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C S W I T C H ----------------- Multitasking Functions for DOS Programs Copyright 1990 by Herb Rose All Rights Reserved Version 1.0 release date : Aug 10, 1990 This file is an introduction to the Cswitch functions. These functions provide limited multitasking capabilities for programs running under MS-DOS or PC-DOS. The term 'limited multitasking' is used because these routines do not provide a multitasking user interface like DESQVIEW or WINDOWS. These routines are programming tools that will enable a C programmer to write applications that consist of independent, simultaneously executing programs. Using Cswitch, you can execute functions as independent tasks, which execute concurrently in a time-slicing environment. You may also load and execute programs separately. Cswitch provides you with a priority driven scheduler and dispatcher, DOS interface, and task control functions to implement true multitasking under your program's control. Before I continue the explanation of Cswitch, please bear with me as I detail the legalities of using these routines. WARRANTY -------- This software is distributed as is, with no warranty of any kind, either expressed or implied. No warranty or guarantee is made that the software is complete or error free. All liability and risk associated with the use of this software is assumed by the user. Under no circumstances will ADEPT Software, its owners, or its agents be responsible for any damages arising from the use, or the inability to use this product. This applies to all damages and remedies including, but not limited to, loss of or damage to property, and loss of profits. This software has been tested in many configurations, under various conditions, and has consistently performed acceptably. To the best of my knowledge, there are no errors or omissions in the software that will pose a problem to the user. The user must be aware, however, that this is a programmer's tool, and by it's very nature will affect the software that controls the user's computer. All reasonable precautions should be taken to protect your computer from the affects of an errant operating system. In short - BACK UP YOUR DISKS, AND BE CAREFUL WHEN TESTING NEW PROGRAMS THAT USE CSWITCH!!!!! Shareware Distribution ---------------------- CSWITCH is distributed as shareware. You may freely use and distribute these routines under the following conditions : 1. All files on this diskette must be distributed as a set, and no modifications or additions may be distributed as part of the set unless specifically approved in writing by Herb Rose. 2. All copyright and legal notices must remain intact and unaltered. 3. Copying and distribution fees may not exceed $6, and must be clearly specified as such. 4. You must register Cswitch if you want a copy of the source code, or if you wish to distribute a software product that makes use of any of the Cswitch functions. Note that registration is not required if you simply want to use Cswitch in the privacy of your own home, or just to see what it can do. As an incentive to registration, I include C and assembly source code when you register. The source code is easy to read and well documented. Make files are included for Microsoft C and Microsoft Assembler. Registered users are informed of new releases of the software, and may download new releases (including source code) from the INFO*SHARE BBS. If you plan to distribute any software product that makes use of Cswitch functions, then registration is mandatory. Registration provides you with an unlimited, nonexclusive binary distribution license, with no royalties. This is only fair, I think. If you distribute a program, you should pay for the tools you used to create that program. Registration is for use on a single computer. Contact ADEPT Software for site licensing, or for multiple copy discounts. Due to practical constraints, I cannot provide telephone assistance except to registered users. I operate a 24-hour Bulletin Board System to handle technical assistance. Telephone support is available for registered users, but I prefer to handle non-emergency questions via the BBS. The number is (703) 803-8000. Registration Form ----------------- Please use the following form, or a reasonable facsimile, when registering Cswitch. Mail to : ADEPT Software P.O. Box 2181 Woodbridge, Va. 22193 CSWITCH Registration ..................... ______ @ $30 = ______ Virginia Residents add 4.5% sales tax ................. = ______ Overseas Air Delivery ...............................