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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Looking for C functions to access PC memory
Message-ID: <9307@mimsy.UUCP>
Date: Wed, 11-Nov-87 19:55:47 EST
Article-I.D.: mimsy.9307
Posted: Wed Nov 11 19:55:47 1987
Date-Received: Sat, 14-Nov-87 05:58:48 EST
References: <24261F3U@PSUVMB> <2312@emory.uucp> <151@fxgrp.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 54

In article <151@fxgrp.UUCP> ljz@fxgrp.UUCP (Lloyd Zusman) writes:
[example `peek' and `poke' routines using `movedata' deleted]

>Shorter versions of these could be written ...

[shorter examples deleted]

>However, these shorter versions violate strict typing conventions, as
>they make use of the fact that a "char far *" takes up the same space
>on the stack as two unsigned int's (on the PC).  A syntax checker like
>'lint' wouldn't like this code, nor would many strict-typing purists.

`peek' and `poke' are already utterly machine dependent, hence it
does not matter how machine-dependently you code them.  Were I
to use an IBM PC, I might write something like this:

	#define	peek(addr)	(*(unsigned char far *)(addr))
	#define poke(addr, value) (*(char far *)(addr) = (value))

If I had to construct addresses from segment+offset, I might use
some variation on this:

	#define	buildaddr(seg, off) (((long)(seg) << 16) + (off))
	/* I thought segments were << 4 ...? */

Using `movedata' to copy one byte from one location is overkill.

Now then, as to style and machine-dependency: the routines that
actually *use* peek and poke should be hidden away inside other
routines that do whatever is really desired.  Someone mentioned
controlling DTR on an on-board modem:

	#define	DTRADDR	((char far *)0x12345678)
		/* note that the L suffix is not strictly necessary */
		/* the value above is certain to be wrong */
	#define	DTR_BIT	0x20	/* also likely wrong */

	/*
	 * Turn DTR on (if onoff!=0) or off (if onoff==0).
	 */
	set_dtr(onoff)
		int onoff;
	{

		if (onoff)
			*DTRADDR |= DTR_BIT;
		else
			*DTRADDR &= ~DTR_BIT;
	}

=> Hide machine dependent operations inside machine-independent routines. <=
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris