EMBEDDED SYSTEMS & MICROCONTROLLERS · PAGE 88

Assembly: Shifts, Multiply and Divide

Fast arithmetic tricks.

Shifting left by one doubles a number, and shifting right by one halves it, using one fast instruction. Small chips often have no divide instruction, so shifts matter.

Multiplying by 10 can be done as (x shifted left 3) plus (x shifted left 1). Check for overflow and carry when working with several bytes.

EXAMPLE

; generic teaching syntax: R0 = R0 * 10
        MOV  R1, R0
        SHL  R0, 3     ; x * 8
        SHL  R1, 1     ; x * 2
        ADD  R0, R1    ; x * 10

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