Load exe file and call function from them in dos

Question

I have a program(A) and there is anather executable file(B) in the same folder. I must call function from this anther program(B) in my program(A). And all this must be done in dos. How can i do it or what i should read to do this? Please help.

Answer 1

If your two programs are separate executables files then will most likely run in two different processes, You cannot just call functions accross two different processes, you need to use some Inter Process communication mechansim.

You need to start understanding the basics & make a start somewhere and this seems to be a good place to do so.

Since you mention DOS as the target platform, DOS is a non-preempted single user single processing environment but still TSR's in DOS environment emulate the phenomenon of multiprocessing. To implement IPC in DOS you will have to arrange for the TSR to collar a software interrupt, and then communicate with it through that.

Answer 2

MS-Dos is a 16 bit OS. The executables that run in MS-Dos come in two flavours: ".exe" and ".com". Think of the ".com" as a ".exe" with lots of default values assumed by the OS. The ".exe" files contain a header which is read by the OS to determine various parameters. One of these parameters is the entry point address. Only one entry point address is defined (and for a ".com" it is always cs:0x100) and that is the address the OS jumps to when the program has been loaded.

MS-Dos has functions to load another executable and run it, but it can only run from the address given in the header. No other function address is exported so you can't just call some arbitrary function in the other executable. There is no DLL system in MS-Dos.

So, in order to call some arbitrary function in the second executable, you need to create your own DLL style system. This is not trivial since the OS uses a segmented memory model, that is, the memory is divided into 64k pages and addresses are formed from the segment address added to an offset, eg segment*16 + offset. So, there are 2^12 ways to express the same physical address. During the loading process, MS-Dos has to fix-up these segment values to reflect the actual location in memory the program has been loaded to. Remember, in MS-Dos there is no virtual memory. If you were to create your own DLL system, you will need to do this fixing-up yourself for code that's bigger than 64k (code+data less than 64k can ignore segments and treat all address as just 16bit offsets).

If you knew the addres, loading the ".exe" using the MS-Dos API would still be tricky as you'd need to know the CS (code segment) address the executable has been loaded to.