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# hyphenrf's solution

## to Octal in the MIPS Assembly Track

Published at Jul 25 2020 · 0 comments
Instructions
Test suite
Solution

Convert an octal number, represented as a string of ascii values (e.g. "1735263"), to its decimal equivalent using first principles. For instance, given the input "10", your program should return the integer 8.

## Note

• Treat invalid input as octal 0.

Decimal is a base-10 system.

A number 233 in base 10 notation can be understood as a linear combination of powers of 10:

• The rightmost digit gets multiplied by 10^0 = 1
• The next number gets multiplied by 10^1 = 10
• ...
• The nth number gets multiplied by 10^(n-1).
• All these values are summed.

So:

``````   233 # decimal
= 2*10^2 + 3*10^1 + 3*10^0
= 2*100  + 3*10   + 3*1
``````

Octal is similar, but uses powers of 8 rather than powers of 10.

So:

``````   233 # octal
= 2*8^2 + 3*8^1 + 3*8^0
= 2*64  + 3*8   + 3*1
= 128   + 24    + 3
= 155
``````

## Source

All of Computer Science http://www.wolframalpha.com/input/?i=base+8

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### runner.mips

``````#
# Test octal_convert with some examples
#
# s0 - num of tests left to run
# s1 - address of input word
# s2 - address of expected output word
# s3 - char byte
# s4 - output word
#
# octal_convert must:
# - be named octal_convert and declared as global
# - follow the convention of using the t0-9 registers for temporary storage
# - (if it uses s0-7 then it is responsible for pushing existing values to the stack then popping them back off before returning)
# - write integer result to v0

.data

# number of test cases
n: .word 6
# input values (null terminated) & expected output values (word sized ints)
ins:  .asciiz "1", "10", "17", "130", "2047", "1234567"
outs: .word     1,    8,   15,    88,   1063,    342391

failmsg: .asciiz "failed for test input: "
expectedmsg: .asciiz ". expected "
tobemsg: .asciiz " to be "
okmsg: .asciiz "all tests passed"

.text

runner:
lw      \$s0, n
la      \$s1, ins
la      \$s2, outs

run_test:
move    \$a0, \$s1                # move address of input str to a0
jal     octal_convert           # call subroutine under test
move    \$v1, \$v0                # move return value in v0 to v1 because we need v0 for syscall

lw      \$s4, 0(\$s2)             # read expected output from memory
bne     \$v1, \$s4, exit_fail     # if expected doesn't match actual, jump to fail

scan:
lb      \$s3, 0(\$s1)             # load byte
beq     \$s3, \$zero, done_scan   # if char null, break loop
j       scan                    # loop

done_scan:
addi    \$s1, \$s1, 1             # move input address on byte past null

addi    \$s2, \$s2, 4             # move to next word in output
sub     \$s0, \$s0, 1             # decrement num of tests left to run
bgt     \$s0, \$zero, run_test    # if more than zero tests to run, jump to run_test

exit_ok:
la      \$a0, okmsg              # put address of okmsg into a0
li      \$v0, 4                  # 4 is print string
syscall

li      \$v0, 10                 # 10 is exit with zero status (clean exit)
syscall

exit_fail:
la      \$a0, failmsg            # put address of failmsg into a0
li      \$v0, 4                  # 4 is print string
syscall

move    \$a0, \$s1                # print input that failed on
li      \$v0, 4
syscall

la      \$a0, expectedmsg
li      \$v0, 4
syscall

move    \$a0, \$v1                # print actual that failed on
li      \$v0, 1                  # 1 is print integer
syscall

la      \$a0, tobemsg
li      \$v0, 4
syscall

move    \$a0, \$s4                # print expected value that failed on
li      \$v0, 1                  # 1 is print integer
syscall

li      \$a0, 1                  # set error code to 1
li      \$v0, 17                 # 17 is exit with error
syscall

# # Include your implementation here if you wish to run this from the MARS GUI.
# .include "impl.mips"``````
``````.globl octal_convert

octal_convert:
li    \$v0, 0

LOOP:
lb    \$t1, 0(\$a0)
beqz  \$t1, STOP

subi  \$t1, \$t1, '0'
sll   \$v0, \$v0, 3
or    \$v0, \$v0, \$t1

j     LOOP

STOP:
jr    \$ra``````