mov ax, 0x0405 ; unpacked 45 (AH=04, AL=05) add al, 0x03 ; add unpacked 3 → AL=08 aaa ; adjusts if AL>9, increments AH The 6502 has no dedicated BCD arithmetic instructions, but it includes a Decimal Mode flag (D flag in status register). When set, all ADC and SBC instructions automatically perform BCD arithmetic.
ld a, $45 add a, $27 daa ; A becomes $72 The Z80 DAA uses carry and half-carry flags to correct the accumulator. ARM (especially Thumb and Cortex-M) does not have dedicated BCD arithmetic instructions. BCD operations must be implemented via software routines, using bit manipulation and conditional addition/subtraction. bcd commands
| 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 | mov ax, 0x0405 ; unpacked 45 (AH=04, AL=05)
mov al, 0x45 ; BCD 45 add al, 0x27 ; BCD 27 → binary result 0x6C daa ; adjusts to 0x72 (BCD 72) ARM (especially Thumb and Cortex-M) does not have
seconds = rtc_read(0x00); // BCD 0x45 = 45 sec // Convert to binary for computation: bin_sec = ((seconds >> 4) * 10) + (seconds & 0x0F); Multiplexed displays often receive BCD directly from a 74LS47 decoder. Processor outputs BCD values to port. 5.3 Financial/Cash Registers Currency amounts must be exact to the cent. BCD arithmetic prevents floating-point errors: