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.
Decimal is a base-10 system.
A number 233 in base 10 notation can be understood as a linear combination of powers of 10:
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.
233 # octal = 2*8^2 + 3*8^1 + 3*8^0 = 2*64 + 3*8 + 3*1 = 128 + 24 + 3 = 155
All of Computer Science http://www.wolframalpha.com/input/?i=base+8
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
# # 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 # - read input address of string from a0 # - 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: addi $s1, $s1, 1 # move input address on byte forward 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"
# read a string representation of an octal number and # return its integer equivalent # a0:String -> v0:Signed octal_convert: li $v0, 0 loop: lb $t0, 0($a0) beq $t0, $0, return addi $a0, $a0, 1 sll $v0, $v0, 3 addi $t0, $t0, -48 bltz $t0, fail # below the '0' - '7' range addi $t1, $t0, -8 bgez $t1, fail # above the '0' - '7' range add $v0, $v0, $t0 b loop fail: li $v0, -1 return: jr $ra
A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.