From: Nate Eldredge <neldredge AT hmc DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: self-mod code and DJGPP - writable code segment?
Date: 14 Mar 2000 00:26:28 -0800
Organization: InterWorld Communications
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apsh AT ecr DOT mu DOT oz DOT au (Alistair_P SHILTON) writes:

> I was wondering if it is possible to link self-modifying assembler
> code to DJGPP.  When I try, I get an error message.  So I checked
> the documentation, which says that the code segment is not writable.
> Is there some way around this?
> 
> I was planning to use the self-mod to make the code act differently
> after the first call to it (ie. setup on first call, normal operation
> on subsequent calls).  Now I know that there are other ways of achieving
> the same end, but I like the concept of self-modding code.  Any
> suggestions?

The code segment, as addressed via the CS register, is not writable,
true.  This is a feature of the cpu.  However, the DS segment
addresses exactly the same memory (in DJGPP) and is writable.  So
something like this should work:

movb interrupt_number, %al
movb %al, modify_here+1
modify_here:
int 0x42

Do be careful to make sure you write it maintainably, though.
Self-modifying code is dangerous in that respect.

-- 

Nate Eldredge
neldredge AT hmc DOT edu