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From: hemphill@over.world (Scott Hemphill)
Newsgroups: comp.sys.apple2
Subject: Re: 65C02 ADC in Decimal Mode
Message-ID: <HEMPHILL.91Feb18205201@over.world>
Date: 19 Feb 91 04:52:00 GMT
References: <lee.666923694@chsun1>
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Organization: California Institute of Technology
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In-Reply-To: lee@chsun1.uchicago.edu's message of 19 Feb 91 00:34:54 GMT
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>>>>> On 19 Feb 91 00:34:54 GMT, lee@chsun1.uchicago.edu (Dwight A Lee) said:

> I'm writing a function in C which needs to behave exactly like a 65C02
> ADC (add with carry).  Unfortunately, I am unable to find out just how
> the N (negative) and V (overflow) processor flags are set when the
> processor is in decimal mode.

Here is some C code that is straight out of a simulator I have written for
the 65c02.  I have verified that it gives the correct answers for all possible
operands.  (Even when adding invalid bcd in decimal mode!)

Variables:
   A   the accumulator (an unsigned char)
   b   the other operand (an unsigned char)
   C   the carry flag (0 or 1)
   D   the decimal mode flag (0 or 1)
   V   the overflow flag (0 or 1)
   w   (an unsigned int temporary)
   NZ  the N and Z flags are derived from this unsigned int

A word of explation about "NZ": one optimization my simulator makes is to
avoid computing the N and Z bits unnecessarily.  I do this by storing an
intermediate value in NZ.  If I need to compute "N" or "Z" later, I can
do so.  "N" is equivalent to (NZ >= 0x80) and "Z" is equivalent to
((Z & 0xff) == 0).  Anyway, here's the code:

#define ADC()                                        \
      do {                                           \
        if ((A^b) & 0x80) V = 0; else V = 1;         \
        if (D) {                                     \
          w = (A & 0xf) + (b & 0xf) + C;             \
          if (w >= 10) w = 0x10 | ((w+6)&0xf);       \
          w += (A & 0xf0) + (b & 0xf0);              \
          if (w >= 160) {                            \
            C = 1;                                   \
            if (V && w >= 0x180) V = 0;              \
            w += 0x60;                               \
          } else {                                   \
            C = 0;                                   \
            if (V && w < 0x80) V = 0;                \
          }                                          \
        } else {                                     \
          w = A + b + C;                             \
          if (w >= 0x100) {                          \
            C = 1;                                   \
            if (V && w >= 0x180) V = 0;              \
          } else {                                   \
            C = 0;                                   \
            if (V && w < 0x80) V = 0;                \
          }                                          \
        }                                            \
        A = w;                                       \
        NZ = A;                                      \
      } while (0)

--
Scott Hemphill	hemphill@csvax.cs.caltech.edu	...!ames!elroy!cit-vax!hemphill