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From: gnu@hoptoad.UUCP
Newsgroups: comp.arch,comp.sys.m68k
Subject: Re: Bit operations on 68K
Message-ID: <1703@hoptoad.uucp>
Date: Sat, 31-Jan-87 07:57:23 EST
Article-I.D.: hoptoad.1703
Posted: Sat Jan 31 07:57:23 1987
Date-Received: Sun, 1-Feb-87 12:15:32 EST
References: <760@orcisi.UUCP> <112@lmi-angel.UUCP> <816@drivax.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 35
Xref: watmath comp.arch:283 comp.sys.m68k:211

The 68020 introduced a new set of 8 instructions that operate on bit
FIELDS rather than BITS.  The old BIT instructions were:

	BTST, BSET, BCLR, BCHG

and they numbered bits in little endian order, modulo 32 in registers
or modulo 8 on a memory address.  The little endian order was to avoid
confusing hardware engineers who knew that pin "D0" was the low order
pin; they could code "BTST #0" and get the same bit.  Unfortunately it
confused the programmers who didn't know anything about pins but they
sure knew that the rest of the machine was big endian.  These instructions
only operate on a single bit at a time.

In the 68020, these BIT FIELD instructions were added:

	BFTST, BFSET, BFCLR, BFCHG, BFINS, BFEXTS, BFEXTU, BFFFO

(the last four are insert, extract signed, extract unsigned, and find
first one).  These number bits big-endian and will accept any bit
number relative to a byte address and figure out where it really
belongs (e.g. you can ask for bit 1005 and it will find it.)  Finally
the bit order is the same as the byte order.  This makes arrays of bit
fields work just fine and is also nice for accessing bitmapped
displays.  These instructions take both a starting bit number and a
width -- they operate on 1 to 32 bits, and are smart about 32 bit
fields lying across multiple words of memory and such.

Probably the guy who mentined "BSET.L" just meant "BFSET".  Various other
instructions got extended with ".L" so this is an easy mistake to make.

Can we stop talking about this?
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   gnu@ingres.berkeley.edu
Love your country but never trust its government.
		     -- from a hand-painted road sign in central Pennsylvania