3.20. PACKING 12-BIT VALUES INTO ARRAY
; jump if $4=idx&1 is not zero (i.e., if idx is odd):
bne $4,$0,$L11
addu $2,$3,$2 ; branch delay slot, this instruction is executed before BNE
; $2 = $3+$2 = idx&(~1) + idx>>1 = idx*1.5
; idx is even, go on
lw $3,%got(array)($28)
; $3 = array
addiu $4,$2,1
; $4 = $2+1 = idx1.5+1, i.e., offset of middle byte in array
srl $6,$5,4
; $6 = $5>>4 = val>>4
addu $2,$3,$2
; $2 = $3+$2 = array + idx1.5 (offset of left byte)
; store $6 (val>>4) as left byte:
sb $6,0($2)
addu $2,$3,$4
; $2 = $3+$4 = array + idx1.5 + 1 (absolute address of middle byte)
; load middle byte at $2+0 = array + idx1.5 + 1
lbu $3,0($2)
; $3 = middle byte
andi $5,$5,0xf
; $5 = $5&0xF = val&0xF
andi $3,$3,0xf
; $3 = $3&0xF = middle_byte&0xF
sll $5,$5,4
; $5 = $5<<4 = (val&0xF)<<4
or $5,$3,$5
; $5 = $3|$5 = middle_byte&0xF | (val&0xF)<<4 (new version of middle byte)
j $31
; store $5 (new middle byte) at $2 (array + idx*1.5 + 1)
sb $5,0($2) ; branch delay slot, this instruction is executed before J
$L11:
; idx is odd, go on
lw $4,%got(array)($28)
; $4 = array
addiu $3,$2,1
; $3 = $2+1 = idx1.5+1 (offset of middle byte)
addu $3,$4,$3
; $3 = $4+$3 = array + idx1.5+1
; load middle byte at $3 (array + idx1.5+1)
lbu $6,0($3)
; $6 = middle byte
srl $7,$5,8
; $7 = $5>>8 = val>>8
andi $6,$6,0xf0
; $6 = $6&0xF0 = middle_byte&0xF0
or $6,$6,$7
; $6 = $6|$7 = middle_byte&0xF0 | val>>8
addu $2,$4,$2
; store updated middle byte at $3 (array + idx1.5+1)
sb $6,0($3)
j $31
; store low 8 bits of val at $2+2=idx*1.5+2 (place of right byte)
sb $5,2($2) ; branch delay slot, this instruction is executed before J
3.20.12 Difference from the real FAT12
The real FAT12 table is slightly different:https://en.wikipedia.org/wiki/Design_of_the_FAT_file_
system#Cluster_map.
For even elements:
+0 +1 +2
|23|.1|..|..
For odd elements: