INMOS TN68 - Developing parallel C programs for
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1. 4 Using the Dx214 C toolset 4 1 Introduction In this chapter we shall use the IMS Dx214 C toolset to build and configure the upper case example We shall use the example program in the following different ways e Upper casing on a single transputer all in C e Upper casing on two transputers using the icconf configuration tool 4 2 C parallel processing library extensions As well as being an ANSI standard C compiler icc provides a rich set of parallel processing library calls to enable us to write parallel programs all in C for more in depth coverage of these routines the reader is referred to 2 The extensions are similar to the ones provided in the IMS D711 INMOS 3L Parallel C compiler 1 4 3 Parallel version on one transputer all In C In this section we shall look at how we use the C parallel library functions to create a parallel program running on a single transputer The best way to understand how these libraries work is by the use of an example so here goes MODULE Upper Casing Example All in C using icc FILE system c NAME Richard Onyett Santa Clara RTC PURPOSE To create and run two parallel processes to perform the upper casing function include lt stdlib h gt include lt stdio h gt include lt channel h gt New headers for the channel IO include lt process h gt New headers for the processes The Master and worker processes are2. For now this serves as a guide to where the tools fit into the grand scheme of things 2 2 Software toolset summary The following is a brief list of all the tools provided in the toolset Tool Description icc The ANSI C compiler icconf The configurer icollect The code collector icvlink The TCOFF file convertor idebug The network debugger idump The memory dumper Used by idebug iemit The transputer memory configuration tool ieprom The EPROM program formatter tool ilibr The toolset librarian ilink The toolset linker ilist The binary lister imakef The Makefile generator iserver The host file server isim The IMS T425 simulator iskip The skip loader tool Table 1 Summary of toolset components 2 3 Software design cycle single transputer systems This section will take a look at the major tools and steps involved in devel oping software for a single transputer system Lets take a look at each of the steps Edit The Edit phase consists of writing the source code for your program this will include all of the source modules and header files Any standard text editor can be used for the C toolset e g MicroEmacs Microsoft Word and even edlin Compile The compilation phase consists of submitting the source code to the com piler The compiler is called icc which stands for Inmos C Compiler It requires the name of the source file and which processor type you wish to
3. components are built using the following makefile Module Simple Parallel C Examples Name Richard Onyett Santa Clara RTC File makefile cc icc CFLAGS t8 SRC master tco worker tco system tco LINK ilink f startup lnk SRC t8 BOOT icollect system lku t 15 system btl SRC LINK o system 1ku BOOT system tco system c CC system c CFLAGS master tco master c CC master c CFLAGS worker tco worker c CC worker c CFLAGS To compile we say make This will invoke the C compiler icc and the linker ilink the output of the linker is then used to produce a bootable file which can be downloaded to the transputer Running the single processor version To run the program we type iserver se sb system btl Booting Root Transputer Upper Casing EXAMPLE STARTS Master C Process STARTING Enter some text and it will be upper cased To quit type EOF CTRL Z a A hello HELLO Z Master C Process ENDING Upper Casing EXAMPLE ENDS In detail e iserver Invoke the host server program e se sb system btl Boot the file system btl to the transputer and monitor the error flag e Booting Root Transputer Message from the server as it starts 16 4 4 Configuring a multi processor version using icconf Introduction Now that we have the two process version working we shall map this onto two transputers using the configurer The IMS Dx214 Too
4. example runs two processes Master and Worker in parallel The sys tem c file contains the set up and calls to the two processes Each process in the system is declared by saying include lt process h gt Process WorkerPtr Declare a process The process is defined as a function call i e 11 void Worker Process Ptr Param1 Param2 ParamN Do some things The Process parameter must always be supplied it is needed so that the process can be executed by the system To create the process we must use the ProcAlloc function i e WorkerPtr ProcAlloc Worker 0 2 ToWorker FromWorker The parameters are as follows e Worker The name of the function process e 0 Default workspace size 4Kbyte on a 32 bit transputer and 1 Kbyte on a 16 bit transputer e 2 Number of parameters to be passed in this case its two e ToWorker First parameter in this case a channel called ToWorker e FromWorker Second parameter in this case a channel called From Worker Note that WorkerPtr is the pointer to our process or NULL if no space ProcAlloc Worker 0 2 ToWorker FromWorker is the function that builds processes Running the processes in parallel Now that we have set up the processes we must run them in parallel To achieve this we use the ProcPar library call i e ProcPar MasterPtr WorkerPtr NULL The parameters to this are as follows e MasterPtr The master process e Worke
5. stacksize 1k heapsize 50k interface input FromMaster output ToMaster Worker Describe how things are wired up input from_host output to_host connect Master fs from_host connect Master ts to_host connect Master ToWorker Worker FromMaster connect Master FromWorker Worker ToMaster Pull in the real code use master lku for Master use worker lku for Worker 22 Place the processES onto processORS place place place place place place Master Worker from_host on host to_host on host fs on Root Master Master on Root on Slave ts on Root Run Master on the first transputer Run Worker on the second transputer Wire up to the HOST machine link 0 link 0 Configured for IMS B008 place place place place Master Master Worker Worker ToWorker FromWorker ToMaster FromMaster on Root link 3 on Root link 3 on Slave link 0 on Slave link 0 Declaring the physical hardware We must declare all the transputers nodes in the network In detail e T800 Type of processor e memory 2M Describe amount of memory available on this processor e Root Name of the transputer node Showing physical interconnect We must describe how the links of our root and slave processors are con nected The root node is connected by its Link 0 to the host computer In detail e connect Root link 0 host Sign up for
6. the host services e connect Root link 3 Slave link 0 Connect root node to Slave node 23 Interface description This section describes how our process is joined to all others in the sys tem We pass in channel parameters and data concerning the size of the stack and heap required In detail e process Configuration keyword e stacksize 1k Size of the stackspace e heapsize 50k Size of the heap e interface Configuration keyword Here comes the interface descrip tion e input fs Pass input channel called fs Channel parameter 1 e output ts Pass input channel called ts Channel parameter 2 The first two items in the interface description for a process commu nicating with the host must be the from server fs and to server ts channels in that order e input FromWorker Pass input channel called FromWorker Channel parameter 3 e output ToWorker Pass output channel called ToWorker Channel parameter 4 e Master Name of this process This interface section is how channel parameters are passed into the C pro cesses although the parameters do not have to be channels The textual declaration of the channels determines which number should be used in the get_param call WARNING There is no checking that the parameter number is wired to the correct channel Wiring things up The next section wires up the soft channels to the processes on the transputer nodes In detail e c
7. Developing parallel C programs for transputers INMOS Technical Note 68 INMOS Limited 72 TCH 068 00 BONE plu t e r nfelt You may not 1 Modify the Materials or use them for any commercial purpose or any public display performance sale or rental 2 Remove any copyright or other proprietary notices from the Materials This document is distributed in the hope that it will be useful but WITHOUT ANY WARRANTY without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE INMOS IMS OCCAM are trademarks of INMOS Limited INMOS Limited is a member of the SGS THOMSON Microelectronics Group Contents 1 Introduction 2 What is the C toolset 2 1 Introduction e 2 2 Software toolset summary e 2 3 Software design cycle single transputer systems 2 4 Software design cycle multiple transputer systems 3 Example problem description 3 1 Introduction 4 Using the Dx214 C toolset 4 1 Introduction eca 0 0 2 2 00000022 ae 4 2 C parallel processing library extensions 4 3 Parallel version on one transputer all In C 4 4 Configuring a multi processor version using icconf Conclusions Differences between 3L and icc concurrency library Useful hints when writing C toolset programs B 1 What if the program will not run References 27 27 28 29 30 1 Introduction This document pr
8. compile to Linking Linking is achieved using the ilink tool This pulls together all of the object files and any libraries that have been created When compiling for a single transputer then the whole of the C Run Time library is used to give access to the host services Edit MicroEmacs Librarian ilibr Boot Strap icollect Run iserver Debug idebug Figure 1 Typical software development route single transputer systems Boot Strap To make the program run a transputer a Bootstrap must be added This is a small piece of code which is added to the front of your code and con tains instructions to reset the transputer and get ready to load and run your program After the load has occurred the bootstrap is overlayed and disappears To add this boot strap we use the icollect tool Run To load the program onto the transputer we use the iserver program This program sits on the host and enables the transputer to access the hosts services these include the file system keyboard and screen Debugging If the program failed to run correctly then we enter the debugging phase of development The toolset provides an interactive single stepping debugger called idebug The user can specify breakpoints and trace through the code 2 4 Software design cycle In the previous section we saw how multiple transputer systems to develop programs using single trans puters this section
