ARM VERSION 1.2 manual Ratio DivMod r0,r5,r4,r2

Writing ARM and Thumb Assembly Language

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

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