Example of 4x4 loop:
li $t1, 0 # y
li $t2, 0 # x
loop:
beq $t1, 4, loop_end
loop_x:
# BODY
# END
loop_end_x:
addi $t2, $t2, 1
blt $t2, 4 loop_x
loop_x_end:
li $t2, 0
addi $t1, $t1, 1
j loop
loop_end:
li $t1, 0