8 Dec
2005
8 Dec
'05
2:14 p.m.
Subject: Re: Legacy apps in Windows/OS X was Re: Old MS-DOS & Win Software
From: Jim Leonard <trixter at oldskool.org>
Date: Thu, 08 Dec 2005 15:55:58 -0600
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at
classiccmp.org>
Chuck Guzis wrote:
You lived a shelterd life.
8080/8085 have a flag that is set when ther eis a carry from the low 4
bits to the upper as part of the DAA (Decimal Adjust Accumulator). Not
only is it mostly undocumented therre are a bunch of instructions like
Jumps that work with it also undocumented. While I've never deleved
deeper than I had to with 8086/88 I'd bet it exits there and and with
a few more permutations added. It's not unique to Intel!
Allison
codes. Inspection of the 808x instruction set
can be very illuminiating,
particularly when it comes to arcana like which instructions modify (and
how) the half-carry flag, for example.
The "half-carry flag"? Huh? 
8 Dec
8 Dec
2:26 p.m.
Allison wrote:
You lived a shelterd life.
I lived a young life. I first became of programming age in late 1970s so my
first CPUs were 6502, Z80, 68000, and 808x.
I'm in my 30s, it's not like I was born in 1985 or anything :)
--
Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/
Want to help an ambitious games project? http://www.mobygames.com/
Or check out some trippy MindCandy at http://www.mindcandydvd.com/
2:33 p.m.
On 12/8/2005 at 5:14 PM Allison wrote:
8080/8085 have a flag that is set when ther eis a carry
from the low 4
bits to the upper as part of the DAA (Decimal Adjust Accumulator). Not
only is it mostly undocumented therre are a bunch of instructions like
Jumps that work with it also undocumented. While I've never deleved
deeper than I had to with 8086/88 I'd bet it exits there and and with
a few more permutations added. It's not unique to Intel!
It's still there--and is one of the reasons this little tibit still works
to convert a 4-bit binary value to ASCII hex:
and al,15
add al,90h
daa
adc al,40h
daa
Of course, on the '86 it's probably faster to just translate the thing
(XLAT) or even use an indexed move--and rather than use a shift and mask to
separate the two hex nibbles, one could use an AAM with a 10H in the second
byte of the opcode instead of the default 0AH. But on the 8080, a cute
trick saved a bunch of code because indexing wasn't easy.
BTW, where was the first place this little trick was employed? I think I
can remember seeing it in the Intel MDS monitor code.
Cheers,
Chuck
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