$5 = ______ (U.S. delivery and Overseas Surface delivery is free) TOTAL ................................................. = ______ Please send check or money order, in U.S. funds. Your Name : _____________________________________ Company Name : _____________________________________ Address : _____________________________________ City, State, Zip : _____________________________________ (or country) Introduction to Multitasking ---------------------------- Multitasking is not a new concept. Computers have been programmed to execute multiple tasks concurrently for many years. It was observed a long time ago that most programs spent the majority of their time waiting for data input or output. Since users and most peripheral devices are notoriously slow compared to the execution speeds of modern computers, most programs simply sit there 'twiddling their thumbs' waiting for data. It was observed that the computer's computational capabilities could be used more efficiently if another program were allowed to use the processor while a program that was just waiting was put into a dormant state. Carrying this observation one step farther, we find true multitasking. Multitasking is the ability to have several programs loaded into the computers memory, and apparently executing simultaneously. In reality, there is a controlling program, called the Kernel, or Scheduler, that causes the computer to execute some of each programs instructions in a round-robin fashion. Programs that are waiting for something to happen, such as waiting for a keystroke, or for a disk transfer to complete, are normally placed into a 'waiting' state, and do not get placed back into execution until the event they are waiting for has actually happened. Notice that I said programs were 'apparently' running simultaneously. It is impossible for a microcomputer, such as the PC, to execute more that one program simultaneously. Other computers may have several micro- processors, and can actually execute more than one task simultaneously. When there is only one microprocessor, the tasks must be executed one at a time. The term 'time-slicing' is used to describe the operation of a multi-tasking operating system, meaning that the processor will execute each task's instructions for a short time, then will execute another task's instructions for a while, eventually allowing all the tasks to get some of their work done in a round-robin fashion. If the amount of time given to each task is very large, say one or two seconds, there could be a significant amount of time for each task to remain idle while it awaited it's turn to execute. If the amount of time given to each task (called the 'time slice') is too short, then too much of the computer's time will be spent saving and restoring task context information, diminishing the efficiency of the system. A good time slice setting is one that allows many different tasks to execute in a short period of time, without noticably affecting the efficiency of the system. Cswitch uses the timer interrupt for the PC to trigger a task switch. This occurs approximately 18 times per second, or once every 55 milliseconds. Since a normal time slice on a real multitasking operating system is usually around 50 microseconds, our time slice is actually very large. In fact, 55 milliseconds is way too big for a real-time operating system, which has to react to external stimulus as quickly as possible. With 18 tasks loaded into Cswitch, each task will only get to run once per second, for about 1/18 second. If some of the tasks are higher priority than others, the lower priority tasks may not run at all for several seconds. This is clearly not acceptable for programs that must react to external stimulus. The 55 millisecond time slice, however, is fine for running one or two tasks at a high priority, and allowing several other tasks to run at a lower priority. This gives the effect of 'background' processes running. If you find the 55 millisecond time slice inadequate, you should consider altering the timer chip's programming so that it provides a timer interrupt more often. Your interrupt handler will have to count the number of interrupts, and activate the DOS time-of-day routine (the normal destination of interrupt vector 8) once every 55 milliseconds, otherwise the computer's internal date and time settings will not be accurate. Now we will focus on how you get multiple tasks to cooperate so that a given job will get done. Basically, there are only 2 areas that we are concerned with - 1. Passing information from one task to another, and 2. Preventing another task from interfering with a resource you are using. Moving Data Between Tasks ------------------------- The first area is handled in Cswitch via Message Queues. A queue is simply a list of messages. In this case, Cswitch has 64 queues, numbered 0-63. Any task may place a message onto any of the queues, and any task may fetch a message from any of the queues. If there is more than one message waiting on a queue, they will be fetched sequentially, on a first-in, first-out basis. A message may be any format, containing any information that may be needed by another task. When placing a message on a queue, you must tell Cswitch which queue to put it on, how long the message is (in bytes), and the address of the first byte of the message. Cswitch will copy as many bytes as you have told it into a temporary