SBC


Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	E9	2/3 bytes	2 cycles*
Absolute	ED	3 bytes	4 cycles*
Absolute Long	EF	4 bytes	5 cycles*
Direct Page	E5	2 bytes	3 cycles*
Direct Page Indirect	F2	2 bytes	5 cycles*
Direct Page Indirect Long	E7	2 bytes	6 cycles*
Absolute Indexed by X	FD	3 bytes	4 cycles*
Absolute Long Indexed by X	FF	4 bytes	5 cycles*
Absolute Indexed by Y	F9	3 bytes	4 cycles*
Direct Page Indexed by X	F5	2 bytes	4 cycles*
Direct Page Indexed Indirect by X	E1	2 bytes	6 cycles*
Direct Page Indirect Indexed by Y	F1	2 bytes	5 cycles*
Direct Page Indirect Long Indexed by Y	F7	2 bytes	6 cycles*
Stack Relative	E3	2 bytes	4 cycles*
Stack Relative Indirect Indexed by Y	F3	2 bytes	7 cycles*


Flags Affected
N	V	M	X	D	I	Z	C
N	V	.	.	.	.	Z	C

SBC (Subtract with Carry/Borrow) is the main 65x subtraction instruction. The accumulator serves as both the minuend and where the difference is stored. The operand serves as the subtrahend. For example, SBC #6 does "a minus 6" but there is this caveat about the carry flag:

If the carry flag (aka borrow flag) is clear, one more is subtracted. As there is no subtract-without-carry instruction, SEC should be run beforehand or the carry flag should otherwise be ensured to be set, otherwise the difference may be one greater than expected.

The size of the accumulator determines whether this is an 8 or 16 bit operation. If 16-bit and not using immediate addressing, the low-order byte is located at the effective address and the high-order byte at the effective address plus one.

If the difference is negative, the carry flag will be cleared, otherwise it will be set. If the decimal flag is set, then binary coded decimal subtraction is performed, otherwise binary subtraction.

Syntax

SBC #const
SBC addr
SBC long
SBC dp
SBC (dp)
SBC [dp]
SBC addr, X
SBC long, X
SBC addr, Y
SBC dp, X
SBC (dp, X)
SBC (dp), Y
SBC [dp], Y
SBC sr, S
SBC (sr, S), Y

If you only need to subtract one, consider DEC instead.

Cycle Penalties

SBC takes an extra cycle for each of the following:

if the accumulator is 16 bits wide, in all addressing modes
when utilizing the direct register (opcodes E5, F2, E7, F5, E1, F1, and F7), if the low byte of the direct register is nonzero
In both Absolute Indexed and Direct Page Indirect Indexed by Y addressing modes if adding an index crosses a page boundary

Unlike the 65c02, the 65c816 does not have a cycle penalty when subtracting in decimal mode.

In Eyes & Lichty on page 498, opcode F5 has a "0" superscript typo on the # of cycles column. On page 533 opcode F5 does not have this superscript.

External Links

Eyes & Lichty, page 497 on SBC
Labiak, page 179 on SBC
2.2.2 on MCS6500 Manual, page 14 on SBC
Carr, page 271 on SBC
Leventhal, page 3-91 on SBC
snes9x implementation of SBC: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1133
undisbeliever on SBC: https://undisbeliever.net/snesdev/65816-opcodes.html#sbc-subtract-with-borrow-from-accumulator
Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#SBC
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.1

SBC

Contents

Syntax

Cycle Penalties

See Also

External Links