.data
# the following are used by the subroutines
bitbuf: .byte 0, 0
cumulus: .word 0
rbitbuf: .byte 0, 0
rfree: .word 0
# these are instead for testing
outbuf: .space 16
abuf: .space 16
txt: .asciiz "This is a test"
.text
main:
# ---- encode txt to buf -----
la $s0, txt
la $s1, outbuf
encloop:
lbu $a0, ($s0)
beq $a0, $0, txtend
li $a1, 7
move $a2, $s1
jal bits_write
addu $s0, $s0, 1
move $s1, $v1
b encloop
txtend:
move $a1, $s1
jal bits_flush
move $s1, $v0 # update buf, will be used as "terminator"
la $s0, outbuf
# ---- write buffer
wrtchar:
beq $s0, $s1, wrtend # while $s0 < $s1 {
lb $a0, ($s0)
li $v0, 11
syscall # print_char($a0)
addi $s0, $s0, 1 # inc $s0
b wrtchar # }
wrtend:
# ---- decode from buffer in bufferb----
# $s1 is still our end marker
la $s3, outbuf # prev. output becomes input
li $s7, 7 # commodity
la $s4, abuf # current output buf
decloop:
bge $s3, $s1, decend # while $s3(input) < $s1(limit)
move $a0, $0 # clean acc.
move $a1, $s7 # 7 bits
move $a2, $s3 # from input buf
jal bits_read # read 7 bits into acc.
move $s3, $v0 # update input buf
sb $v1, ($s4)
addi $s4, $s4, 1 # write char and update outp buf ptr
b decloop # end while
decend:
# lets output abuf; $s4 points to end of abuf,
# we need to mark it with 0 to print it as a str
sb $0, ($s4)
la $a0, abuf
li $v0, 4
syscall # print_string(abuf)
li $v0, 10 # exit
syscall
# read_bits
# $a0 acc.
# $a1 how many (max 32)
# $a2 input buffer
# > $v1 (new acc)
# > $t0 num of read bits or less (failure)
# > $v0 updated input buffer
bits_read:
sub $sp, $sp, 20 # local vars
sw $ra, 0($sp) # save regs
sw $s0, 4($sp)
sw $s1, 8($sp)
sw $s2, 12($sp)
sw $s3, 16($sp)
move $s3, $a2 # $s3 input buf
move $s0, $a0 # $s0 acc
move $s1, $a1 # $s1 n
move $s2, $0 # rbit = 0
li $t0, 32
bgt $a1, $t0, brexit # if n > 32 then return
brloop:
ble $s1, $0, brexit # while n > 0 (if <= 0 break)
move $a0, $s0
move $a1, $s3
jal read_bit # read 1 bit into acc.
move $s3, $v0 # update input buf
li $t1, 1
bne $t0, $t1, brexit # exit if not read
move $s0, $v1 # get updated acc
sub $s1, $s1, 1 # n--
addi $s2, $s2, 1 # rbit++
b brloop
brexit:
move $t0, $s2 # currently read bits
move $v1, $s0 # new accumulator
move $v0, $s3 # updated input ptr
lw $ra, 0($sp) # restore regs
lw $s0, 4($sp)
lw $s1, 8($sp)
lw $s2, 12($sp)
lw $s3, 16($sp)
addu $sp, $sp, 20 # and free stack
jr $ra # return
# get last (maybe incomplete) accumulated byte
# $a0 accumulator
# >$v0 free storage
bits_getlast:
lw $t0, rfree
li $t1, 8
sub $t1, $t1, $t0 # sochar - rfree
sllv $a0, $a0, $t1 # *d << (8 - rfree)
lbu $t2, rbitbuf
srlv $t2, $t2, $t0 # rbitbuf >> rfree
sb $0, rbitbuf # rbitbuf = 0
sw $0, rfree # rfree = 0
move $v0, $t1
jr $ra # return sochar - rfree
# read a single bit
# $a0 accumulator
# $a1 input buffer
# > $v1 new accumulator
# > $t0 bit read (1 or not 1; in this impl, reading
# can't fail unless input buffer is an invalid ptr
# > $v0 updated input buffer
read_bit:
sub $sp, $sp, 20 # local stack,
sw $s0, 0($sp) # save regs
sw $s1, 4($sp)
sw $s2, 8($sp)
sw $s3, 16($sp)
move $s0, $a0 # save args
