Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pacbell!att-ih!ihnp4!ihlpa!jimx
From: jimx@ihlpa.ATT.COM (Harris)
Newsgroups: comp.sys.ibm.pc
Subject: Passing pointers in C to 8086 assembler
Message-ID: <7759@ihlpa.ATT.COM>
Date: 26 Apr 88 20:25:14 GMT
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 61


I'm getting myself confused, and need some pointers (pun intended)
on passing pointers from a C function to an 8086 assembly
language procedure.

Let's assume that I pass a pointer to a string of unknown length,
and that the asm procedure then copies that string to a local variable:

C:
	char msg[80];
	foo( msg );

ASM, leaving out header stuff:

	local_msg	db	80 dup (0)
	_foo		proc	near
			push	bp
			mov	bp, sp
			push	di
			push	si

Here's where I get confused.  My pointer (16-bit offset, I believe)
is at [bp+4].  How do I address the second character of msg[]?
[bp+5] is one address above [bp+4], right?  Or is it the address
pointed to by bp+5?  If I want msg[4], how do I address it?

	ex: does [si+cx] = [bp+4+cx]??
			mov	si, [bp+4]
			mov	di, local_msg
			mov	cx, 4
			mov	al, [si+cx]
			mov	[di+cx], al

	or graphically:
                 ____                ____
	bp+4 -->|____|-->[bp+4]---->|____|
		|____|              |____|
		|____|              |____|
		|____|              |____|
      bp+4+4 -->|____|-->[bp+8]-->? |____|<--[bp+4][4] == [bp+4+4]???

I think we can agree that obviously [bp+8] is not the same thing
as [bp+4]+4.  But the book I have seems to imply that [bp+4][4] is
the same as [bp+4+4], which I have trouble accepting.  So what is
the solution?  

If you have a good reference for passing pointers to assembly language,
please let me know.  I have "Supercharging C with Assembly Language",
by Chesley and Waite.  I haven't found where (if) they deal with this,
so even a page reference would help.  The more I look at this, the
more confusing it seems to get!

---
As an alternative, how can I get a segment:offset pair for a variable
to put into the dos.h register structures?  If I could do this, it would
solve my problem.  
---

Thanks in advance, and you can email me or post, as you like.

			Jim Harris
			ihlpa!jimx