Difference between revisions of "DAA"
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If an addition operation is performed between 15 (BCD) and 27 (BCD), simple decimal arithmetic gives this result: | If an addition operation is performed between 15 (BCD) and 27 (BCD), simple decimal arithmetic gives this result: | ||
Latest revision as of 08:55, 25 May 2012
This is an article about the GB-Z80 CPU.
Opcodes:
Arithmetic and logical: ADD - ADD (16-bit) including LD HL,SP+nn - ADC - AND - CP - CPL - DAA - DEC - DEC and INC (16-bit) - INC - SBC - SUB - OR - XOR
Conditional: CALL - JP - JR - RET
Load: LD (8-bit) - LD (16-bit)
Extended Set: BIT - RES - RL - RLC - RR - RRC - SET - SLA - SRA - SRL - SWAP
If an addition operation is performed between 15 (BCD) and 27 (BCD), simple decimal arithmetic gives this result:
15 + 27 -- 42
But when the binary representations are added in the Accumulator according to standard binary arithmetic.
0001 0101 + 0010 0111 --------- 0011 1100 = $3C
the sum is ambiguous. The DAA instruction adjusts this result so that the correct BCD representation is obtained:
0011 1100 + 0000 0110 0100 0010 = $42
Instructions
DAA - $27 - 1 Machine Cycle
Flags
Z: 1 if Accumulator is zero after operation; 0 otherwise N: not affected H: 0 C: 1 if N is 0 and A is greater than $99, 0 otherwise
Pseudo-code
void DAA(void)
{
if (n)
{
if (c) { reg.a -= 0x60; }
if (h) { reg.a -= 0x06; }
}
else
{
if (c || (reg.a & 0xFF) > 0x99) { reg.a += 0x60; c = 1; }
if (h || (reg.a & 0x0F) > 0x09) { reg.a += 0x06; }
}
z = reg.a == 0;
h = 0;
}