9. e configured onto processors we must add some code to form a process interface Each process must have a main wrapped around it The C code for the Master process is as follows MODULE Simple Parallel C example FILE master c NAME Richard Onyett RTC Santa Clara PURPOSE To generate a stream of ascii characters on a channel and pass them to the upper case worker process include lt stdio h gt include lt stdlib h gt include lt channel h gt tinclude lt misc h gt int main void Channel ToWorker Channel FromWorker int cC int Going 1 Access the external configuration channels FromWorker Channel get_param 3 ToWorker Channel get_param 4 Intro printf Master C Process STARTING n printf Enter some text and it will be upper cased n 18 printf nTo quit type EOF CTRL Z n while Going c getchar Get a character from the stdin ChanOutInt ToWorker c Pass to the worker if c EOF Going 0 else c ChanInInt FromWorker Get the character back putchar c Qutput to the screen printf Master C Process ENDING n Shake the needles exit_terminate EXIT_SUCCESS from your back The worker process The code for the worker process is as follows MODULE Simple Parallel C example FILE worker c NAME Richard Onyet
10. esents a cook book approach to writing parallel C programs for single and networks of transputers Using the IMS Dx214 series C toolset the user can write programs which contain many parallel processes these processes can then be mapped onto a number of transputers using a method called configuration Although this technical note is aimed primarily at new users to the INMOS development systems advanced users may find this useful to come quickly up to speed with the new IMS Dx214 C toolset Also users of the IMS D711 3L INMOS toolset may find this note of interest as there is a section on how to convert existing IMS D711 C code over to the IMS Dx214 toolset All of the examples presented were developed using the following equipment e IMS D7214 C toolset for IBM PC e IMS B008 Motherboard for IBM PC e IMS B404 TRAM Two were required for the network example 2 What is the C toolset 2 1 Introduction The IMS Dx214 is a software cross development system for transputers hosted on a variety of platforms e g PC SUN3 SUN4 or VAX The devel opment system consists of a set of tools to enable users to write programs for single or multiple transputer networks In this section we shall look at the typical development cycle for code running on single or multiple transputers A brief outline of each tool used at each stage is given Later on in the document we shall show by means of worked examples exactly how to use the tools at each step
11. han_init process h timer h ChanReset chan_reset ProcAfter timer_delay ChanIn chan_in_message ProcWait timer_wait ChanInChar chan in byte ProcTime timer_now ChanInInt chan_in_word ProcTimeA fter timer_after ChanInTimeFail chan_in_message_t ProcTimePlus Not available ChanInChanFail Not Available ProcTimeMinus Not available ChanOut chan_out_message semaphor h sema h ChanOutChar chan_out_byte SemAlloc sema_alloc ChanOutInt chan_out_word SemInit sema_init ChanOutTimeFail chan_out_message_t Sem Wait sema_wait ChanOutChanFail Not Available SemSignal sema signal process h thread h SEMAPHOREINIT Not Available Process Not Available Not available sema signal_n ProcRun thread_create Not available sema_wait_n ProcPar Not Available Not available net h ProcParList Not Available Not available net_send ProcAlloc thread_create Not available net_receive Proclnit thread_create Not available par h ProcRunHigh thread_create printf par_printf ProcRunLow thread_create free par_free ProcReschedule Not Available malloc par_malloc ProcParam Not Available fprintf par_fprintf ProcStop thread_stop Table 2 Comparison of icc vs 3L concurrency library functions Upgrading code to IMS Dx214 from IMS D711 If you are recompiling code from IMS D711 with the IMS Dx214 C toolset you should include the conndx11 h file This contains macros to convert the IMS D711 channel and thread calls to equivalent IMS Dx214 calls 27 B Useful hints when writing C t
12. hat the transputers in the network have enough memory Do not specify 1 MByte in the configuration script when only 32K is available on the TRAM e Does the processor type in the network match what you have compiled for e Make sure that there is enough stack and heap space 29 References 1 IMS D711 3L Parallel C user manual INMOS Limited February 1989 INMOS document number 72 TDS 179 00 2 IMS Dx214 ANSI C toolset user manual INMOS Limited August 1990 INMOS document number 72 TDS 224 00 3 Program design for concurrent systems INMOS Technical Note 5 Philip Mattos INMOS Limited Bristol 30