buffer, and will place a pointer to that buffer into the message list for that queue. When you fetch a message from a queue, you must tell Cswitch which queue to read a message from, the maximum number of bytes you are willing to receive, and where to put the message. After Cswitch has copied the message (or as much of the message as you will allow) into your memory, the temp buffer is released, and the message pointer is deleted from the queue's list. Note : if the message contained more bytes than you received (by specifying a maximum byte count lower than the actual size of the message), the message will still be removed from the queue. You should make certain you allow enough room to receive any message your task is likely to receive. It is up to the individual tasks to coordinate which queue numbers will be used to pass messages back and forth. In the Falken BBS, a multi- line Bulletin Board System program written with Cswitch, the main BBS program assigns input and output queues to each task when it is started. Queue number 1 is reserved, and is used to send the queue assignments to the tasks. In other words, when a task first runs under control of Falken, it must read a message from queue number 1. That message will tell it which queue it must read messages from, and which queue it can write messages to, for the rest of the time it is running. Semaphores ---------- The second area of concern is to prevent another task from interfering with a resource that you are using. A resource may be almost anything, from a disk file to a memory buffer, to a peripheral device. Cswitch uses a semaphore system to control access to these resources. You must know in advance which resources must be controlled with semaphores. If you are going to allow several tasks to access a memory area simultaneously, it may be a good idea to control it, since one task may try to write new information to the memory while another task is trying to read the old information. Semaphores work like this - A semaphore has an owner and a waiting list. If your task attempts to 'attach' to the semaphore, but another task already 'owns' the semaphore, your task gets placed onto the 'waiting list', and does not get to run anymore (it is taken out of the ready queue). Your task will remain dormant until it is the 'owner' of the semaphore. When the owner of the semaphore 'releases' it, the first task in the 'waiting list' is made the new owner, and is allowed to run again. Note : When you 'attach' to a semaphore, you will become the owner of the semaphore before you are allowed to continue. Your program remains dormant until it is the owner, and you must release the semaphore when you are done with it, to allow other programs to run, which may be waiting for the semaphore. If you attempt to 'attach' to a semaphore, and it is unowned, your task becomes the owner and is allowed to continue running immediately. You must still release the semaphore when you are done. Cswitch Functions ----------------- Cswitch allows tasks to perform the following functions : - All normal DOS and BIOS functions - Send information between tasks with Message Queues - Control system resources with Semaphores - Alter task priority - Suspend / Resume task execution - Delay for a time period (sleep) - Start new tasks, either sharing existing code, or loading a new program file from disk. - Load small programs into expanded memory to allow more programs to execute simultaneously. To use Cswitch, you must link your main program with the CSWITCH1.OBJ, CSWITCH2.OBJ and LMTC.OBJ files. These files provide interrupt level control for task switching, and the operating system functions, such as message passing and semaphore control. In addition, programs which will be loaded and executed under control of Cswitch must be linked with LMTC.OBJ or SMTC.OBJ (for large or small model, respectively). These files contain interface routines for calling system services. Your main program must call the function START_MT(). START_MT() initializes the Cswitch data areas and enables multitasking. It intercepts several interrupt vectors, and builds Task Control Blocks for your main program and for an 'idle task'. The idle task runs at a low priority, and checks the delay and termination queues regularly to service tasks waiting for those functions. After the call to START_MT(), multitasking is fully operational, and you may spawn tasks and load programs. Note that your main program has a Task Control Block (TCB) and gets scheduled for time- slicing just as every other task does. Your main program runs at a priority of 1 (the highest priority allowed). Before terminating, your main program MUST call the function END_MT(). END_MT restores the interrupt vectors, releases all unnecessary memory, and halts all multitasking. Essentially, it restores the computer to the same state it was in prior to the START_MT() call. ** WARNING ** WARNING ** WARNING ** WARNING ** WARNING ** WARNING ** If you allow your