Subtracts the data in the operand with the contents of the accumulator. Subtract 1 from the result if the carry flag is clear. Store the final result in the accumulator.

Binary/Decimal mode

If the d flag is clear then binary subtraction is performed. If the d flag set then Binary Coded Decimal (BCD) subtraction is performed.

Data size

On all processors, the data subtracted from memory is 8-bit. However, for 16-bit processors with the m flag is clear then the data subtracted is 16-bit with the low-order 8-bits at the effective address and the high-order 8-bits at the effective address plus one.

Multi-precision arithmetic

In multi-precision (multi-word) arithmetic, the carry flag should be set before the low-order words are subtracted. The subtraction will generate a new carry flag value based on that subtraction which will then be passed on to the next word.

For example, to subtract 1 from a 16-bit value at &70 on 8-bit processors:

1  SEC      ; Set carry before first subtraction
2  LDA &70  ; Subtract 1 from low-order byte
3  SBC #1
4  STA &70
5  LDA &71  ; Subtract 0 to high order byte
6  SBC #0   ; This will subtract 1 if carry was clear
7  STA &71  ; from the low-order byte