Date: Saturday, December 31, 1994
From: INTERNET: kirkland@cs.utah.edu (Dan Kirkland)
  Re: Re: hardware bug with A=A+CON fs,expr ?                              

|> The Jazz v2.7 documentation states that there
|> is a hardware bug with the A=A+CON fs,expr instruction 
|> when fs specifies a single nibble (WP,P,S,XS).  This
|> is the first I've heard of this bug.  What is the bug?
|> I assume it affects C=C+CON fs,expr as well.

Actually it should be 'r=r+CON fs,expr' and  'r=r-CON
fs,expr' where r is register A ,B ,C ,or D.  SASM does
not allow fields WP, P, S, and XS. The bug affects all
single nibble fields WP (with P=0), P, S, and XS. WP
works normal when P does not equal zero.

With multi-nibble fields, 'r=r+CON fs,expr' will add expr
to the fs field of register r.  There is no carry
generated from the most significant nibble.

So, if A.A equals FFFFF, then A=A+CON A,5 will give 00004
in A.A (A.A rolls over to 0, then 1, 2, 3, 4).

With single nibble fields, 'r=r+CON fs,expr' affects the
WHOLE register! With nibble fs being the least
significant nibble and fs+15 (fs = nibble 0) or fs-1
being the most significant nibble.  AND, there IS a carry
generated from the most significant nibble which wraps
around and affects the least significant nibble.

So, if A.W equals FFFFFFFFFFFFFFFF, then A=A+CON P,5 will
give 0000000000000005 in A.W (A.W rolls over to 1, then
2, 3, 4, 5).

If A.W equals FFFFFFFFFFFFFFFF, then A=A+CON XS,5 will
give 0000000000000500 in A.W.

So basicly, single nibble fields will increase (or
decrease) the whole register and at rollover it will
rollover to 1 instead of to 0. 'r=r-CON fs,expr' will go
from 0...00 to F...FE instead of to F...FF.

For timing, the single nibble fields are just more than
the W field. So with the W field timing at 21 cycles, the
single nibble fields are about 22 cycles.

DanK