## LSU EE 4720 Computer Architecture Spring 2015
#
 ## LIVE Classroom example for review of MIPS:  Convert int to hex string.
#
#   This file contains the state of the code after class on Wednesday, 21
#   January 2015.  The program at this point is complete.


        .data
msg:
        .asciiz "The value of %/s4/5d = 0x%/s4/x is %/a1/s\n"
str:
        .align 4
        .space 12               # Storage for string.
values:                         # Table of test inputs.
        .word 1234              # First entry in table of test inputs.
        .word 0x1234
        .word 0xa9
        .word 0x0
        .word 0xf
        .word 0xf00d
        .word -1

###############################################################################
#
 ##  Test Code
#
#  This code calls the hex to string routine multiple times using
#  data appearing above.
#


        .text
        .globl __start
__start:
        la $s2, values          # Address of table of test inputs.
        addi $s3, $0, -1;       # Value that marks end of table.
MLOOP:
        lw $a0, 0($s2)          # Load a test input from the table.
        la $a1, str             # Set $a1 to address of storage for string.
        lui $t0, 0x2020         # Prepare an initial value for string.
        ori $t0, $t0, 0x2020    # t0 will contain four blanks.
        sw $t0, 0($a1)          # Write string with initial value (blanks).
        sw $t0, 4($a1)
        sw $t0, 8($a1)
        sb $0, 11($a1)
        jal itos
        addi $a2, $0, 10        # Set length of string.
        lw $s4, 0($s2)          # Load test input again.
        la $a1, str             # Address of string.
        la $a0, msg             # Address of message (printf-like format str)
        addi $v0, $0, 11        # Code indicating we want to run printf.
        syscall                 # Make a system call to do printf.
        addi $s2, $s2, 4        # Advance to next entry in table.
        bne $s4, $s3 MLOOP      # Continue if we are not at end of table.
        li $v0, 10              # Exit system call. (Not used if branch taken.)
        syscall
        nop


###############################################################################
#
# Unsigned Integer to Hexadecimal String
#

itos:
        ## Register Usage
        #
        # CALL VALUES:
        #  $a0: Unsigned integer, to convert.
        #  $a1: Address of storage. 
        #       Initialized to null-terminated string of blanks.
        #  $a2: Length of storage.
        #
        # RETURN:
        #       There is no return register value ..
        #       .. instead write storage at $a1 with hex representation of $a0.
        #
        # NOTE: Registers $a0-$a4 and $t0-$t7 can be modified.
        # NOTE: 'a' = 97, 'z' = 122,  'a' - 'A' = 32
        # NOTE: '0' = 48 = 0x30


        add $t6, $a1, $a2
LOOP:
        andi $t3, $a0, 0xf

        slti $t4, $t3, 10
        bne $t4, $0  SOMEWHERE
        addi $t5, $t3, 48
IS_A_LETTER:
        addi $t5, $t3, 87

SOMEWHERE:
        addi $t6, $t6, -1
        sb $t5, 0($t6)
        srl $a0, $a0, 4
        bne $a0, $0 LOOP
        nop

DONE:
        jr $ra
        nop