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SAM47 INSTRUCTION SET KS57C2308/P2308/C2316/P2316
5-50
DECS — Decrement and Skip on Borrow
DECS dst
Operation: Operand Operation Summary Bytes Cycles
R Decrement register (R); skip on borrow 1 1 + S
RR Decrement register pair (RR); skip on borrow 2 2 + S
Description: The destination is decremented by one. An original value of 00H will underflow to 0FFH. If a
borrow occurs, a skip is executed. The carry flag value is unaffected.
Operand Binary Code Operation Notation
R 0 1 0 0 1 r2 r1 r0
R ← R–1; skip on borrow
RR 1 1 0 1 1 1 0 0
RR ← RR–1; skip on borrow
1 1 0 1 1 r2 r1 0
Examples: 1. Register pair HL contains the value 7FH (01111111B). The following instruction leaves the
value 7EH in register pair HL:
DECS HL
2. Register A contains the value 0H. The following instruction sequence leaves the value 0FFH
in register A. Since a "borrow" occurs, the “CALL PLAY1” instruction is skipped and the “CALL
PLAY2” instruction is executed:
DECS A ; "Borrow" occurs
CALL PLAY1 ; Skipped
CALL PLAY2 ; Executed