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From: pipkins@qmsseq.imagen.com (Jeff Pipkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: i/o redirection in msdos (DOS BUG IN EXEC)
Message-ID: <65@qmsseq.imagen.com>
Date: 13 Dec 89 16:29:30 GMT
References: <8157@cg-atla.UUCP>
Reply-To: pipkins@imagen.com (Jeff Pipkins)
Organization: QMS Inc., Mobile, Alabama
Lines: 26

It sounds like the problem may actually be with the way you are spawning
the child process.  There is a small bug in some versions of MS-DOS in
the EXEC function call that causes ALL the registers to be destroyed on
return, even the stack registers SS & SP.

    < Set up all other registers for EXEC call >
    < Save all registers here >
    Mov  CS:SaveSS, SS    ; Save SS in code segment.
    Mov  CS:SaveSP, SP    ; Save SP in code segment.
    Int  21h              ; Do the EXEC call
    CLI                   ; Compensate for bug in some early 8088s.
    Mov  SS, CS:SaveSS    ; Restore SS
    Mov  SP, CS:SaveSP    ; Restore SP
    < Stack is okay now.  Restore other registers.>

Some ancient MS-DOS manuscripts recommend calling DOS via a long
indirect call to your PSP at offset 50h.  It is best to NEVER DO THIS.
Always use the Int 21h method.

If you are using the MSC library functions to spawn it, I doubt you will
have to worry about this bug at all.  But since you didn't say, I
thought it would be worthwhile to post this for other unsuspecting
assembly-language programmers.

-Jeff
pipkins@imagen.com