move $s3, $a1 # $s0 = acc, $s3 = inbuf
lw $s1, rfree # preload rfree and
lbu $s2, rbitbuf # rbitbuf
bne $s1, $0, skipif # if rfree == 0 then
lbu $s2, ($s3) # rbitbuf = nextchar()
addi $s3, $s3, 1 # update it inp buf
li $s1, 8 # rfree = 8
skipif:
sll $s0, $s0, 1 # *d << 1
srl $t0, $s2, 7 # rbitbuf >> (8-1)
andi $t0, $t0, 1 # & 1
or $s0, $s0, $t0 # | *d
sll $s2, $s2, 1 # rbitbuf << 1
sub $s1, $s1, 1 # rfree--
sw $s1, rfree
sb $s2, rbitbuf # update rfree, rbitbuf
li $t0, 1 # $t0 returns 1
move $v1, $s0 # return the acc. in $v1
move $v0, $s3 # and updated inp buf in $v0
lw $s0, 0($sp) # restore regs
lw $s1, 4($sp)
lw $s2, 8($sp)
lw $s3, 16($sp)
add $sp, $sp, 20 # .. and stack
jr $ra # return($v0, $v1)
# bits_write; write upto 32 bits
# $a0 holds the bits (right aligned)
# $a1 how many bits are significative
# $a2 dest buf
# > $v1 updated buf ptr
# > $v0 written bits (-1 on error)
bits_write:
sub $sp, $sp, 28 # some local storage
sw $fp, 20($sp)
move $fp, $sp
sw $s1, 4($fp) # save regs and values
sw $s0, 0($fp)
sw $s2, 8($fp)
sw $s3, 12($fp)
sw $ra, 16($fp)
sw $s4, 24($fp)
move $s4, $a2
move $s1, $a1 # n is $s1
li $t0, 32
bgt $a1, $t0, error # n > 32 => return(-1)
sub $t0, $t0, $a1 # 32 - n
sllv $s0, $a0, $t0 # d << (32-n)
move $s3, $0
loop:
blez $s1, exit0 # if n <= 0 then break
sub $s1, $s1, 1 # n--
addu $s3, $s3, 1 # wbit++
move $a0, $s0
rol $a0, $a0, 1 # place "last" bit in first pos.
move $a1, $s4
jal appendbit # append it
sll $s0, $s0, 1 # dpad << 1
move $s4, $v1 # update buf ptr
b loop # loop
error:
li $v0, -1
b exit
exit0:
move $v0, $s3
exit:
lw $s1, 4($fp) # restore regs
lw $s0, 0($fp)
lw $s2, 8($fp)
lw $s3, 12($fp)
move $v1, $s4
lw $s4, 24($fp)
lw $ra, 16($fp)
move $sp, $fp # and release stack
lw $fp, 20($sp)
addi $sp, $sp, 28
jr $ra # return wbit, ptr to byte beyond last
# written
# flush bits
# $a1 output buf
# >$v0 adv. ptr to buf.
bits_flush:
li $t0, 8
lw $t1, cumulus
sub $t0, $t0, $t1 # 8 - cumulus
lbu $a0, bitbuf
sllv $a0, $a0, $t0 # bitbuf <<= (8 - cumulus)
sb $a0, ($a1) # write to buf
addi $v0, $a1, 1 # inc buf ptr
sb $0, bitbuf # bitbuf := 0
sw $0, cumulus # cumulus := 0
jr $ra
# add a single bit.
# $a0 holds the bit (as LSB i.e. "rightmost bit")
# $a1 output buf
# > $v0 number of written bits (always 1)
# > $v1 incremented buf ptr
appendbit:
sub $sp, $sp, 16 # save s-regs on stack
sw $s0, 0($sp)
sw $s1, 4($sp)
sw $s2, 8($sp)
sw $s3, 12($sp)
move $s3, $a1 # save buf ptr
move $s0, $a0 # copy argument
lw $s1, cumulus # get cumulus
lbu $s2, bitbuf # preload bitbuf
li $t1, 8
bne $s1, $t1, mroom # if cumulus = 8 then
sb $s2, ($s3) # write bitbuf to buf
add $s3, $s3, 1 # inc buf.
move $s2, $0 # bitbuf := 0
move $s1, $0 # cumulus := 0
mroom: # endif
sll $s2, $s2, 1 # bitbuf <<= 1
andi $s0, $s0, 1 # arg & 1
or $s2, $s2, $s0 # bitbuf |= arg
sb $s2, bitbuf # store bitbuf
addi $s1, $s1, 1 # cumulus++
sw $s1, cumulus # store cumulus
lw $s0, 0($sp) # restore regs
lw $s1, 4($sp)
lw $s2, 8($sp)
move $v1, $s3
lw $s3, 12($sp)
addi $sp, $sp, 16 # restore stack
li $v0, 1
jr $ra # return(1)