program to terminate without calling END_MT, almost anything can happen, since the interrupt vectors are still pointing to Cswitch code which may or may not be overwritten by DOS. At a minimum, your system will lock up. A worst-case scenario could involve unrecoverable damage to disk files. It is highly recommended that you test all your programs that use multitasking on a 'test' computer, a separate machine that does not contain any critical files or programs, to prevent loss of valuable data or programs. At a minimum, make sure all your important files are backed up before testing new programs that use Cswitch routines. Executing Tasks --------------- There are 2 methods of starting new tasks under Cswitch. The first method is to SPAWN a new task that will execute a portion of your code independently. Normally, this facility will be used to execute a function as a separate task. The new task will be given it's own stack and TCB, and will be scheduled for time-slicing normally. It will begin executing code at the address you specify (normally a function name). It may call other functions, and may manipulate global data normally. Any automatic (local) variables will reside on the new tasks stack, so there will not be any interference when a function is called by 2 different tasks. In fact, you can spawn several new tasks which execute the same function if you like. When the function being executed returns to the caller, the task will be terminated. Any functions that are called by the spawned task will return normally, of course. Only when the function being spawned returns will the task terminate. The second method of starting new tasks is to load a program from disk and execute it. The function LOADTASK() is used to load an EXE or COM file from disk and execute it. You must supply 3 parameters to the LOADTASK function - the command line to run the program and the priority to be assigned to the task, and a flag indicating whether the task may be loaded into expanded memory or not. For instance, to load and run the program MYPROG.EXE with the parameter 'myfile.dat', you would normally enter a DOS command line like this : C:>myprog myfile.dat To load and run the program under Cswitch, you would call the LOADTASK function like this : loadtask("myprog myfile.dat",4,1); This loads and runs 'myprog.exe', gives it the command line parameter 'myfile.dat', and allows it to run at priority 4, and allows it to be loaded into expanded memory, if possible. The .EXE extension is assumed if no extension is provided. You can load and execute .COM files by specifying the .COM extension. You must specify the full path of the program to be executed, if it is not in the current default directory. Cswitch will not search your PATH for the program. Scheduling ---------- The Cswitch scheduler maintains a list of tasks that are waiting to execute, called the 'ready queue'. When a task switch occurs, the TCB at the head of the ready queue is fetched, and allowed to run. Normally, the TCB that was just executing is inserted back into the ready queue. In some cases, such as waiting for semaphore, delaying, etc. the task will not be re-inserted into the ready queue. Tasks are inserted into the ready queue according to their relative priority. Each task is assigned a 'base priority' from 1 to 10. 1 is the highest priority, and 10 is the lowest. Each task also has a 'current priority', which is used to insert TCBs into the ready queue. Each time a task is fetched from the ready queue, the 'current priority' of all the other TCBs on the ready queue is decremented by 1. When a task is re-inserted into the ready list, it's current priority is set equal to it's base priority, then it is inserted into the ready queue so that it is in front of all tasks whose current priority is lower. In this way, tasks with a higher priority will be allowed to execute more often than tasks of lower priority. The tasks with lower priority will remain on the ready queue until their current priority increases to the point that the other tasks get inserted behind them, and they work their way to the head of the queue. Using Expanded Memory --------------------- Cswitch can use expanded memory to load and run tasks that require less than 64K. By calling START_SWAPPING(), you tell Cswitch to start using expanded memory. Any task that is loaded after calling START_SWAPPING() is eligible to be loaded into expanded memory. Cswitch System Services ----------------------- Cswitch must replace some of the DOS system services with it's own services. In every case, the calling sequence and register setup is identical do the DOS call. Return values are also identical to DOS return values. In short, if your program ran OK under DOS, it should run OK under Cswitch. Cswitch replaces the following DOS (interrupt 21) services : 48h : allocate memory Cswitch controls memory allocation for all tasks. DOS uses a 'first fit' algorithm for allocating memory, meaning that it allocates a chunk of memory from the first block that is large enough to fulfill the request. This leads to memory fragmentation, limiting the number of tasks that can run. Cswitch uses a 'best fit' algorithm, searching all the available memory blocks to find a block that is exactly the size needed, or is closest to it. This way, memory fragmentation is kept to a minimum. Note : starting with DOS 3.3, you can alter the memory allocation strategy that DOS uses, but 'first fit' is still the default. 49h : release memory Cswitch must also handle the memory release function. When a block of memory is released, all memory is re-combined as much as possible to limit memory fragmentation. 4ah : set memory block size This DOS function is used to adjust the size of a memory block. Since DOS tasks expect to be the only task running, they assume that a memory block can be altered by simply increasing the size of the block when they need more memory. Since several tasks may occupy memory blocks adjacent to the block to be modified, this is not always possible under Cswitch. Cswitch will attempt to fulfill the request by looking for a free block adjacent to the specified block, and combining them to form a larger block. If unsuccessful, it will return an error. Note that programs that allocate memory dynamically with CALLOC or MALLOC calls may run into problems with this limitation. The problem is that the library routines for allocating memory (MALLOC, et al) use a table of structures to control the memory blocks they obtain from DOS. Normally, this table will only hold 20 entries (this is true for Microsoft C and QuickC). Since Cswitch will return an error on some of the 'set memory block' calls, the library routines may execute an 'allocate memory' call every time you execute MALLOC. After 20 calls, the table is full, and you can't allocate any more memory! You can avoid problems by using the EXEMOD program to change the minimum memory requirement of your task, so that enough memory will be reserved when your task is loaded to handle dynamic memory allocation demands without having to request more memory from DOS. Or, you can avoid using the MALLOC/CALLOC routines wherever possible, and instead call the DOS allocate memory call directly. Be careful, though, because Cswitch can only control a total of 512 memory blocks. When a task in expanded memory wants to extend the memory block, it is much easier to do, provided the limit of 64K is not exceeded. Since each task loaded into expanded memory is allocated the entire 64K expanded memory region to work in, it is usually possible to extend the memory allocation to the desired size. 4ch : terminate task Cswitch handles task termination internally. It will release memory held by the task, close all open files, and release the Task Control Block. 1ah : set new DTA area Cswitch records the address of the tasks new DTA in the TCB, then passes this request to DOS normally. This function is normally handled by the library routines, and is seldom called explicitly by a programmer. 2fh : get DTA address Cswitch returns the DTA address from the tasks TCB. Cswitch Functions ----------------- The following functions are provided in the file CSWITCH1.C, and may only be called by the main program. i.e. these functions may only be called by the program that is linked with Cswitch1 and Cswitch2. START_MT() Parameters : none Description : Starts the multitasking kernel, and enables all system functions. This function must be called once, and only once, before executing any of the other functions or system services. Return Value : none Notes/Comments : START_SWAPPING() Parameters : none Description : This function sets Cswitch up to swap tasks out to expanded memory. Return Value : The amount of expanded memory available is returned, Notes/Comments : All tasks started prior to this call are memory-resident, and all spawned tasks are memory resident. This routine may only be called once, and there is no way to turn off usage of expanded memory once it is started. END_MT() Parameters : none Description : Ends multitasking. The program that called START_MT() must call END_MT() before terminating. Return Value : none Notes/Comments : See the warning above concerning the dire consequences of terminating without restoring the interrupt vectors and turning off multitasking. LOADPRG(cmd_string,pri,expmemflag) char *cmd_string; int pri; int expmemflag; Parameters : cmd_string is a character pointer to a command string to load the program, including command line parameters just as you would type in to load the program under DOS. pri is the base priority of the new task if expmemflag is true, the task is eligible to be loaded into expanded memory. if set to 0, the task may not be loaded into expanded memory, even if START_SWAPPING() has been called previously. Description : Loads and executes an .EXE or .COM file from disk. The full path to the program file must be specified, and command line parameters may be used. Return