## Q5.ASM
## AUTHOR: BRAD LINDSAY
## STUDENT ID: 001199212
## ABOUT: Application finds max of 10 values
##        Using the MAX3 function


## Variable Mapping:
#   $t7, base address for T20Numbers
#   $t6, max index
#   $t5, current index
#   $t4, unused
#   $t3, unused
#   $t2, Value of Max loaded from address.
#   $t1, Address to Max
#   $t0, address of word to extract

.data

T20Numbers:     .word  12,20,-9,13,15,18,-8,10,20,-14 # The 10 numbers
Max:        .space 4

.macro getnum(%ax)
sll $t0, $t5, 2     # multiply our index by 4 to get word offset
add $t0, $t7, $t0   # add to get word address.
lw %ax, 0($t0)      # load word into ax (a0,a1,a2)
.end_macro

.text

la $t7, T20Numbers  # get address of first element of T20Numbers
li $t6, 9       # maximum address for our word list.
li $t5, 0       # track which element we're starting on.

## MAIN PROGRAM LOOP ##############################################################################
getmax:         getnum($a0)     # populate $a0 with number from T20Numbers.
            addi $t5, $t5, 1    # add 1 to our index.
            getnum($a1)     # populate $a1 with number from T20Numbers.
            addi $t5, $t5, 1    # add 1 to our index.
            getnum($a2)     # populate $a2 with number from T20Numbers.

            jal max3            # check max between these three numbers
            jal checkmax        # set the max value if need be.

            beq $t5, $t6, exit  # exit procedure if finished.

                        # note: this is dumb, but it works.
            addi $t5, $t5, -1   # otherwise decrement by 1 and repeat
                        # end note

            j getmax        # jump to label for repeat
## END MAIN PROGRAM LOOP ##########################################################################

## CHECK MAX SUBROUTINE ###########################################################################
checkmax:       la $t1, Max         # load address of max into $t1
            lw $t2, 0($t1)          # load content of max into $t2
            bge $v0, $t2, setmax        # check if new max > old max if so set.
            jr $ra              # otherwise just return
setmax:         sw $v0, 0($t1)          # store $v0 as new max
            jr $ra
## END CHECK MAX SUBROUTINE #######################################################################

## MAX SUB ROUTINE ################################################################################
max3:           add $v0, $a0, $zero     # copy $a0 to $v0
            sub $t0, $a1, $v0       # if $a1 < $v0 then $t0 is negative
            bltz    $t0, good       # if $t0 is < 0 nothing changes
            add     $v0, $a1, $zero     # otherwise $a1 is highest
good:           sub $t0, $a2, $v0       # if $a2 < $v0 then $t0 is negative
            bltz    $t0, done       # if $t0 is < 0 nothing changes
            add $v0, $a2, $zero     # $a2 becomes highest
done:           jr $ra              # return to caller
## END MAX SUB ROUTINE ############################################################################

## EXIT HANDLING ##################################################################################
exit:           lw $a0, 0($t1)          # load the max value into $a0
            addi $v0, $zero, 1      # request system call function 1 output
            syscall             # issue systemcall.
## END EXIT HANDLING ##############################################################################