Convert a hexadecimal number, represented as a string (e.g. "10af8c"), to its decimal equivalent using first principles (i.e. no, you may not use built-in or external libraries to accomplish the conversion).
On the web we use hexadecimal to represent colors, e.g. green: 008000, teal: 008080, navy: 000080).
The program should handle invalid hexadecimal strings.
All of Computer Science http://www.wolframalpha.com/examples/NumberBases.html
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
# # Test hex_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 # # hex_convert must: # - be named hex_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 9 # input values (null terminated) & expected output values (word sized ints) ins: .asciiz "1", "C", "10", "af", "100", "19ACE", "000000", "ffff00", "00fff0" outs: .word 1, 12, 16, 175, 256, 105166, 0, 16776960, 65520 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 hex_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 a hexadecimal number and # return its integer equivalent # a0::String -> v0::Signed hex_convert: li $v0, 0 loop: lb $t0, 0($a0) beq $t0, $0, return addi $a0, $a0, 1 sll $v0, $v0, 4 addi $t1, $t0, -57 bgtz $t1, alpha # alpha if above '9' addi $t0, $t0, -48 # if not alpha, shunt '0' down to 0 val alpha_done: bltz $t0, fail # either < '0' or an invalid in-between range add $v0, $v0, $t0 b loop # reduce ascii vals in A-F or a-f to contiguous num vals alpha: addi $v0, $v0, 10 # add offset of being above '9' directly addi $t1, $t0, -70 bgtz $t1, alpha_lcase # lcase if above 'F' alpha_continue: addi $t0, $t0, -65 # shunt 'A' down to 0 val addi $t1, $t0, -5 bgtz $t1, fail # > 'f' b alpha_done alpha_lcase: addi $t0, $t0, -32 # shunt 'a' down to 'A' b alpha_continue 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.