Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83 SMI; site sun.uucp
Path: utzoo!linus!decvax!decwrl!sun!gnu
From: gnu@sun.uucp (John Gilmore)
Newsgroups: net.micro.68k,net.arch,net.lang
Subject: Re: consistent bit addressing in the 68020
Message-ID: <1638@sun.uucp>
Date: Thu, 23-Aug-84 05:12:10 EDT
Article-I.D.: sun.1638
Posted: Thu Aug 23 05:12:10 1984
Date-Received: Sat, 25-Aug-84 02:51:14 EDT
References: <491@turtlevax.UUCP>
Distribution: net
Organization: Sun Microsystems, Inc.
Lines: 27

The 68000 addressed bits the wrong way (low order bit == bit 0, but
high order byte == byte 0).  This was reportedly fought over internally
at Motorola and the people who said "Let's make  BTST #0  test the data
from pin D0" won out -- they didn't want to confuse hardware types.

Well, all us software types beat on them mercilessly, so they fixed it
for the 68020.  However, they had to keep the old instructions for
backward compatability.  So they defined eight new instructions, which
all operate in a consistent way on bit FIELDS (not just bits).  You
provide a byte address, a bit number, and a bit width.  The bit number
is a signed 32-bit number; the width is 1 thru 32.  They can be
immediate fields or in data registers.  The instructions will figure
out which byte or bytes the relevant bits are in, get them, and process
them.  The 8 things you can do with them are:

	Clear them		\
	Set them		 | Same as old 68000 instructions
	Invert them		 |
	Test them		/
	Zero-extend and put them in a register
	Sign-extend and put them in a register
	Take low-order bits from a register and put them in the bitfield
	Find the leftmost 1-bit in the field, put its bit# in a register

This is especially neat for bitmapped graphics, since you can address
the screen with bit numbers on random bit boundaries and the CPU will take
care of finding and lining up the data.