application should ne
44. ver use this procedure A Manipulating One Trap Dispatch Table Obsolete Routines This section describes two obsolete Trap Manager routines Get TrapAddress and Set TrapAddress Though a description of the routines are included here any use of these routines is discouraged GetTrapAddress 8 32 The Get TrapAddress function is obsolete and is documented here only for the sake of completeness FUNCTION GetTrapAddress trapNum Integer LongInt trapNum Toolbox A line instruction or a trap number If you specify an A line instruction the function extracts the trap number for you Trap Manager Reference DESCRIPTION CHAPTER 8 Trap Manager The Get TrapAddress function was used when both the Operating System trap addresses and Toolbox trap addresses were located in the same trap dispatch table Today any system software routine with the trap number 00 to 4F 54 or 57 is drawn from the Operating System dispatch table any other software routine is taken from the Toolbox dispatch table WARNING The Get TrapAddress function is not supported under Power PC a WARNING The Get TrapAddress procedure ignores the high order bits in the t rapNum parameter the procedure is not able to differentiate between Operating System routines and Toolbox routines The Get TrapAddress procedure is not reliable on any computer today A SetTrapAddress DESCRIPTION The Set TrapAddress procedure is obsolete and is documented
45. xecution of the system software routine control is transferred back to the trap dispatcher Tail Patch A tail patchis a routine that gets executed before the original system software routine and regains control after the execution of the system software routine A tail patch uses a jump subroutine instruction JSR to transfer control to the system software routine After the system software routine returns control to the tail patch the tail patch returns control to the trap dispatcher WARNING You should never install tail patches in system software versions earlier than System 7 Tail patches may conflict with come from patches installed by Apple a Come From Patch Used Only by Apple A come from patch also called a system patch is a type of patch used only by Apple Come from patches are used to replace erroneous code or to add capabilities not in ROM When a come from patch is invoked it examines the stack to determine where it was called from If the come from patch was invoked from a particular place in ROM a spot where the code needs to be augmented or deleted the come from patch executes the About the Trap Manager CHAPTER 8 Trap Manager modifying code Otherwise if the come from patch was called from a part of the system that does not need to be augmented it transfers control to the next routine in the daisy chain This routine could be another patch or the system software routine Beginning with System
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