I am just about done rewriting original 6502 code over onto another processor. Part of the dissasembly of the original code (source code not available) has several (4 or 5) reads and / or writes directly to specific, fixed stack memory locations (between $120 and $140).

My question, does anyone know a reason a program would do this?

PS this is not:
TAS

It is:
LDA $0132

Since the 6502 starts the stack at $01FF and builds it down as needed, its possible that your program was using the space below the expected maximum size the system needed for stack use. This could be especially true if the original hardware had limited RAM. On my system, my OS only uses about 16 bytes of the stack.

This is just one possibility.

Another possible answer (slight as it may be), is that some sort of I/O device might have been mapped to the lower portion of the stack space as well.

Daryl

Thanks Daryl.

I think you hit the nail on the head. The original design had 00-FF only for RAM. It did have 3 PIAs at 200, 280 and 300H that did have some RAM; However, they were only used for I/O.

Thanks again. Guess I'll just move them up to upper RAM.

I'm glad to help!