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MICROPROCESSOR INSTRUCTION REFERENCE->SUBB

SUBB           Subtract with Borrow A,<source byte>

Subtract source byte and carry flag from accumulator and store result in accumulator.

Formats:
    

Description	Mnemonic	Bytes	Cycles

Subtract Immediate Data:	SUBB A,#<data>	2	1
Subtract Indirect Address:	SUBB A,@Ri	1	1
Subtract Register:	SUBB A,Rn	1	1
Subtract Memory:	SUBB A,<direct address>	2	1

where Ri is R0 or R1 and Rn is R0 - R7
    

PSW:	C	AC	F0	RS1	RS0	OV	P
	*	*	N/A	N/A	N/A	*	*

(N/A: Not affected)

Carry (C) (borrow) is set if a borrow is needed for bit 7, else it is cleared.
Auxiliary carry (AC) is set if a borrow is needed for bit 3, else it is cleared.
Overflow (OV) is set if a borrow is needed into bit 6 but not into bit 7 or into bit 7 but not into bit 6.