Value : 0 = no error -1 = not enough memory to create new task stack -2 = no free task control blocks -3 = no free memory control blocks Notes/Comments : This function performs the same service as LOADTASK() below. The difference is that this service is called directly as a function. LOADTASK is invoked via the INT 62H. Functionally, there is no difference, since LOADTASK() eventually calls LOADPRG to do the work. Cswitch Global Variables ------------------------ The following variables are available to the main program. These are provided for information only. int dos48, dos49, dos4a, dos4c; These integers represent a running count of how many DOS calls (INT 21H) have been made for these function codes. unsigned int base_mem_segment; This is the segment value of the memory segment controlled by Cswitch. int idlecount; This is a running count of how many times the idle task has run. The idle task is spawned when START_MT() is called, and handles the tasks that are delaying and terminating. int swap_count; This is a running count of how many task switches have taken place. When the scheduler is called, this count is incremented even if no task switch takes place due to a task locking out switching or some other reason. int extmem_pages; This is the total number of expanded memory pages available for task swapping. This variable is loaded after a successful call to START_SWAPPING(). int exmemfree; This is the number of free pages of expanded memory left in the system. Pages are allocated as needed by memory allocation calls, so this number may vary somewhat during operation. Cswitch Internal Services ------------------------- The following functions are provided in the file LMTC.OBJ and SMTC.OBJ, and may be called by any task executing under Cswitch control. These functions allow access to the semaphore, messaging, and task control functions of Cswitch. These functions are actually invoked via an INT 62H call. The parameters are loaded into registers, and the INT 62H transfers control to an assembly language routine. The registers are then pushed onto the stack, and the C function for system calls is executed. Most of the work done by Cswitch is done by C routines, with assembly language used only for interrupt control. All of these functions are of type INT, although some do not have return values assigned. RELINQ() Parameters : none Description : Gives up the rest of the tasks time slice. This is graceful way of not wasting processor time if your task has nothing else to do for a short while. Return Value : none Notes/Comments : none SPAWN(func_addr,pri) int *func_addr(); int pri; Parameters : func_addr is the address of a function or subroutine that is to be executed as a separate task pri is the base priority for the new task (1-10) Description : Causes the specified function or subroutine to be executed as a separate task. A new stack is allocated for the task, allowing it to have exclusive access to local variables. It will still share global variables with the calling task. STDIN, STDOUT, and STDERR are inherited from the caller. The function can be spawned more than once, allowing multiple tasks to execute the same code thread. Return Value : >0 = the tcb number of the new task, indicating no error -1 = not enough memory to create new task stack -2 = no free task control blocks -3 = no free memory control blocks Notes/Comments : A task that is swappable to expanded memory (i.e. one that was loaded after a call to START_SWAPPING() ) cannot spawn new tasks. LOADTASK(cmd_string,pri,expmemflag) char *cmd_string; int pri; int expmemflag; Parameters : cmd_string is a character pointer to a command string to load the program, including command line parameters just as you would type in to load the program under DOS. pri is the base priority of the new task if expmemflag is non-zero, the task may be loaded into expanded memory, if it is enabled. If exmemflag is 0, the task will not be loaded into expanded memory. Description : Loads and executes an .EXE or .COM file from disk. The full path to the program file must be specified, and command line parameters may be used. STDIN, STDOUT, and STDERR are inherited from the caller. Return Value : 0 = task loader queue is full 1 = no error Notes/Comments : This call actually places a load request onto a queue to be executed by the 'idle' task. The status of the load may be checked with the TEST_LOAD() call. .COM files are given exactly 64K memory. .EXE files are sized according to the information in the file header. The MINALLOC parameter is used to determine how much memory, in addition to the task size and data space, is needed for the program. SLEEP(seconds) int sleep; Parameters : seconds is the number of seconds the task is to remain alseep. Description : puts the calling task into a sleeping state, where it will not execute at all, for a specified number of seconds. Return Value : none Notes/Comments : SEND_MD_MSG(queue,message_addr,count) int queue; char *message_addr; int