# Driver program for Hello World!
# Ima X Traterrestrial
# ixterres
#
# This program runs the Hello World subroutine.
# It also demonstrates/tests the preservation of registers 2, 3 & 4
# by outputting their contents to the terminal before and after
# the call to Hello.  Lastly, note that a subroutine (printReg) is
# used for printing register contents, but that it fails to preserve
# register 1 (this is not good practice).  Also note that this subroutine
# is declared AFTER the end of program- this is not necessary, but it helps
# keep this modular.
# 
# Registers:
#   $1 -- a constant value (-1) to demonstrate preservation
#   $2 -- a constant value (-2) to demonstrate preservation
#   $3 -- a constant value (-3) to demonstrate preservation
#   $4 -- the values to be output

.globl main

# initialize $2 and $3, and print out their values
main:   addi    $1, $0, -1      # initialize $1
        addi    $2, $0, -2      # initialize $2
        addi    $3, $0, -3      # initialize $3

# Print out the contents of registers 2-4 as integers on their own lines
        add     $4, $0, $1      # read $1 into $4
        jal     printReg        # print out register

        add     $4, $0, $2      # read $2 into $1
        jal     printReg        # print out register

        add     $4, $0, $3      # read $3 into $1
        jal     printReg        # print out register

# Actually call Hello World
        jal     Hello           # call the HelloWorld subroutine

# Print out the contents of registers 2-4 as integers on their own lines
        add     $4, $0, $1      # read $1 into $4
        jal     printReg        # print out register

        add     $4, $0, $2      # read $2 into $1
        jal     printReg        # print out register

        add     $4, $0, $3      # read $3 into $1
        jal     printReg        # print out register

# Quit running the program
        trap    10              # end program


# Prints out the contents of register 4 on its own line
# Note that printReg overwrites the contents of $4 with the value 10
# 	- this is an example of poor register preservation
printReg:	
        trap    1               # output $4 as an integer
        addi    $4, $0, 10      # put ascii value for newline in r$
        trap    101             # output newline
        jr      $31             # return to caller