13. in different files extern void Worker Process p Channel FromMaster Channel ToMaster extern void Master Process p Channel FromWorker Channel ToWorker int main void Process WorkerPtr MasterPtr Declare some processes Channel ToWorker FromWorker Connect em up with channels printf Upper Casing EXAMPLE STARTS n Allocate and initialise some channels if ToWorker ChanAlloc NULL printf ERROR Cannot allocate space for channel ToWorker n exit EXIT_FAILURE 10 if FromWorker ChanAlloc NULL 4 printf ERROR Cannot allocate space for channel FromWorker n exit EXIT_FAILURE Allocate the processes needed if WorkerPtr ProcAlloc Worker 0 2 ToWorker FromWorker NULL gt 4 printf ERROR Cannot allocate space for process Worker n exit EXIT_FAILURE if MasterPtr ProcAlloc Master 0 2 FromWorker ToWorker NULL gt 4 printf ERROR Cannot allocate space for process Master n exit EXIT_FAILURE Now we have some processes lets run em all in parallel This will not return until ALL of the processes have finished ProcPar MasterPtr WorkerPtr NULL All processes have successfully terminated lets say so printf Upper Casing EXAMPLE ENDS n exit EXIT_SUCCESS I m Outta here Setting up a process This
14. k master tco t8 o master lku LINK2 ilink f startrd lnk worker tco t8 o worker 1ku CONFIG icconf upc cfg BOOT icollect upc cfb upc btl 0BJS master lku worker 1ku CONFIG BOOT master lku master tco LINK1 worker lku worker tco LINK2 master tco master c CC master c CFLAGS worker tco worker c CC worker c CFLAGS The configurer is run by icconf upc cfs In detail e icconf Invoke the configurer e upc cfs The name of the configuration script The final stage to collect all of the code is done by icollect upc 1ku Running the mufti processor version This is done exactly as described previously 26 5 Conclusions This document has taken at look at the various methods for writing parallel C programs The user has the choice between a program written and con figured completely in C or using a Mixed language approach by wrapping the C programs in occam All of the methods allow the user to initially test the system on a single transputer once this is working then a configuration script is written and the system is parcelled out onto multiple transputers A Differences between 3L and icc concurrency li brary Table 2 provides a comparison between the IMS D711 and IMS Dx214 concurrency libraries icc 3L icc 3L channel h chan h ProcGetPriority thread_priority Channel CHAN PROC _HIGH THREAD_URGENT ChanAlloc Not available PROC_LOW THREAD_NOTURG Chan Init c
15. later on how we obtain the value for param number So to obtain a connection to the outside world we write Channel ToWorker Declare a local channel 20 Connect to the external config channel ToWorker Channel get_param 4 It is worth mentioning that the configuration channels zero and one are reserved for by the C run time library Writing a configuration script Now we are in a position to look at the configuration script required to build the example on two processors The configuration script is held in a file called upc cfg MODULE Configuration script file for a simple two processor program NAME Richard Onyett RTC Santa Clara PURPOSE To configure using INMOS C configuration language the UPPER CASE program Declare all of the physical hardware in the system We have 2 x T800 TRAMSs in ours T800 memory 2M Root T800 memory 2M Slave Tell the world how these two are intimately bonded connect Root link 0 host connect Root link 3 Slave link 0 Describe the interface between our processes 21 Master task fs gt 3 gt ToWorker ts lt lt FromWorker process stacksize 1k heapsize 50k interface input fs output ts input FromWorker output ToWorker Master Worker Slave task FromMaster gt ToMaster lt Se process
16. lset provides a C like configuration language to specify how the mapping is to occur The configurer tool is called icconf which stands for Inmos C Configurer The physical system was shown earlier in figure 3 The reader should refer to this diagram as it will make this section clearer As before intimate details of the configuration language syntax is ignored the interested reader is referred to the IMS Dx214 ANSI C toolset User Manual 2 Introducing some new tools In section 2 4 we saw how to use the tools to develop a multi transputer program The main difference between this development and the develop ment for single processors is the use of the configuration tools to map the processes onto processors The following diagram shows the steps presented in section 2 4 but shows the filename conventions used INPUT FILENAME STEP OUTPUT FILENAME Compile Boot Strap icollect e Figure 5 Filename conventions for multiple transputer development The icconf tool takes as input a textual description of the transputer net work with a file suffix of cfs and outputs a configuration data file with a cfb suffix This file is then passed into a code collector tool called icollect The output from this tool is our bootable file that we can run on our network 17 of transputers using the iserver The inc files are predefined descriptions of transputer modules TRAMS The master process To enable the processes to b
