Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!hp4nl!philapd!ssp11!dolf
From: dolf@idca.tds.PHILIPS.nl (Dolf Grunbauer)
Newsgroups: comp.sys.amiga.tech
Subject: Re: 32 bit RAM
Message-ID: <676@ssp11.idca.tds.philips.nl>
Date: 9 Mar 90 15:03:39 GMT
References: <1778@crash.cts.com> <8720@netcom.UUCP>
Organization: Philips Information Systems, Apeldoorn, The Netherlands
Lines: 43

In article <8720@netcom.UUCP> hue@netcom.UUCP (Jonathan Hue) writes:
>In article <1778@crash.cts.com> hawk@pnet01.cts.com (John Anderson) writes:
>>I thought each instruction was 32 bits in length and needed to be stored at
>>two 16 bit addresses so when it was used in 32 bit RAM it took up 32 bits for
>>that same one instruction.  Am I right so far in saying the first is right and
>>the second (what I used to think) is wrong?  
>
>They're both wrong.  68k instructions can be 1, 2, or 3 16-bit words long,
>although I'm pretty sure that only the 68020 and 68030 support the 3
>word long ones (CAS2 and LINK (long) come to mind).

Hmm I think this is not quite correct. The instruction word is always one
16-bit word. If necessary there are extra extension words, e.g. to hold
some immediate data or address etc. For example:
JSR   (a0)          will translate to: 4E90
JSR   0x12345678    will translate to: 4EB9 1234 5678

The longest possible instruction on a MC68000/MC68008/MC68010/MC68070 is:
MOVE.L #0x12345678,0x9ABCDEF0  which translates to: 23FC 1234 5678 9ABC DEF0

The longest possible instruction on a MC68020/MC68030/MC68040 is:
MOVE.L ([0x11111111,A0,D0.L*8],0x22222222), ([0x33333333,A1,D1.L*8],0x44444444)
which translates to: 21B1 0F23 1111 1111 2222 2222 1F23 3333 3333 4444 4444

Note that the very short: MOVE.L A0,D0  just translates to: 2010

When a co-processor is involved (e.g. MC68881/MC68882), the actual instruction
length depends on the co-processor, but is limited to 256 (extionsion) bytes,
giving a total of 129 words. The instruction word indicates that the
instruction is actually for the co-processor, the next word is the co-processor
instruction and then there may be at most 254 extension bytes for the co-pro.
The actual limit for a MC68881 is I think 8 words:
FMOVE.X #0x123456789ABCDEF012345678,FP0
which translates to: F23E 4800 1234 5678 9ABC DEF0 1234 5678

Disclaimer: I hope the hexnumbers of the assembler translations are correct,
            I did not do a compiler check, but tried to reproduce the
            MC68030 and MC68881 User's Manual.
-- 
Dolf Grunbauer          Tel: +31 55 433233  Internet dolf@idca.tds.philips.nl
Philips Telecommunication and Data Systems  UUCP ....!mcvax!philapd!dolf
Dept. SSP, P.O. Box 245, 7300 AE Apeldoorn, The Netherlands         n   n   n
It's a pity my .signature is too small to show you my solution of  a + b = c