[Simh] DAA Emulation

[Timothe Litt]

I was not defending the simh code, and agree that it does not conform.  
 
Interestingly, a version of altair_cpu.c c.a. 2003 seems to be more correct,
so it appears that some "code cleanup" introduced this form of the bug at a
later date.  The old code still wasn't correct, as it didn't clear AC if the
add is NOT done...
 
Obviously this needs a regression test!
case 047: { /* DAA */

DAR = A & 0x0F;

if (DAR > 9 || AC > 0) {

DAR += 6;

A &= 0xF0;

A |= DAR & 0x0F;

if (DAR & 0x10)

AC = 0200000;

else

AC = 0;

}

DAR = (A >> 4) & 0x0F;

if (DAR > 9 || AC > 0) {

DAR += 6;

if (AC) DAR++;

A &= 0x0F;

A |= (DAR << 4);

}

if ((DAR << 4) & 0x100)

C = 0200000;

else

C = 0;

if (A & 0x80) {

S = 0200000;

} else {

S = 0;

}

if ((A & 0xff) == 0)

Z = 0200000;

else

Z = 0;

parity(A);

A = A & 0xFF;

break;

}


---------------------------------------------------------
This communication may not represent my employer's views,
if any, on the matters discussed.
  

 

  _____  

From: Al Williams [mailto:al.williams at awce.com] 
Sent: Saturday, December 11, 2010 12:08
To: Timothe Litt
Cc: Richard Cini; simh at trailing-edge.com; Vince Briel
Subject: Re: [Simh] DAA Emulation


I agree with your interpretation and by that interpretation the code in both
emulators is currently busted. In the case of SIMH adding 6 does not always
result in a half carry.  

Granted if LSD is >9 it will cause AC to set. But in the case when AC is
already set this isn't always the case. Assuming the last add was BCD you
could have X9+Y9 = [X+Y+1]2.

So the LSD is 2 and AC is set. Adding 6 gives you 8 and no half carry. So
setting the flag every time you add 6 is incorrect.


On Sat, Dec 11, 2010 at 10:51 AM, Timothe Litt <litt at ieee.org> wrote:


According to the Sep 75 Intel 8080 Microcomputer Systems manual:
 
P 4-3: Auxiliary Carry: If the instruction caused a carry out of bit 3 and
into bit 4 of the resulting value, the auxiliary carry is set; otherwise it
is reset.  This flag is affected by single precision additions,
subtractions, increments, decrements, comparisons and logical operations,
but is principally used with additions and increments preceding a DAA
(Decimal Adjust Accumulator) instruction.
 
P 4-8: DAA description has the text previously cited, plus
    NOTE: All flags are affected
    Cycles: 1
    States: 4
    Flags: Z,S,P,CY,AC
 
I read this as AC should be set if the first add 6 (to the LSB of A)
happened and caused a carry and should be cleared otherwise.  (Only this add
can cause a carry from bit 3 to bit 4.)

 



---------------------------------------------------------
This communication may not represent my employer's views,
if any, on the matters discussed.
  

 

  _____  

From: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-edge.com]
On Behalf Of Al Williams
Sent: Saturday, December 11, 2010 10:30
To: Richard Cini
Cc: simh at trailing-edge.com; Vince Briel
Subject: Re: [Simh] DAA Emulation


Yeah I've read the docs, but even what you pasted in doesn't directly answer
the question: What is the state of the AC flag after a DAA? So I think the
code correctly implements the additions you mentioned. But I think we all 3
disagree on the AC flag code. Notice I didn't copy the whole function in
either case, so the handling of the MSD is "not shown". 




On Sat, Dec 11, 2010 at 8:16 AM, Richard Cini <rich.cini at verizon.net> wrote:


Al --

    Thanks a lot for the email. I'm sure you looked at this, but I pulled
the Intel 8080 Users Manual and under DAA, it says the following (snipping a
bit):

    The 8-bit number in the accumulator is adjusted to form two 4-bit [BCD]
digits by the following process:

        (1) If the value of the least significant 4 bits is greater than 9
or if the AC flag is set, 6 is added to the accumulator.
        (2) if the value of the most significant 4 bits is now greater than
9 or if the CY flag is set, 6 is added to the most significant 4 bits of the
accumulator.

    Based on this, I would say that the second part of the test in the
Altair32 code is wrong. Further, it looks like the SIMH code may be wrong as
well because it doesn't test the CY flag in the second test.

    As far as the register display, I'll make that change - oddly no one has
ever reported it.

    Thanks again for locating this bug.

Rich

--
Rich Cini
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator
http://www.altair32.com
http://www.classiccmp.org/cini 




On 12/11/10 1:32 AM, "Al Williams" <al.williams at awce.com> wrote:



Hi Rich and Bob,

I've been doing some work on Vince Briel's excellent AVR emulation of the
8080 and while rewriting DAA I came across what I think to be a harmless bug
but thought you might want to comment on it.

>From what I can glean DAA effects all flags including half carry. And I
_think_ that half carry occurs from the +6 (if it happens at all). So if you
don't adjust the LSD you get AC=0. If you add 6 then if adding the 6 gives
you a carry out of Bit 3 you get AC set. Note that the carry might ripple so
that's NOT to say Bit 4 is necessarily 1.

Here's part of Altair32's code:

static void daa ( OP_ARG_U ) 
{
// Decimal Adjust Accumulator
// DAA:: A=BCD format
// Flags: SZAPC
// *****

register /* FJS */ word tmp = ACCUM;

if ( (( tmp & 0x0f ) > 0x09 ) || ( FLAGS & AC_FLAG ))
tmp += 0x06;

if (tmp > 0x0f)
FLAGS |= AC_FLAG; // if adjusted LSB > 0xf, set AC
else
FLAGS &= ~AC_FLAG; // else clear SC 


So since tmp is not masked off, any value >0xF gets AC set even if no carry
or add occurred! In other words, pretend the value of ACCUM is 90 (and thus
temp is 90) with AC=0. The first if does not fire. The second if does and AC
gets set. That's got to be wrong. Granted, who checks AC after DAA? But
still.... 


So why copy Bob? Well... I think SIMH has a similar but different problem.
Here's a snip from the DAA code:

/*opcode=0x27*/
static void i86op_daa(PC_ENV *m)
{
   uint16 dbyte;
   dbyte = m->R_AL;
   if (ACCESS_FLAG(m,F_AF)|| (dbyte&0xf) > 9)
     {
    dbyte += 6;
    if (dbyte&0x100)
      SET_FLAG(m, F_CF);
    SET_FLAG(m, F_AF);
     }
   else


Here we add 6 to dbyte and then AC is always set. If no +6 then AC is
cleared. This COULD be correct behavior, but I can't find any reference
material that says it is. In any event, SIMH and Altair32 are doing
something different here, so they both can't be right. Meanwhile I have my
own version of DAA in AVR assembler that I will spare you unless you ask.
The only real silicon I have even close to operational at the moment is a
Z80 and not only is it not operational, the DAA is one place where it is a
lot different so I don't trust the result there.

Oh. One other note about Altair32. In the debugger, the A and FLAGS display
is swapped in the debug console window. The Register display up top left is
ok though. I bet you've heard that one before.

If either of you can show documentation on the AC flag after DAA or point to
a real piece of silicon's behavior I'd like to know so I can fix the DAA
code in the Briel emulator to match. Otherwise, I thought you'd like to know
about this bug even though it is pretty innocuous as far as I can tell.


Al Williams
http://www.ddj.com/embedded (among others)








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