17. nput from the keyboard via stdin and passes it onto the Worker via the ToWorker channel The Worker processes the data and passes it back to Master via the FromWorker channel The Worker simply takes the keyboard characters and turns them into upper case characters these are then displayed by the Master process 3 2 What is configuration Configuration is the process by which individual components of the system are mapped onto physical processors For the simple upper casing example we have two parallel communicating processes This can be run as two par allel processes on one processor or as two processes running on two separate processors Figure 4 shows how the two processes are mapped onto two separate processors Each transputer has four links for this simple example we shall only use two links for communication these are marked on the diagram as LINK1 and LINK2 The LINKO is connected to the host machine this is the link by which the system is booted and all communication with the host is passed back through this channel For our example all of the keys and output messages are passed via LINKO Each INMOS link is bi directional this means that you can map one input channel and one output channel onto one physical link ToHost FromHost LINKO IMS T800 Master Processor Master Process ToWorker LINK2 LINK1 Worker Processor Worker Process Figure 4 Example two processor system
18. onnect Master fs from host Connect to the server on the host 24 e connect Master ts to host e connect Master ToWorker Worker FromMaster Connect the Mas ter to the Worker e connect Master FromWorker Worker ToMaster Pulling In the real code To pull in the actual compiled and linked code we use the use directive e use master lku for Master Pull in the master process e use worker lku for Worker Pull in the worker process Placing the processes onto processors The final stage is to place all of our processors onto a transputer node and place the soft channels onto physical transputer links e place Master on Root Run Master on the first transputer e place Worker on Slave Run Worker on the second transputer e place Master fs on Root link 0 Connect up the server chan nels e place Master ts on Root link 0 e place Master ToWorker on Root link 3 Connect up the Mas ter using channel 2 e place Master FromWorker on Root link 3 e place Worker ToMaster on Slave link 0 Connect up the Worker using channel 1 e place Worker FromMaster on Slave link 0 Building the example Now we have configuration script written we must build the whole lot To do this we use the following make file Module Simple Parallel C Examples Name Richard Onyett Santa Clara RTC 25 File makefile CC icc CFLAGS t8 OBJS master tco worker tco LINK1 ilink f startup ln
19. oolset programs Here are a few simple rules that you may find useful when writing parallel programs using the IMS Dx214 C toolset Program design methodology Writing parallel programs can be a confusing task Good program design is essential when writing software that may execute on tens or even hundreds of separate processors Further details on writing concurrent software are available in 3 Let us assume that you have broken the problem down into parallel blocks an easy way to test the system is to write a software simulation model This means running many parallel processes on a single transputer We have done this already with our upper case example see section 4 where we used the ProcPar command to run the Master and Worker processes on a single transputer Once we have this system working we simply write a configuration script and use that to map the processes onto processors Always have your processes defined in separate files This means that you can reuse the code in both a simulation and configured modes For each of your processes add the following Master process ifdef CONFIG int main void Channel ToWorker Channel FromWorker Access the external configuration channels FromWorker Channel get_param 3 ToWorker Channel get_param 4 else void Master Process Mptr Channel FromWorker Channel ToWorker endif Rest of the process This code segment make
20. rPtr The worker process e NULL No More processes 12 Channel communications To enable communication between our two processes we must declare some channels This is done by saying Channel ToWorker FromWorker Connect em up with channels We must now allocate some space for this channel by saying ToWorker ChanAlloc This also initialises the channel The master process The master process takes a character from the standard input channel and passes it onto the worker process The code for this process is as follows MODULE Simple Parallel C example FILE master c NAME Richard Onyett RTC Santa Clara PURPOSE To generate a stream of ascii characters on a channel and pass them to the upper case worker process include lt stdio h gt include lt stdlib h gt include lt channel h gt include lt process h gt Declare a procedure called MASTER void Master Process p Channel FromWorker Channel ToWorker int cC int Going 1 Pp P Takes care of unused variable warning 13 Intro printf Master C Process STARTING n printf Enter some text and it will be upper cased n printf nTo quit type EOF CTRL Z n while Going c getchar Get a character from the stdin Chan0utInt ToWorker c Pass to the worker if c EOF Going 0 else c ChanInInt FromWorker Ge
