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Writing ARM and Thumb Assembly Language
2-50 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
The macro checks that no two parameters use the same register. It also optimizes the
code produced if only the remainder is required.
To avoid multiple definitions of labels if
DivMod
is used more than once in the assembler
source, the macro uses local labels (90, 91). Refer to Local labels on page 2-13 for more
information.
Example 2-15 shows the code that this macro produces if it is invoked as follows:
ratio DivMod r0,r5,r4,r2
Example 2-15
ASSERT r5 <> r4 ; Produce an error if the
ASSERT r5 <> r2 ; registers supplied are
ASSERT r4 <> r2 ; not all different
ASSERT r0 <> r5 ; These three only matter if $Div
ASSERT r0 <> r4 ; is not null ("")
ASSERT r0 <> r2 ;
ratio
MOV r2, r4 ; Put divisor in $Temp
CMP r2, r5, LSR #1 ; double it until
90 MOVLS r2, r2, LSL #1 ; 2 * r2 > r5
CMP r2, r5, LSR #1
BLS %b90 ; The b means search backwards
MOV r0, #0 ; Initialize quotient
91 CMP r5, r2 ; Can we subtract r2?
SUBCS r5, r5, r2 ; If we can, do so
ADC r0, r0, r0 ; Double r0
MOV r2, r2, LSR #1 ; Halve r2,
CMP r2, r4 ; and loop until
BHS %b91 ; less than divisor