count; Parameters : queue is the message queue to place a message on message_addr is a pointer to the data to be placed on queue count is the number of bytes to place on the queue Description : places the specified number of bytes onto a message queue. queues are numbered 0-63. Return Value : -1 = bad queue number other wise, returns number of bytes transferred Notes/Comments : TESTMSG(queue) int queue; Parameters : queue is the queue number to test Description : tests whether there are any messages waiting on the specified queue. Return Value : -1 = bad queue number 0 = nothing on queue else, returns the number of bytes contained in the first message on the queue Notes/Comments : RECVMSG(queue,message_addr,count) int queue; char *message_addr; int count; Parameters : queue is the message queue to place a message on message_addr is a pointer a data area to receive the data count is the maximum number of bytes to transfer Description : reads the first message from the specified queue into the data area pointed to by message_addr. only the first 'count' bytes of the message are transferred. if more than 'count' bytes are in the message, the extra is lost. If there are no messages on the specified queue, the task is placed into a dormant state until a message is put on the queue. If you do not want your task to remain dormant if the queue is empty, you should call TESTMSG() to make sure something is available before calling this function. Return Value : -1 = bad queue number 0 = no message was on queue else, returns the number of bytes transferred Notes/Comments : SEM_ATTACH(semaphore_number) int semaphore_number; Parameters : semaphore number is the number of the semaphore to attach (0-63) Description : attaches to the semaphore. see semaphore description elsewhere in documentation Return Value : -1 = invalid semaphore number all other values = success Notes/Comments : SEM_RELEASE(semaphore_number) int semaphore_number; Parameters : semaphore number is the number of the semaphore to be released (0-63) Description : releases the semaphore. see semaphore description elsewhere in documentation Return Value : -1 = invalid semaphore number all other values = success SEM_TEST(semaphore_number) int semaphore_number; Parameters : semaphore number is the number of the semaphore to test (0-63) Description : tests to see if the semaphore is already owned by another task. see semaphore description elsewhere in documentation Return Value : -1 = semaphore is unowned all other values = semaphore is owned SETPRI(new_priority) int new_priority; Parameters : new_priority is the new base priority of the calling task. Description : allows a task to change its priority, affecting how often the task gets to execute. Return Value : none Notes/Comments : only values 1-10 are legal. the lower the number, the more often a task will run. the main program is assigned a prioroty of 1, and usually should be the only task with such a low priority. SUSPEND() Parameters : none Description : suspends the task until it is awakened by a WAKEUP() call. Return Value : none Notes/Comments : WAKEUP(tcb_number) int tcb_number; Parameters : tcb_number is the tcb identifier for the task that is to be awakened. Description : allows a task that was suspended with a SUSPEND() call to continue processing. Return Value : none Notes/Comments : This is the ONLY way to wake up a SUSPENDed task. This call will also wake up a task that is in a SLEEP state (see SLEEP() above). In this way, a task may go into a sleep state, and another task can cause it to start executing again before the sleep delay is over. HOG() Parameters : none Description : prevents this task from being swapped out until a corresponding NOHOG() is issued. In effect, this halts multitasking, allowing the caller to run forever, if desired. Return Value : none Notes/Comments : Use this function when your task must not be interrupted. DOS functions like I/O and other critical functions are already protected, so this function should not be used too often. NOHOG() Parameters : none Description : allows multitasking to continue after being halted by a HOG() call. Return Value : none Notes/Comments : SPAWN_EXIT() Parameters : none Description : halts execution of a SPAWNed task. Return Value : none Notes/Comments : this is the preferred method of halting execution of a spawned task. When the spawned task is finished, it should call this function to terminate. GET_TCB_INFO(pointer_address) struct tcb_rec far **pointer_address; Parameters : pointer_address is the address of a pointer into which the address of the calling tasks TCB will be placed. Description : this function is used to access your Task Control Block you pass in the address of a pointer, and the address of your TCB is placed into the pointer. The pointer must be a LONG pointer, as supported in the LARGE memory model. Return Value : the TCB identifier (an index into the TCB array) is returned. Notes/Comments : Normally, tasks do not need