SBC Subtract with Borrow from Accumulator

Subtract with Borrow
Tags:

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
Flags Affected
Flags
nv----zc
nSet if most-significant bit of result is set
vSet if signed overflow
zSet if result is zero
cSet if unsigned borrow not required, clear if required
Instructions
SyntaxOpcode Available on: # of # of Addressing Mode
(hex) 6502 65C02 65816 bytes cycles
SBC #constE9 x x x 21 22, 5 Immediate
SBC addrED x x x 3 42, 5 Absolute
SBC longEF x 4 52, 5 Absolute Long
SBC dpE5 x x x 2 32, 3, 5 Direct Page
SBC (dp)F2 x x 2 52, 3, 5 Direct Page Indirect
SBC [dp]E7 x 2 62, 3, 5 Direct Page Indirect Long
SBC addr,XFD x x x 3 42, 4, 5 Absolute Indexed X
SBC long,XFF x 4 52, 5 Absolute Long Indexed X
SBC addr,YF9 x x x 3 42, 4, 5 Absolute Indexed Y
SBC dp,XF5 x x x 2 42, 3, 5 Direct Page Indexed X
SBC (dp,X)E1 x x x 2 62, 3, 5 Direct Page Indexed Indirect X
SBC (dp),YF1 x x x 2 52, 3, 4, 5 Direct Page Indirect Indexed Y
SBC [dp],YF7 x 2 62, 3, 5 Direct Page Indirect Long Indexed Y
SBC sr,SE3 x 2 42, 5 Stack Relative
SBC (sr,S),YF3 x 2 72, 5 Stack Relative Indirect Indexed Y

Notes:

  1. 65816: Add 1 byte if m=0 (16-bit memory/accumulator)
  2. 65816: Add 1 cycle if m=0 (16-bit memory/accumulator)
  3. 65816: Add 1 cycle if low byte of Direct Page register is not 0
  4. Add 1 cycle if adding index crosses a page boundary
  5. 65C02: Add 1 cycle if d=1
Last modified November 5, 2021: Add instruction categories (6b74ff9)