The FreeDOS Project

The "CALL 5" CP/M compatibility entry crashes when called. It doesn't adjust the stack properly. If it did, it would also branch on the wrong function numbers.

Attached diff merges in some code I've written for an upcoming RxDOS release (also GNU GPL, so you can remove the additional notice in the diff).

Note that a better solution may be to write the string A9h,F0h,FEh,CDh,20h at address 5 of each PSP. These bytes are the opcodes "test ax, FEF0h" and "int 20h", terminating all programs that try to "call 5". The useless test opcode allows the data at address 6 of the PSP to remain valid (even though that's just more CP/M compatibility crap). The upcoming RxDOS release will include a "compile"-time option that adds this into the PSP copy/create code and removes all other "CALL 5" code.

On a side note, I've relocated RxDOS's "CALL 5" entry into the section called HMA_TEXT for DOS-C (DOSCODE for RxDOS) as well.


Where the former DOS-C approach failed:

> ; Stack is:
> ; return offset
> ; psp seg
> ; 000ah
> ;
> push bp ; trash old return address
> mov bp,sp
> xchg bp,[2+bp]
> pop bp

Additionally "trashing" (overwriting with bp) the useless value of 000Ah is not required for anything.

> pushf ; start setting up int 21h stack
> ;
> ; now stack is
> ; return offset
> ; psp seg
> ; flags

The above "trashing" code didn't remove the useless value from the stack.

> cmp cl,024h
> jbe cpm_error

Should branch to cpm_error on values >24h, 00h-24h are allowed.