3.20. PACKING 12-BIT VALUES INTO ARRAY
; EDX now is (idx>>1)*3
; point EAX to the middle byte:
lea eax, [rdx+1]
; sign-extend EAX value to RDX:
cdqe
; get middle byte into EAX:
movzx eax, BYTE PTR array[rax]
; finally check the value of the lowest bit in index.
; jump if index is odd (NE is the same as NZ (Not Zero)):
jne .L9
; this is even element, go on
; sign-extend EDX to RDX:
movsx rdx, edx
; shift middle byte 4 bits right:
shr al, 4
; AL now has 4 highest bits of middle byte
; load left byte into EDX:
movzx edx, BYTE PTR array[rdx]
; sign-extend AL (where high 4 bits of middle byte are now)
movzx eax, al
; EAX has 4 high bits bits of middle byte
; EDX now has left byte
; shift left byte 4 bits left:
sal edx, 4
; 4 lowest bits in EDX after shifting is cleared to zero
; finally merge values:
or eax, edx
ret
.L9:
; this is odd element, go on
; calculate address of right byte:
add edx, 2
; isolate lowest 4 bits in middle byte:
and eax, 15 ; 15=0xF
; sign-extend EDX (where address of right byte is) to RDX:
movsx rdx, edx
; shift 4 bits we got from middle bytes 8 bits left:
sal eax, 8
; load right byte:
movzx edx, BYTE PTR array[rdx]
; merge values:
or eax, edx
ret
Setter
put_to_array:
; EDI=idx
; ESI=val
; copy idx to EAX:
mov eax, edi
; calculate idx>>1 and put it to EAX:
shr eax
; isolate lowest bit in idx:
and edi, 1
; calculate (idx>>2)3 and put it to EAX:
lea eax, [rax+rax2]
; jump if index is odd (NE is the same as NZ (Not Zero)):
jne .L5
; this is even element, go on
; sign-extend address of triplet in EAX to RDX:
movsx rdx, eax
; copy val value to ECX:
mov ecx, esi
; calculate address of middle byte:
add eax, 1