Converting numbers to hexadecimal ASCII characters in MIPS assembly

Question

I am currently working on an assignment where I need to write an assembly-language subroutine called "hexasc" that converts numbers in the range of 0 through 15 into a printable ASCII-coded character ('0' through '9', or 'A' through 'F', depending on the number). I am using the MARS simulator.

I have attempted to write the subroutine using basic assembly language instructions such as "li" and "move", but I am having trouble getting the desired output. Here is the code I have written so far:

hexasc: li $v0, 48 add $v0, $a0 jr $ra

I would greatly appreciate any help or guidance on how to correctly implement this subroutine. I am particularly struggling with converting the input numbers to their corresponding ASCII characters without using any registers such as s0-s7, gp, sp, fp, and ra.

This is what I have right now:

`# hexmain.asm

.text

main: li $a0,0 # change this to test different values

jal hexasc      # call hexasc
nop         # delay slot filler (just in case)  

move    $a0,$v0     # copy return value to argument register

li  $v0,11      # syscall with v0 = 11 will print out
syscall         # one byte from a0 to the Run I/O window

stop: j stop # stop after one run nop # delay slot filler (just in case)

hexasc: # You can write your own code for hexasc here`

Answer 1

Let's think about this for a moment.

Number    Ascii (hex)
0            30
1            31
2            32
3            33
4            34
5            35
6            36
7            37
8            38
9            39
A            41
B            42
C            43
D            44
E            45
F            46

So we see a simple pattern: If our 4-bit number is greater than 0x39, add 0x37 to the digit. Otherwise, add 0x30. (You may need to do andi $a0,$a0,0x0F to chop off everything beyond the lowest 4 bits.)

EDIT: I didn't really do a good job of explaining why we add 0x37 to the number if it's greater than 9.

0x0A + 0x37 = 0x41 (ascii code of 'A')
0x0B + 0x37 = 0x42 (ascii code of 'B')
etc.