# shmibs's solution

## to Binary in the MIPS Assembly Track

Published at Jul 13 2018 · 2 comments
Instructions
Test suite
Solution

Convert a binary number, represented as a string (e.g. '101010'), to its decimal equivalent using first principles.

Implement binary to decimal conversion. Given a binary input string, your program should produce a decimal output. The program should handle invalid inputs.

## Note

• Implement the conversion yourself. Do not use something else to perform the conversion for you.

Decimal is a base-10 system.

A number 23 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: `23 => 2*10^1 + 3*10^0 => 2*10 + 3*1 = 23 base 10`

Binary is similar, but uses powers of 2 rather than powers of 10.

So: `101 => 1*2^2 + 0*2^1 + 1*2^0 => 1*4 + 0*2 + 1*1 => 4 + 1 => 5 base 10`.

## Source

All of Computer Science http://www.wolframalpha.com/input/?i=binary&a=*C.binary-_*MathWorld-

## Submitting Incomplete Solutions

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

### runner.mips

``````#
# Test binary_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
#
# binary_convert must:
# - be named binary_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 9
# input values (null terminated) & expected output values (word sized ints)
ins:  .asciiz "0", "1", "10", "11", "100", "1001", "11010", "10001101000", "000011111"
outs: .word     0,   1,   2,     3,    4,       9,      26,          1128,          31

failmsg: .asciiz "failed for test input: "
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     binary_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

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"``````
``````# convert a string representation of a binary number
# to its corresponding integer value
# a0::String -> v0::Signed
binary_convert:
li	\$v0, 0

loop:
lb	\$t0, 0(\$a0)
beq	\$t0, \$0, return
sll	\$v0, \$v0, 1
li	\$t1, 48 # '0'
beq	\$t0, \$t1, loop
li	\$t1, 49 # '1'
bne	\$t0, \$t1, fail
b	loop

fail:
li	\$v0, -1

return:
jr	\$ra``````