access to their TCB. Since TCBs contain critical task control information, great care must be taken when accessing the TCB. GET_TCB_ADDRESS(pointer_address) struct tcb_rec far **pointer_address; Parameters : pointer_address is the address of a pointer into which the address of the TCB array will be placed. Description : similar to 'GET_TCB_INFO()' above, but this call returns the address of the TCB array in memory, to allow your task to access any given TCB, based on the task number. Return Value : the TCB identifier (an index into the TCB array) of the calling task returned. Notes/Comments : Normally, tasks do not need access to their TCB. Since TCBs contain critical task control information, great care must be taken when accessing the TCB. GET_LOAD_STATUS() Parameters : none Description : returns the status of the last 'LOAD_TASK()' call made by your program. Return Value : 0 = load in progress (not done yet) -1 = cannot open file -2 = insufficient memory -3 = no TCBs available -4 = no Memory Control Blocks available otherwise, the TCB number (index into the TCB array) of the new task is returned. Notes/Comments : Sample Programs --------------- 2 sample program are included, with full source code. MTTEST.C will load and execute the programs TEST1.EXE, TEST2.EXE, TEST3.EXE, TEST4.EXE and TEST5.EXE. These 5 programs read and write a disk file. Each time they finish reading and writing the file, they send a message to MTTEST. MTTEST receives and displays the messages. It's not particularly impressive to watch, but it does illustrate that several programs can perform concurrent I/O, and that programs can easily be loaded and executed from disk. MT.C is another sample program. It demonstrates one use of the semaphores, and the sleep() function. The source files for these programs, and a make file for Microsoft C is included. Writing Programs With CSWITCH ----------------------------- When you write your multitasking program, there are several things to remember : 1. You must use the LARGE memory model. Programs that are linked with SMTC, or that are simply loaded and run, may be small model. But your main program must be LARGE model. 2. You must compile your main program, and any program that will SPAWN tasks, with the /Gs option to turn off stack checking. This is imperative, and without it, your program will not run. 3. C library routines are not re-entrant. Therefore, you cannot do a PRINTF() from your main program, and one from a SPAWNED task at the same time. In general, if a spawned task needs to do I/O, do it with DOS interrupts rather than with the library calls. If this is impractical or impossible, then make the spawned function a separate program and execute it with LOADTASK(). If several different programs perform simultaneous I/O there is no problem. It is only when the same PRINTF or FOPEN routines are called from 2 different places in the same program at the same time that there is a problem. This probably applies to other languages as well. Also, most library routines perform stack checking upon entry. Since a SPAWNED task is assigned a stack from free memory, the stack check will often fail, causing SPAWNED tasks to abort with a false stack overflow error. If SPAWNed tasks are going to be performing complex functions and making a lot of library calls, they probably should be made into separate programs and executed with LOADTASK(). Language Support ---------------- Currently, the only language directly supported is C. Most languages can be made to mimic C's function calls, so there should not be a problem with linking these routines to other languages. Soon, I will be adding interface modules for Turbo Pascal and QuickBasic. Identifiers are prefixed with an underscore ( _ ) by the C compiler, so if your compiler does not automatically add the underscore, you will have to use it explicitly in the module names, and in the global data names. Cswitch Limitations ------------------- Cswitch routines will not be appropriate in every situation requiring multitasking. In some cases, products like Desqview, Omniview, Windows, OS/2, and Unix are more appropriate. Cswitch does give a programmer an option, however. Writing programs that multitask in the DOS environment is a clean solution to many problems. In general, if your application must somehow do several things all at once, then writing your application as a set of cooperating tasks is a lot easier than trying to build every function into a single task. Let your imagination take it from there. As stated previously, Cswitch does not attempt to provide a multi- tasking user interface. You must prevent tasks from writing to the monitor or reading the keyboard simultaneously. This can easily be done with semaphores or I/O redirection. Tasks that are created using the SPAWN call inherit their owners STDIN, STDOUT and STDERR, as do tasks loaded from disk. Normally these handles refer to the CON device - the keyboard and monitor. No other file handles are inherited.