.text
    #initialize $sp
    lui     $sp,            0x4000
    addi    $sp,            $sp,                    0x7ff

    # load digit to mem
    lui     $s0,            0x4000
    addi    $s0,            $s0,                    0x20
    #0
    addi    $s1,            $zero,                  0x3f
    sw      $s1,            0($s0)
    #1
    addi    $s1,            $zero,                  0x6
    sw      $s1,            4($s0)
    #2
    addi    $s1,            $zero,                  0x5b
    sw      $s1,            8($s0)
    #3
    addi    $s1,            $zero,                  0x4f
    sw      $s1,            12($s0)
    #4
    addi    $s1,            $zero,                  0x66
    sw      $s1,            16($s0)
    #5
    addi    $s1,            $zero,                  0x6d
    sw      $s1,            20($s0)
    #6
    addi    $s1,            $zero,                  0x7d
    sw      $s1,            24($s0)
    #7
    addi    $s1,            $zero,                  0x7
    sw      $s1,            28($s0)
    #8
    addi    $s1,            $zero,                  0x7f
    sw      $s1,            32($s0)
    #9
    addi    $s1,            $zero,                  0x6f
    sw      $s1,            36($s0)
    #A
    addi    $s1,            $zero,                  0x77
    sw      $s1,            40($s0)
    #B
    addi    $s1,            $zero,                  0x7c
    sw      $s1,            44($s0)
    #C
    addi    $s1,            $zero,                  0x58
    sw      $s1,            48($s0)
    #D
    addi    $s1,            $zero,                  0x5e
    sw      $s1,            52($s0)
    #E
    addi    $s1,            $zero,                  0x79
    sw      $s1,            56($s0)
    #F
    addi    $s1,            $zero,                  0x71
    sw      $s1,            60($s0)

    lui     $s1,            0x4000
    addi    $s1,            $s1,                    0x10
    addi    $s0,            $zero,                  0xa80
main_loop:
    addi    $s0,            $s0,                    1
    addi    $a0,            $s0,                    0
    addi    $a1,            $s1,                    0
    jal     display_number
    j       main_loop


    # a0 is the number to be displayed, a1 is the bcd control addr
display_number:
    addi    $sp,            $sp,                    -32
    sw      $ra,            4($sp)
    sw      $s0,            8($sp)
    sw      $s1,            12($sp)
    sw      $s2,            16($sp)
    sw      $s3,            20($sp)
    sw      $s4,            24($sp)
    sw      $s5,            28($sp)
    sw      $s6,            32($sp)

    # s0 is the number to be displayed
    addi    $s0,            $a0,                    0

    # s1 is the bcd_control addr
    addi    $s1,            $a1,                    0

    # s2 is the base addr for display digit
    lui     $s2,            0x4000
    addi    $s2,            $s2,                    0x20

    # s3 is the counter for a complete scan of 4 digit
    addi    $s3,            $zero,                  0xbe2
all_digit_scan_loop:
    # s4 is the num being shifted gradually
    addi    $s4,            $s0,                    0
    # s6 is the enable digit
    addi    $s6,            $zero,                  0x100

    addi    $s5,            $zero,                  4
single_digit_loop:
    # get a digit
    andi    $t0,            $s4,                    0xf
    # digit * 4 is the memory shift
    sll     $t0,            $t0,                    2
    add     $t0,            $s2,                    $t0
    lw      $t0,            0($t0)
    or      $t0,            $t0,                    $s6
    sw      $t0,            0($s1)

    srl     $s4,            $s4,                    4
    sll     $s6,            $s6,                    1
    # jal     wait_for_digit_time

    addi    $t0,            $zero,                  0x400
wait_time_loop:
    addi    $t0,            $t0,                    -1
    bgtz    $t0,            wait_time_loop

    addi    $s5,            $s5,                    -1
    bgtz    $s5,            single_digit_loop
    # end of single digit loop
    
    # ** depends on the clock **
    # four digit, 328.68us. 
    # 1s / 328.68us = 3042 = (be2)hex

    addi    $s3,            $s3,                    -1
    bgtz    $s3,            all_digit_scan_loop

    lw      $ra,            4($sp)
    lw      $s0,            8($sp)
    lw      $s1,            12($sp)
    lw      $s2,            16($sp)
    lw      $s3,            20($sp)
    lw      $s4,            24($sp)
    lw      $s5,            28($sp)
    lw      $s6,            32($sp)
    addi    $sp,            $sp,                    32
    jr      $ra