Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!mcnc!rti!h-three!tony
From: tony@h-three.UUCP (me)
Newsgroups: comp.sys.intel
Subject: 386 "real" mode access beyond 1M using data_32 instruction prefix
Keywords: 80386,real mode
Message-ID: <529@h-three.UUCP>
Date: 12 Feb 89 20:25:39 GMT
Organization: h-three Systems, Research Triangle Park,  NC
Lines: 50

I need to write a routine for a 386 in "real" mode (actually a firm-
ware application) that can read/write data above 1M.  At first glance,
the instruction and address prefixes (0x66, 0x67) would seem to be
the way to do this; but I can't get them to work, and even have a 
suspicion they won't work.

Here is the current test routine (assembler is right to left w operands).
The routine takes a physical address and a 32 bit value to put there.
Technique is to zero 'es' and stick the address into 'esi'.  Theory is
that will yield:    effective address == (0 << 4) + esi

This routine fails with General Prot Fault (13).

| void m32poke(addr, value)
| u32 addr, value;

#include	"sys/mch.h"
#include	"asm.h"

	.globl	_m32poke
_m32poke:	
	push	bp		    | C linkage
	mov	bp,sp
	push	si

	xor	ax,ax		    | es to zero as base
	mov	es,ax

	.byte   0x66		    | 32bit prefix
	mov	si,*4(bp)	    | si=address
	.byte	0x66
	mov	ax,*8(bp)	    | ax=value

	.byte	0x66		    | GP 13 fault points here!
	.byte	0x67
	seg	es
  	mov	(si),ax	

	pop	si
	pop	bp
	ret

Is this the wrong way to do this?

Is it even possible to access above 1M without going into full protected
mode?
-- 
--tony	 ...!uunet!h-three!tony	 (919) 549-8334	 h-three  Systems  Corporation
	 h-three!tony@uunet.uu.net		 POB 12557; RTP, NC 27709; USA
						 t. e. bennett