Path: utzoo!mnetor!uunet!husc6!ut-sally!nather
From: nather@ut-sally.UUCP (Ed Nather)
Newsgroups: comp.sys.ibm.pc
Subject: Re: "abort, retry, or ignore?" : help needed
Message-ID: <10423@ut-sally.UUCP>
Date: 21 Feb 88 20:42:05 GMT
References: <2358@chalmers.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 105

In article <2358@chalmers.UUCP>, johnsson@chalmers.UUCP (Thomas Johnsson) writes:
> I'm in dire need of some help from the PCDOS expertise out there ...
> 
> When issuing a read or a write to a diskette station or the printer
> and the output unit in question is not ready (e.g. the diskette
> station has its gate open, or the the power to the printer is off),
> one get the message on the screen
> 
>     device not ready: abort, retry, or ignore?
> 
> Now I would like to disable this, and simply let the read or write
> operation return with an error indication.  (I'm using the DOS file
> handle operations, if that's of any help.)
> 
> Had any one out there in netland succeded in this?
> Help would be greatly appreciated!
> 

Well, you can do it, but it isn't easy.  What you must do is to
write a "critcal error handler" in assembly code that intercepts
the interrupt generated by the error, and returns control to the
calling program.  I wrote one that is dead simple -- it just moves
the error status information to locations available to the main
program, and then exits.  

For this to work, though, you must change the interrupt vectors so
they point to your routine and not DOS's when you start up your
program(s), and restore them when you exit. 

Here's the code I use:
---------------------------------------------------------------------
EXTRN	_errax:word, _errdi:word, _dskerr:word	

crit1	dw	?			; save orig int vectors here
crit2	dw	?

;code to intercept critical error vectors

	push	es
	.
	.
	.
	mov	ax,es:90h		; int 24h - critical error
	mov	crit1,ax
	mov	ax,es:92h
	mov	crit2,ax
	lea	ax,crerr		; point to critical error handler
	mov	es:90h,ax
	mov	ax,_TEXT		; code segment set by C compiler
	mov	es:92h,ax
	.
	.
	.
	pop	es
	.
	.
	.
; code to uninstall the intercept 

	push	es
	.
	.
	.
	mov	ax,crit1		; remove crit error intercept
	mov	es:90h,ax
	mov	ax,crit2
	mov	es:92h,ax
	.
	.
	.
	pop	es

; the interrupt service routine

crerr	PROC	FAR			; disk error handler

	push	ds
	push	ax
	mov	ax,DGROUP		; to get at variables
	mov	ds,ax
	pop	ax
	mov	_errax,ax		; save causes
	mov	_errdi,di
	mov	_dskerr,1
	mov	al,20h			; end-of-interrupt to 8259
	out	20h,al
	mov	al,0
	pop	ds
	iret
crerr  endp

---------------------------------------------------------------------

The error information is in the AX register and the DI register in
a weird format described in the IBM PC Tech Reference manual, and is
also listed in Norton's book for programming the IBM PC.  These are
just saved (and a flag is set) so the C program can take whatever
action it wishes.  It must define the variables (same name sans the
underscore) as globals.  It's ugly, but it works.  It was also much 
too much trouble to do such a simple thing. 
-- 
Ed Nather
Astronomy Dept, U of Texas @ Austin
{allegra,ihnp4}!{noao,ut-sally}!utastro!nather
nather@astro.AS.UTEXAS.EDU