For our coursework we had to write code in MARS to calculate the memory address of a given coordinate.
.globl main
.data
amountOfRows: .word 16 # The mount of rows of pixels
amountOfColumns: .word 32 # The mount of columns of pixels
promptRows: .asciiz "Please enter the row number:\n"
promptCols: .asciiz "Please enter the column number:\n"
msgShowMemoryAddress: .asciiz "The memory address for the pixel is:\n"
msgErrorValues: .asciiz "The entered value for row/column is invalid, please enter a valid value:\n"
.text
###################################################################################
translate_coordinates:
sw $fp, 0($sp) # push old frame pointer (dynamic link)
move $fp, $sp # frame pointer now points to the top of the stack
subu $sp, $sp, 20 # allocate 16 bytes on the stack
sw $ra, -4($fp) # store the value of the return address
sw $s0, -8($fp) # save locally used registers
sw $s1, -12($fp)
sw $s2, -16($fp)
move $s0, $a0 # $s0 = x coordinate
move $s1, $a1 # $s1 = y coordinate
move $s2, $a2 # $s2 = width for calculations
sll $t3, $s1, 5 # shift y left by 5 which is equal to multiplication of 32
add $t4, $t3, $s0 # (y * 32) + x = index of pixel
sll $t5, $t4, 2 # shift index left by 2 which is equal to multiplication of 4
add $t6, $gp, $t5 # memory address = gp + offset
move $v0, $t6 # place result in return value location
lw $s2, -16($fp) # reset saved register $s2
lw $s1, -12($fp) # reset saved register $s1
lw $s0, -8($fp) # reset saved register $s0
lw $ra, -4($fp) # get return address from frame
move $sp, $fp # get old frame pointer from current fra
lw $fp, ($sp) # restore old frame pointer
jr $ra
###################################################################################
main:
li $v0, 4 # print string
la $a0, promptRows # message to ask the user for the row number
syscall
li $v0, 5 # read integer
syscall # ask the user for a row number
move $t0, $v0
li $v0, 4 # print string
la $a0, promptCols # message to ask the user for the column number
syscall
li $v0, 5 # read integer
syscall # ask the user for a column number
move $t1, $v0
lw $t2, amountOfColumns #load amountOfColumns into $t2
move $a0, $t0 # Put procedure arguments
move $a1, $t1 # Put procedure arguments
move $a2, $t2
jal translate_coordinates # Call procedure
move $t6, $v0 # Get procedure result
move $a0, $t6
li $v0, 1 # syscall code 1 is for print_int
syscall
exit:
li $v0, 10 # syscall to end the program
syscall
I wrote this code to calculate the memory address of a pixel but whenever I run it I get this error:
Error in : invalid program counter value: 0x00000000
We had to use a stackframe for this and I'm quite new to that.
Because I'm quite new to it, it might be that I made some mistakes when it comes to that.
By default, MARS starts running the code at the start of .text, and since in your code the function comes before main, it is running the function without calling it properly.
There are three options:
j main immediately after .textmain if defined (off by default)main comes immediately after .text