Enclosed: One new opcode for your 68020 system!

Enclosed: One new opcode for your 68020 system!

Post by David Triss » Thu, 02 Aug 1990 06:21:00




>Number three on my "wish list" of 680XX opcodes is:

>            tst.l   a0

>As we all know, this instruction is documented as illegal in all Motorola
>documentation.  68000 processors take an illegal exception.  The 68020
>and 68030, however will properly execute tst.l a0 (!).

Actually this addressing mode IS documented in the MC68020 and MC68030 manuals
but so pathetically bad that it is misread by almost all readers (initially
including myself.)

Note the following which appears above the supposed table of valid effective
addresses for TST and pay attention to what I've underlined:

---------------
  Effective Address field - Specifies the destination operand. If the
     operation size is word or long, all addressing modes are allowed. If the
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     operation size is byte, only data addressing modes are allowed as
     shown:

      [Table appears here with dashes indicating no An and #<data> modes]
----------------

In fact, I wouldn't be a bit surprized if the table is STILL wrong and that a
tst.b will work with immediate data.

A lot of people besides myself have *ed about this misguiding documentation
but those in charge of it don't seem to care.

 -- Dave Trissel  Motorola Austin

 
 
 

Enclosed: One new opcode for your 68020 system!

Post by roberto div » Thu, 02 Aug 1990 17:47:00


Quoted from the MC68020 & MC68030 User's Manual, TST instruction description,
page B-167 (MC68020UM/AD REV 1) and 3-190 (MC68030UM/AD):                    

"Effective address field - Specifies the destination operand. If the operation
 size is word or long, all addressing modes are allowed. [...]"              

New opcode? Really?                                                          

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|   Roberto Divia`      | Love at  first sight  is one of the greatest |
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Enclosed: One new opcode for your 68020 system!

Post by Ken Niels » Sun, 02 Sep 1990 10:02:00


I seem to be lucky, since I have a version of the MC68020 Users Manual
where TST An is documentet (even in the table!!). The manual is
refered to as MC68020UM/AD REV 2 (Third Edition) from Prentice Hall
ISBN 0-13-567017-9.

--stuff deleted --

Quote:>In fact, I wouldn't be a bit surprized if the table is STILL wrong and that a
>tst.b will work with immediate data.

According to my version of the table it is still NOT possible to use
immediate data.

Quote:>A lot of people besides myself have *ed about this misguiding documentation
>but those in charge of it don't seem to care.
> -- Dave Trissel  Motorola Austin

It just seem to take some time, perhaps the documentation is up to date
when those processors go to museum. :->
---
Ken Nielsen    
Systems Software Development Department
Dansk Data Elektronik A/S, 2730 Herlev, Denmark  Phone: int +4542845011

--
Ken Nielsen    
Systems Software Development Department
Dansk Data Elektronik A/S, 2730 Herlev, Denmark  Phone: int +4542845011

 
 
 

1. Enclosed: One new opcode for your 68020 system!

   >
   >Number three on my "wish list" of 680XX opcodes is:
   >
   >         tst.l   a0
   >
   >As we all know, this instruction is documented as illegal in all Motorola
   >documentation.  68000 processors take an illegal exception.  The 68020
   >and 68030, however will properly execute tst.l a0 (!).

I always thought it was strange that the 68000, which is picky about
odd/even addresses, did not have a bit test instruction for an address
register.

--

---------------------------------------
Scott Amspoker
Basis International, Albuquerque, NM
505-345-5232

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