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From: d88-bli@xbyse.nada.kth.se (Bo Lindbergh)
Newsgroups: comp.sys.sinclair
Subject: Re: DAA (Decimal Adjust Accumalator)
Date: 26 Sep 1995 22:40:10 GMT
Organization: Royal Institute of Technology, Stockholm, Sweden
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References: <446amq$din@griffin.ccc.nottingham.ac.uk>
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In article <446amq$din@griffin.ccc.nottingham.ac.uk> pcxigw@vaxb.nott.ac.uk writes:
> Hi,
>    Does anyone have a description of how DAA works, particularly
> how does one calculate the value to add to the accumalator?

Here's some actual (well, almost...) code from my emulator that reproduces
the behaviour of a physical Z80 exactly.  You probably need to grok full
adders to understand it.

    /* assume these declarations at appropriate locations */
    extern unsigned char parity[256];
    int res,cy;
    unsigned char op8,reg_a,reg_f,flag_z,flag_h,flag_pv,flag_n,flag_c;

    /* first adjust the C and H flags by checking
       if the high or low nibbles of the accumulator exceed 9 */
    cy = (reg_a+0x66) ^ reg_a;	/* add bias and extract carry bits */
    flag_c |= (cy>>8) & 1;	/* carry into bit 8 => set C flag */ 
    flag_h |= (cy>>4) & 1;	/* carry into bit 4 => set H flag */ 

    /* then compute the correction value:
       if the C flag is set, set the high nibble to 6, else to 0.
       if the H flag is set, set the low nibble to 6, else to 0. */
    op8 = (0x88 - ((flag_c<<4) | flag_h)) & 0x66;

    /* but if the last operation was a subtraction, negate it */
    if (flag_n)
	op8 = -op8;

    /* apply the correction */
    res = reg_a+op8;

    /* set the H flag according to the correction addition */
    cy = reg_a ^ res;
    flag_h = (cy>>4) & 1;

    /* save the result */
    reg_a = res;

    /* set the Z flag accordingly */
    flag_z = (0x100-reg_a) >> 8;

    /* set the S flag and the two unused bits accordingly */
    reg_f = reg_a;

    /* set the P/V flag according to the parity */
    flag_pv = parity[reg_a];

Eliminating the one conditional jump is left as an exercise for the reader.
:-)


/Bo Lindbergh