Samsung S3F80JB - Manual (Page 164)

INSTRUCTION SET S3F80JB
6-36
DECW
— Decrement Word
DECW dst
Operation: dst
← dst – 1
The contents of the destination location (which must be an even address) and the operand
following that location are treated as a single 16-bit value that is decremented by one.
Flags: C: Unaffected.
Z: Set if the result is "0"; cleared otherwise.
S: Set if the result is negative; cleared otherwise.
V: Set if arithmetic overflow occurred; cleared otherwise.
D: Unaffected.
H: Unaffected.
Format:
Bytes Cycles Opcode
(Hex)
Addr Mode
dst
opc dst 2 8 80 RR
8 81 IR
Examples: Given: R0 = 12H, R1 = 34H, R2 = 30H, register 30H = 0FH, and register 31H = 21H:
DECW RR0
→
R0 = 12H, R1 = 33H
DECW @R2
→
Register 30H = 0FH, register 31H = 20H
In the first example, destination register R0 contains the value 12H and register R1 the value
34H. The statement "DECW RR0" addresses R0 and the following operand R1 as a 16-bit word
and decrements the value of R1 by one, leaving the value 33H.
NOTE: A system malfunction may occur if you use a Zero flag (FLAGS.6) result together with a DECW
instruction. To avoid this problem, we recommend that you use DECW as shown in the following
example:
LOOP: DECW RR0
LD R2,R1
OR R2,R0
JR NZ,LOOP