21. s use of a conditional compilation flag called CONFIG 28 when this is set it will turn the process into one which can be configured If the flag is not set then the process is assumed to be running on one processor B 1 What if the program will not run Now that you have compiled linked collected configured and submitted your program to a transputer using iserver you discover that It will not function correctly This usually means that the program runs for a time then simply grinds to a halt the following is a checklist you may find useful e Make sure that any external transputers such as in an INMOS ITEM rack have power e Use the checkout tools to see if your transputer network is what you thought it was e If the program requires setting of any IMS C004s has this been done e Compile with the debugging option g on and use the debugger e Run the iserver with the se option on to test if the error flag has been set e Ensure that all ProcPar calls are NULL terminated e Ensure that all ProcAlt calls are NULL terminated e When you specify that there are n parameters passed to a process make sure all n parameters are passed e Make sure you have used exit_terminate for configured programs The server will not terminate until this has been executed e Make sure you have allocated some space for a channel e Check that the get_params are connected to the correct channels in the configuration script e Check t
22. t RIC Santa Clara PURPOSE To receive a stream of ascii characters on a channel and convert them to upper case whether they want to be or not i i i i in nclude lt stdiored h gt nclude lt ctype h gt nclude lt channel h gt nclude lt misc h gt t main void int key int Going 1 Channel FromMaster Channel ToMaster 19 Channel get_param 1 Channel get_param 2 FromMaster ToMaster while Going 4 key ChanInInt FromMaster Get a key from the master if key EOF 4 Going 0 else ChanOutInt ToMaster toupper key Pass it back Notice the use of the get_param function to connect to the external config uration channels Connecting to the external configuration channels Using the IMS D711 configuration language we have used the following code to connect to an external configuration channel the Dx214 still supports this method for compatibility define OUT_CHAN 2 define IN_CHAN 2 int main int argc char argv char envp Channel in int inlen Channel out int outlen int cC Put out the official number on channel 2 ChanOutInt out OUT_CHAN 38 other statements Pull in a INT on channel 2 c ChanInInt in IN CHAN For icconf we use the get_param param_number function call This returns a pointer to the external configuration channel connected to param number we shall see
23. t the character back putchar c Output to the screen printf Master C Process ENDING n Shake the needles from your back This is the master procedure A character is read in and passed along a channel to the worker process Channel communication is done using the ChanOutInt and ChanInInt library calls The worker process The worker process takes a character from an input channel and converts it to upper case it then passes the new character back along an output channel to the master process T he code for the worker process is as follows MODULE Simple Parallel C example FILE worker c NAME Richard Onyett RTC Santa Clara PURPOSE To receive a stream of ascii characters on a channel and convert them to upper case whether they want to be or not include lt stdiored h gt include lt ctype h gt include lt channel h gt include lt process h gt Declare the worker process void Worker Process p Channel FromMaster Channel ToMaster int key int Going 1 P P Takes care of unused variable warning while Going key ChanInInt FromMaster Get a key from the master if key EOF Going 0 else Chan0utInt ToMaster toupper key Pass it back Again channel communication is done using the ChanOutInt and ChanInInt library routines Building the upper case example The C
24. will concentrate on developing programs for multi transputer systems Diagram 2 shows the development cycle E Micro W dit Emacs Link Librarian inky J ilibr igure icconf Strap lect er Debug debug Figure 2 Typical software development route multiple transputer systems This development cycle is almost the same as for the single transputer sys tems the exception is in the use of Configure the configuration tool icconf Configuration is the step taken to map processes onto processors The step consists of writing a configuration script which states how this mapping is to occur we shall take a look at some example scripts later on The tool for this step is called icconf 3 Example problem description 3 1 Introduction Throughout this document one simple example will be used The system consists of the following components Master Process Worker Process FromHost ToWorker ToHost FromWorker Figure 3 Example two process system There are two parallel components Master and Worker these are called pro cesses They communicate with each other using channels these are shown on the diagram as ToWorker and FromWorker To access the host services i e screen and keyboard we use the FromHost and ToHost channels The two processes are written in C we could have easily chosen another language e g Pascal Fortran or Ada The Master process takes i
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