I am cracking a dos exe program. Unfortunately, It is not a single program but a serials programs. The first programs will read the input file(i.e file.ccf),and use system to call the second and third programs to do the job. Maybe in the third program it test whether you are a valid user. How can I trace from the first program into the second program and so on?

Hello robertyonghu !

Do you know if them program only reads file.ccf in the first program?

If it does then the programs must exchange then registration data somehow. This can be done by commandline passed to INT 21/4B00 and at PSP:0081 in the called program, by using the inter-program-communication-area at 0040:00F0 (size 16 bytes) or by hooking one interrupt and calling it from the 'child'-process.

If you don't find the critical point that way, you should search for the 'bad-boy' message in the loaded program, which checks the user-info, and set a breakpoint on RW to this location.

If you're lucky, the program is in C++ and you land in the program at once, else you've to trace until you reach the end of the interrupt handler. (either 10h or 21h)

Now you should look for the condition when the 'bad-boy'-message pops up. (maybe by searching for some data of the file.ccf in memory)