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Path: utzoo!linus!decvax!tektronix!orca!hammer!steveg
From: steveg@hammer.UUCP (Steve Glaser)
Newsgroups: net.micro.68k,net.arch,net.lang
Subject: Re: consistent bit addressing in the 68020
Message-ID: <898@hammer.UUCP>
Date: Tue, 28-Aug-84 11:27:51 EDT
Article-I.D.: hammer.898
Posted: Tue Aug 28 11:27:51 1984
Date-Received: Thu, 30-Aug-84 11:29:45 EDT
References: <491@turtlevax.UUCP>, <1638@sun.uucp> <9903@gatech.UUCP>
Organization: Tektronix, Wilsonville OR
Lines: 29

Actually, the 68020 does number the bits both ways.

Quoting from the 68020 User's Manual page 2-4:

	A bit datum is specified by a base address that selects one
	byte in memory and a bit number that selects the one bit in
	this byte.  The most significant bit of the byte is number
	seven.

	A bit field datum is specified by a base address that selects
	one byte in memory, a bit field offset that indicates the
	leftmost (base) bit of the bit field in relation to the most
	significant bit of the base byte and a bit field width that
	determines how many bits to the right of the base bit are in
	the bit field.  The most significant bit of the base byte is
	bit offset 0, the least significant bit of the base byte is
	offset 7, and the least significant bit of the previous byte
	in memory is offset -1.  Bit field offsets may have values in
	the range of -2^31 to 2^31-1 and bit field widths may range
	between 1 and 32.

Given the high-endian nature of the 68k, it makes sense to do bit
fields this way.  It also makes sense to number the bits in a word
using the natural mathematical rules (bit n is the 2^n bit in an
unsigned int).  The 68000 doesn't have the bit field data type
so it doesn't have this inconsistency.  Oh well, it could be worse.

	Steve Glaser
	tektronix!steveg