matrix_diagonal.s

.globl main

.data
N: .word 6
M: .word 6
line0: .word 1, 2, 3, 4, 5, 6
line1: .word 1, 2, 3, 4, 5, 6
line2: .word 1, 2, 3, 4, 5, 6
line3: .word 6, 5, 4, 3, 2, 1
line4: .word 6, 5, 4, 3, 2, 1
line5: .word 6, 5, 4, 3, 2, 1
matrix2d: .word                 # NxM matrix
    line0, line1, line2,
    line3, line4, line5

.text
main:
    la $s0, matrix2d            # s0 = matrix2d
    lw $s1, N($zero)            # s1 = N[0]
    lw $s4, M($zero)            # s4 = M[0]
    jal lines_loop

lines_loop:
    beq $t0, $s1, exit          # exit if t0 == N
    lw $s2, matrix2d($t1)       # s2 = matrix2d[t1]
    addi $t1, $t1, 4            # t1 += 4
    addi $t0, $t0, 1            # t0 += 1
    move $t2, $zero             # t2 = 0
    move $t3, $zero             # t3 = 0
    jal columns_loop

columns_loop:
    beq $t2, $s4, lines_loop    # go to next line if t2 == M
    add $t4, $s2, $t3           # t4 = s2 + 4
    lw $s3, ($t4)               # s3 = *(s2 + 4)
    addi $t3, $t3, 4            # t3 += 4
    addi $t2, $t2, 1            # t2 += 1
    beq $t0, $t2, add_counter   # update counter if the indexes are equal
    jal columns_loop

add_counter:
    add $s5, $s5, $s3           # s5 += s3 (current number)
    jal columns_loop

exit:
    move $a0, $s5               # a0 = s5
    li $v0, 1                   # print int
    syscall
    li $v0, 10                  # exit
    syscall