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. <div><br></div><div>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.</div>
<div><br></div><div>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.</div><div><br><br><div class="gmail_quote">On Sat, Dec 11, 2010 at 10:51 AM, Timothe Litt <span dir="ltr"><<a href="mailto:litt@ieee.org">litt@ieee.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">According to the Sep 75 Intel 8080 Microcomputer Systems
manual:</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">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.</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">P 4-8: DAA description has the text previously cited,
plus</font></span></div>
<div dir="ltr" align="left"><span> <font color="#0000ff" size="2" face="Arial">NOTE: All flags are affected</font></span></div>
<div dir="ltr" align="left"><span> <font color="#0000ff" size="2" face="Arial">Cycles: 1</font></span></div>
<div dir="ltr" align="left"><span> <font color="#0000ff" size="2" face="Arial">States: 4</font></span></div>
<div dir="ltr" align="left"><span> <font color="#0000ff" size="2" face="Arial">Flags: Z,S,P,CY,AC</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div><font color="#0000ff" size="2" face="Arial"><span>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.)</span></font></div>
<div><font color="#0000ff" size="2" face="Arial"></font></div>
<div><font color="#0000ff" size="2" face="Arial"></font> </div>
<div><font color="#0000ff" size="2" face="Arial"></font><br></div>
<p><font size="2">---------------------------------------------------------<br>This
communication may not represent my employer's views,<br>if any, on the matters
discussed.<br> </font> </p>
<div> </div><br>
<div dir="ltr" lang="en-us" align="left">
<hr>
<font size="2" face="Tahoma"><b>From:</b> <a href="mailto:simh-bounces@trailing-edge.com" target="_blank">simh-bounces@trailing-edge.com</a>
[mailto:<a href="mailto:simh-bounces@trailing-edge.com" target="_blank">simh-bounces@trailing-edge.com</a>] <b>On Behalf Of </b>Al
Williams<br><b>Sent:</b> Saturday, December 11, 2010 10:30<br><b>To:</b> Richard
Cini<br><b>Cc:</b> <a href="mailto:simh@trailing-edge.com" target="_blank">simh@trailing-edge.com</a>; Vince Briel<br><b>Subject:</b> Re:
[Simh] DAA Emulation<br></font><br></div><div><div></div><div class="h5">
<div></div>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".
<div><br></div>
<div><br>
<div><br><br>
<div class="gmail_quote">On Sat, Dec 11, 2010 at 8:16 AM, Richard Cini <span dir="ltr"><<a href="mailto:rich.cini@verizon.net" target="_blank">rich.cini@verizon.net</a>></span>
wrote:<br>
<blockquote style="border-left:#ccc 1px solid;margin:0px 0px 0px 0.8ex;padding-left:1ex" class="gmail_quote">
<div><font face="Calibri, Verdana, Helvetica, Arial"><span style="font-size:11pt">Al --<br><br> 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):<br><br> The 8-bit number in the accumulator is
adjusted to form two 4-bit [BCD] digits by the following
process:<br><br> (1) If the
value of the least significant 4 bits is greater than 9 <b>or </b>if the AC
flag is set, 6 is added to the
accumulator.<br> (2) if the
value of the most significant 4 bits is <b>now </b>greater than 9 <b>or </b>if
the CY flag is set, 6 is added to the most significant 4 bits of the
accumulator.<br><br> 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.<br><br> As far as the register
display, I’ll make that change — oddly no one has ever reported
it.<br><br> Thanks again for locating this
bug.<br><br>Rich<br><font color="#888888"><br>--<br>Rich Cini<br>Collector of
Classic Computers<br>Build Master and lead engineer, Altair32 Emulator<br><a href="http://www.altair32.com" target="_blank">http://www.altair32.com</a><br><a href="http://www.classiccmp.org/cini" target="_blank">http://www.classiccmp.org/cini</a></font>
<div>
<div></div>
<div><br><br><br><br>On 12/11/10 1:32 AM, "Al Williams" <<a href="http://al.williams@awce.com" target="_blank">al.williams@awce.com</a>>
wrote:<br><br></div></div></span></font>
<div>
<div></div>
<div>
<blockquote><font face="Calibri, Verdana, Helvetica, Arial"><span style="font-size:11pt">Hi Rich and Bob,<br><br>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.<br><br>>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.<br><br>Here's part of Altair32's code:<br><br>static void daa ( OP_ARG_U
) <br>{<br>// Decimal Adjust Accumulator<br>// DAA:: A=BCD format<br>//
Flags: SZAPC<br>// *****<br><br>register /* FJS */ word tmp =
ACCUM;<br><br>if ( (( tmp & 0x0f ) > 0x09 ) || ( FLAGS & AC_FLAG
))<br>tmp += 0x06;<br><br>if (tmp > 0x0f)<br>FLAGS |= AC_FLAG; // if
adjusted LSB > 0xf, set AC<br>else<br>FLAGS &= ~AC_FLAG; // else
clear SC <br><br><br>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.... <br><br><br>So why copy Bob?
Well... I think SIMH has a similar but different problem. Here's a snip from
the DAA code:<br><br>/*opcode=0x27*/<br>static void i86op_daa(PC_ENV
*m)<br>{<br> uint16 dbyte;<br> dbyte =
m->R_AL;<br> if (ACCESS_FLAG(m,F_AF)|| (dbyte&0xf) >
9)<br> {<br> dbyte +=
6;<br> if (dbyte&0x100)<br>
SET_FLAG(m, F_CF);<br> SET_FLAG(m,
F_AF);<br> }<br> else<br><br><br>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.<br><br>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.<br><br>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.<br><br><br>Al Williams<br><a href="http://www.ddj.com/embedded" target="_blank">http://www.ddj.com/embedded</a> (among
others)<br><br><br><br><br></span></font></blockquote></div></div></div></blockquote></div><br></div></div></div></div></div>
</blockquote></div><br></div>