ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.9.2 Unsigned integer division macro example

Example2-14 shows a macro that performs an unsigned integer division. It takes four

parameters:

$Bot

The register that holds the divisor.

$Top

The register that holds the dividend before the instructions are executed.

After the instructions are exec uted, it holds the remainder.

$Div

The register where the quotient of the division is placed. It can be

NULL

(

) if only the remainder is required.

$Temp

A temporary register used during the calculation.

Example2-14

MACRO

$Lab DivMod $Div,$Top,$Bot,$Temp

ASSERT $Top <> $Bot ; Produce an error message if the

ASSERT $Top <> $Temp ; registers supplied are

ASSERT $Bot <> $Temp ; not all different

IF "$Div" <> ""

ASSERT $Div <> $Top ; These three only matter if $Div

ASSERT $Div <> $Bot ; is not null ("")

ASSERT $Div <> $Temp ;

ENDIF

$Lab

MOV $Temp, $Bot ; Put divisor in $Temp

CMP $Temp, $Top, LSR #1 ; double it until

90 MOVLS $Temp, $Temp, LSL #1 ; 2 * $Temp > $Top

CMP $Temp, $Top, LSR #1

BLS %b90 ; The b means search backwards

IF "$Div" <> "" ; Omit next instruction if $Div is null

MOV $Div, #0 ; Initialize quotient

ENDIF

91 CMP $Top, $Temp ; Can we subtract $Temp?

SUBCS $Top, $Top,$Temp ; If we can, do so

IF "$Div" <> "" ; Omit next instruction if $Div is null

ADC $Div, $Div, $Div ; Double $Div

ENDIF

MOV $Temp, $Temp, LSR #1 ; Halve $Temp,

CMP $Temp, $Bot ; and loop until

BHS %b91 ; less than divisor

MEND