# Hello World!
# Ima X Traterrestrial
# ixterres
# 
# Implement everyone's favourite first program:
# Output the string "Hello World!" to standard output

# Registers
#   $1 -- number of characters left to print
#   $2 -- the address of the Size "variable"
#   $3 -- address of the next character in the array
#   $4 -- the ascii value of the charac ter to output

.globl Hello

Hello:
# Preserve registers on the stack
        addi    $30, $30, -20   # make room on the stack
        sw      $1, 0($30)      # save $1
        sw      $2, 4($30)      # save $2
        sw      $3, 8($30)      # save $3
        sw      $4, 12($30)     # save $4
        sw      $31, 16($30)    # save $31

# initialize the registers to read the Size and Text variables
        add     $4, $0, $0      # initialize $4
        llo     $2, Size        # get size of the string
        lhi     $2, Size
        llo     $3, Text        # get starting address
        lhi     $3, Text

        lw      $1, 0($2)       # load the Size value

loop:   beq     $1, $0, done    # if string is empty, exit
        addi    $1, $1, -1      # decrement character count
        lw      $4, 0($3)       # load array value 
        trap    101             # output the character
        addi    $3, $3, 4       # increment the string pointer
        j       loop            # do it again

# restore registers and stack, then return to calling procedure
done:   lw      $1, 0($30)      # restore register 1
        lw      $2, 4($30)      # restore register 2
        lw      $3, 8($30)      # restore register 3
        lw      $4, 12($30)     # restore register 4
        lw      $31, 16($30)    # restore register 31
        addi    $30, $30, 20    # free space on stack

        jr      $31             # return to calling procedure

# Values of variables in memory

Size:   .word   13      # the integer variable for the size of Text

# the Text string/character array:
Text:   .word   72      # H
        .word   101     # e
        .word   108     # l
        .word   108     # l
        .word   111     # o
        .word   32      # <space> 
        .word   87      # W
        .word   111     # o
        .word   114     # r
        .word   108     # l
        .word   100     # d
        .word   33      # !
        .word   10      # \n

        .align  4       # just for safety