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From: hughes@locusts.Berkeley.EDU (Eric Hughes)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Int 10H Function 13H - How Do I access BP from C ?
Message-ID: <HUGHES.90Nov20114508@locusts.Berkeley.EDU>
Date: 20 Nov 90 19:45:08 GMT
References: <1990Oct19.202013.28397@nada.kth.se>
	<1990Oct21.204236.22011@nada.kth.se>
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In-Reply-To: d87-vik@dront.nada.kth.se's message of 21 Oct 90 20:42:36 GMT

In article <1990Oct21.204236.22011@nada.kth.se>
d87-vik@dront.nada.kth.se (Ville K{rkk{inen) writes:
>My problem is that Int10h - Func13h requires the offset of my data to
>be stored in BP,that is, the BasePointer. Note this,BP is a pointer,
>not a register defined in the REGS union.

BP is the base pointer from which all auto variables are referenced
and which saves the value of SP.  If you change the value of BP, you
must change it back again before the C function returns.  The only
code which changes BP in compiled code (for MS C 5.1, the compiler in
question) are the function prolog and epilog, which set up the stack
frame.

Really, the easiest way to get at the BIOS service you want is to wrap
it in some assembly language which uses the C calling convention.  See
the mixed model programming guide on how to do this.

Now you *can*, through some total deviousness, get a copy of the
string you want to print in the location pointed by BP and safely
return from that function, but once you've done that, you need to
invoke the interrupt without changing stack frames.  I don't know of a
way to trick MS C 5.1 into generating an int 10h instruction without
using self-modifying code.  (Take a pointer to a function which
contains a dummy instruction, calculate an offset based on an assembly
code listing, make sure you write into the code segment somehow, and
poke in the instruction by hand.)  The upshot of this is that assembly
is pretty much required.

Eric Hughes
hughes@ocf.berkeley.edu