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S3C84E5/C84E9/P84E9 INSTRUCTION SET
6-77
SBC — Subtract with Carry
SBC dst,src
Operation: dst ← dst – src – c
The source operand, along with the current value of the carry flag, is subtracted from the destination
operand and the result is stored in the destination. The contents of the source are unaffected.
Subtraction is performed by adding the two's-complement of the source operand to the destination
operand. In multiple precision arithmetic, this instruction permits the carry ("borrow") from the
subtraction of the low-order operands to be subtracted from the subtraction of high-order operands.
Flags: C: Set if a borrow occurred (src > dst); cleared otherwise.
Z: Set if the result is "0"; cleared otherwise.
S: Set if the result is negative; cleared otherwise.
V: Set if arithmetic overflow occurred, that is, if the operands were of opposite sign and the
sign of the result is the same as the sign of the source; cleared otherwise.
D: Always set to "1".
H: Cleared if there is a carry from the most significant bit of the low-order four bits of the result
set otherwise, indicating a “borrow”
Format:
Bytes Cycles Opcode
(Hex)
Addr Mode
dst src
opc dst | src 2 4 32 r r
6 33 r lr
opc src dst 3 6 34 R R
6 35 R IR
opc dst src 3 6 36 R IM
Examples: Given: R1 = 10H, R2 = 03H, C = "1", register 01H = 20H, register 02H = 03H,
and register 03H = 0AH:
SBC R1,R2 → R1 = 0CH, R2 = 03H
SBC R1,@R2 → R1 = 05H, R2 = 03H, register 03H = 0AH
SBC 01H,02H → Register 01H = 1CH, register 02H = 03H
SBC 01H,@02H → Register 01H = 15H, register 02H = 03H,
register 03H = 0AH
SBC 01H,#8AH → Register 01H = 95H; C, S, and V = "1"
In the first example, if the working register R1 contains the value 10H and the register R2 the value
03H, the statement "SBC R1,R2" subtracts the source value (03H) and the C flag value ("1") from
the destination (10H) and then stores the result (0CH) in the register R1.