Assembly Language for Beginners

3.20. PACKING 12-BIT VALUES INTO ARRAY

POP {r4,r5,pc}

ENDP

3.20.8 Optimizing Keil 5.05 (ARM mode)

Getter

Getter function for ARM mode has no conditional branches at all! Thanks to the suffixes (like-EQ,-NE),
which can be supplied to many instructions in ARM mode, so the instruction will be only executed if the
corresponding flag(s) are set.

ManyarithmeticalinstructionsinARMmodecanhaveshiftingsuffixlikeLSL #1(itmeans,thelastoperand
is shifted left by 1 bit).

get_from_array PROC
; R0 = idx
LSR r1,r0,#1
; R1 = R0>>1 = idx>>1
; check lowest bit in idx and set flags:
TST r0,#1
ADD r2,r1,r1,LSL #1
; R2 = R1+R1<<1 = R1+R12 = R13
; thanks to shifting suffix, a single instruction in ARM mode can multiplicate by 3
LDR r1,|array|
; R1 = address of array
LSR r0,r0,#1
; R0 = R0>>1 = idx>>1
ADD r0,r0,r0,LSL #1
; R0 = R0+R0<<1 = R0+R02 = R03 = (idx>>1)3 = idx1.5
ADD r0,r0,r1
; R0 = R0+R1 = array + idx*1.5, this is absolute address of triplet
; load middle byte at R0+1:
LDRB r3,[r0,#1]
; R3 = middle byte

; the following 3 instructions executed if index is odd, otherwise all of them are skipped:
; load right byte at R0+2:
LDRBNE r0,[r0,#2]
; R0 = right byte
ANDNE r1,r3,#0xf
; R1 = R3&0xF = middle_byte&0xF
ORRNE r0,r0,r1,LSL #8
; R0 = R0|(R1<<8) = right_byte | (middle_byte&0xF)<<8
; this is the result returned

; the following 3 instructions executed if index is even, otherwise all of them are skipped:
; load at R1+R2 = array + (idx>>1)3 = array + idx1.5
LDRBEQ r0,[r1,r2]
; R0 = left byte
LSLEQ r0,r0,#4
; R0 = R0<<4 = left_byte << 4
ORREQ r0,r0,r3,LSR #4
; R0 = R0 | R3>>4 = left_byte << 4 | middle_byte >> 4
; this is the result returned
BX lr
ENDP

Setter

put_to_array PROC
; R0 = idx
; R1 = val
LSR r2,r0,#1
; R2 = R0>>1 = idx>>1
; check the lowest bit of idx and set flags: