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From: risto@tuura.UUCP (Risto Lankinen)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Writing assembly procedures for higher level languages
Message-ID: <974@tuura.UUCP>
Date: 29 Jan 91 11:49:36 GMT
References: <1991Jan24.133054.8715@wpi.WPI.EDU> <1991Jan24.221332.5802@bradley.bradley.edu>
Organization: Nokia Data Systems Oy
Lines: 44

data@buhub (Mark Hall) writes:

>jhall@wpi.WPI.EDU (John Clinton Hall) writes:

>>AccordingtMicrosoft's _Mixed-Language Programming Guide_, "A procedure
>>caled from any of the Microsoft high-level languages should preserve the
>>values of SI, DI, SS, and DS (in additon to BP, which is already saved)." 
>>However, does one need to save the ES register if it is mdified?  How abo
>>the flags register?

>To save the ES register, you can always:

>	MOV AX,ES
>	PUSH AX

Hi!

Why not PUSH ES directly?

Anyway, I've had bad experience with saving the ES register in programming
in MASM for Windows.  I once thought it was a good idea to...

   PUSH DS
   MOV DS,AX ; This is part of the standard Windows call sequence
   PUSH SI
   PUSH ES
   PUSH DI   ; This order of PUSH:es facilitates LES and LDS at restore-time

Now, severe errors bugged me (literally) quite a while - until I discovered,
that in protected-mode environment the actual segment, whose selector ES
were, has a chance of being invalidated during subsequent calls to Windows'
kernel.  The errors were the result of the LES instruction (or POP ES for
that matter) at the end of the function loading the (at that time) invalid
selector 'back' to a segment register.

This is not a problem with the pushed DS , because there is a mechanism which
keeps the saved DS (and the CS of the far return) automatically up-to-date.

Terveisin: Risto Lankinen
-- 
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