Samsung YP-P10 User Manual download pdf (Page 146)

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101

ARM INSTRUCTION SET S3C2440A RISC MICROPROCESSOR

3-62

Multiplication by 6

ADD Ra,Ra,Ra,LSL #1 ; Multiply by 3

MOV Ra,Ra,LSL#1 ; and then by 2

Multiply by 10 and add in extra number

ADD Ra,Ra,Ra,LSL#2 ; Multiply by 5

ADD Ra,Rc,Ra,LSL#1 ; Multiply by 2 and add in next digit

General recursive method for Rb := Ra*C, C a constant:

1. If C even, say C = 2^n*D, D odd:

D=1: MOV Rb,Ra,LSL #n

D<>1: {Rb := Ra*D}

MOV Rb,Rb,LSL #n

2. If C MOD 4 = 1, say C = 2^n*D+1, D odd, n>1:

D=1: ADD Rb,Ra,Ra,LSL #n

D<>1: {Rb := Ra*D}

ADD Rb,Ra,Rb,LSL #n

3. If C MOD 4 = 3, say C = 2^n*D-1, D odd, n>1:

D=1: RSB Rb,Ra,Ra,LSL #n

D<>1: {Rb := Ra*D}

RSB Rb,Ra,Rb,LSL #n

This is not quite optimal, but close. An example of its non-optimality is multiply by 45 which is done by:

RSB Rb,Ra,Ra,LSL#2 ; Multiply by 3

RSB Rb,Ra,Rb,LSL#2 ; Multiply by 4*3-1 = 11

ADD Rb,Ra,Rb,LSL# 2 ; Multiply by 4*11+1 = 45

rather than by:

ADD Rb,Ra,Ra,LSL#3 ; Multiply by 9

ADD Rb,Rb,Rb,LSL#2 ; Multiply by 5*9 = 45

1 2 ... 141 142 143 144 145 146 147 148 149 150 151 ... 595 596

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Samsung YP-P10 User Manual Page 146

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