| Instruction | Description | Operands | |-------------|-------------|----------| | | Decimal Adjust AL after Addition (packed BCD) | implicit (AL) | | DAS | Decimal Adjust AL after Subtraction (packed BCD) | implicit (AL) | | AAA | ASCII Adjust after Addition (unpacked BCD) | implicit (AL, AH) | | AAS | ASCII Adjust after Subtraction (unpacked BCD) | implicit (AL, AH) | | AAM | ASCII Adjust after Multiply (unpacked) | implicit (AX) | | AAD | ASCII Adjust before Division (unpacked) | implicit (AX) |
| Command | Effect in Decimal Mode | |---------|------------------------| | | Set Decimal Mode (D=1) | | CLD | Clear Decimal Mode (D=0, back to binary) | | ADC | Adds with carry in BCD | | SBC | Subtracts with borrow in BCD | bcd commands
sed ; enable decimal mode lda #$45 ; BCD 45 adc #$27 ; result in A = $72 (BCD 72) cld ; back to binary : The 6502’s decimal mode is famously broken on the CMOS version (65C02) but works on NMOS (original). Many emulators replicate the bug. 3.3 Z80 Processor Z80 lacks automatic BCD mode but provides DAA (Decimal Adjust Accumulator) after addition or subtraction, similar to x86. mov al, 0x45 ; BCD 45 add al,
mov al, 0x45 ; BCD 45 add al, 0x27 ; BCD 27 → binary result 0x6C daa ; adjusts to 0x72 (BCD 72) BCD 45 add al
