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S3C2440A RISC MICROPROCESSOR THUMB INSTRUCTION SET
4-43
DIVISION BY A CONSTANT
Division by a constant can often be performed by a short fixed sequence of shifts, adds and subtracts.
Here is an example of a divide by 10 routine based on the algorithm in the ARM Cookbook in both Thumb and
ARM code.
Thumb Code
udiv10 ; Take argument in a1 returns quotient in a1,
; remainder in a2
MOV a2, a1
LSR a3, a1, #2
SUB a1, a3
LSR a3, a1, #4
ADD a1, a3
LSR a3, a1, #8
ADD a1, a3
LSR a3, a1, #16
ADD a1, a3
LSR a1, #3
ASL a3, a1, #2
ADD a3, a1
ASL a3, #1
SUB a2, a3
CMP a2, #10
BLT %FT0
ADD a1, #1
SUB a2, #10
0
MOV pc, lr
ARM Code
udiv10 ; Take argument in a1 returns quotient in a1,
; remainder in a2
SUB a2, a1, #10
SUB a1, a1, a1, lsr #2
ADD a1, a1, a1, lsr #4
ADD a1, a1, a1, lsr #8
ADD a1, a1, a1, lsr #16
MOV a1, a1, lsr #3
ADD a3, a1, a1, asl #2
SUBS a2, a2, a3, asl #1
ADDPL a1, a1, #1
ADDMI a2, a2, #10
MOV pc, lr