This is the code I have tried and it only works with 3 digits numbers but the sum does not exceed 1000 for adding 3 digit numbers that exceeds 1000 just outputs 1000. I also have a problem fixing adding 1 and 2 digit numbers as inputting 1 and 2 digits numbers results in invalid character. I am confused and been working with this code for 6 hours now and still haven’t fixed it.

sum segment
    assume cs:sum, ds:sum, ss:sum
    org 0100h

program: 
    jmp begin
    var1 db 0Dh, 0Ah, 'ADD NUMBERS PLEASE','$' 
    var2 db 0Dh, 0Ah, 'Add 2 Numbers.','$' 
    var3 db 0Dh, 0Ah, 'Input any number (0-999):', '$' 
    var4 db 0Dh, 0Ah, 'Input any number (0-999):', '$' 
    var5 db 0Dh, 0Ah, 'Sum:','$'

    num11 db 0
    num12 db 0
    num13 db 0
    num21 db 0
    num22 db 0
    num23 db 0
    sum1 db 0
    sum2 db 0
    sum3 db 0
    sum4 db 0

    var6 db 0Dh, 0Ah, 'Invalid Character','$'
    var7 db 0Dh, 0Ah, 'Do you want to repeat (y/n):','$'

    showstr proc
        mov ah, 09h
        int 21h
        ret
    showstr endp

    clearscr proc 
        push bx 
        mov al, 00h
        mov bh, 0Fh
        mov ah, 06h
        int 10h
        pop bx 
        ret
    clearscr endp

checknum1 proc 
    cmp al, '0'
    jb invalid_char1
    cmp al, '9'
    ja invalid_char1 
    ret
valid_char1: 
    ret
invalid_char1:
    mov dx, offset var6 
    call showstr 
    jmp num1
checknum1 endp

checknum2 proc 
    cmp al, '0'
    jb invalid_char1
    cmp al, '0'
    ja invalid_char1
    ret
valid_char2: 
    ret
invalid_char2:
    mov dx, offset var6 
    call showstr 
    jmp num2
checknum2 endp

    add_digit1 proc 
        sub bh, '0'
        sub bl, '0'
        add bh, bl
        cmp bh, 9
        ja carry1 
        jmp nocarry1
    carry1: 
        sub bh, 10
        inc sum2
        ret
    nocarry1:
        add sum1, bh
        ret
    add_digit1 endp

    add_digit2 proc 
        sub bh, '0'
        sub bl, '0'
        add bh, bl
        cmp bh, 9
        ja carry2
        jmp nocarry2 
    carry2:
        sub bh, 10
        inc sum3
        ret
    nocarry2:
        add sum2, bh
        ret
    add_digit2 endp

    add_digit3 proc 
        sub bh, '0'
        sub bl, '0'
        add bh, bl
        cmp bh, 9
        ja carry3 
        jmp nocarry3 
    carry3:
        sub bh, 10
        inc sum4
        ret
    nocarry3:
        add sum3, bh 
        ret
    add_digit3 endp

    repeatprog proc 
        cmp al, 'y'
        je begin 
        jmp exit
    repeatprog endp

begin:
    mov ax, sum
    mov ds, ax
    mov sum1, 0
    mov sum2, 0
    mov sum3, 0
    mov sum4, 0
    call clearscr
    mov dx, offset var1 
    call showstr

num1:
    lea dx, var3 
    call showstr
    mov ah, 01h 
    int 21h
    call checknum1
    mov num13, al
    mov ah, 01h 
    int 21h
    call checknum1
    mov num12, al
    mov ah, 01h 
    int 21h
    call checknum1
    mov num11, al

num2:
    lea dx, var4 
    call showstr
    mov ah, 01h
    int 21h
    call checknum2
    mov num23, al
    mov ah, 01h
    int 21h
    call checknum2
    mov num22, al
    mov ah, 01h
    int 21h
    call checknum2
    mov num21, al
    mov bl, num11 
    mov bh, num21 
    call add_digit1
    mov bl, num12 
    mov bh, num22 
    call add_digit2
    mov bl, num13 
    mov bh, num23 
    call add_digit3
    lea dx, var5
    call showstr
    mov dl, sum4
    add dl, '0' 
    mov ah, 02h 
    int 21h

    mov dl, sum3 
    add dl, '0'
    mov ah, 02h 
    int 21h

    mov dl, sum2
    add dl, '0'
    mov ah, 02h 
    int 21h

    mov dl, sum1
    add dl, '0'
    mov ah, 02h 
    int 21h

    lea dx, var7 
    call showstr 
    mov ah, 01h 
    int 21h
    call repeatprog

exit:
    mov ah, 4ch
    int 21h

sum ends 
end program

I have tried this fix but it still won’t work:

checknum1 proc 
    cmp al, '0'
    jb invalid_char1
    cmp al, '9'
    ja invalid_char1 
    ret
valid_char1: 
    ret
invalid_char1:
    mov dx, offset var6 
    call showstr 
    jmp num1
checknum1 endp

checknum2 proc 
    cmp al, '0'
    jb invalid_char2
    cmp al, '9'
    ja invalid_char2 
    ret
valid_char2: 
    ret
invalid_char2:
    mov dx, offset var6 
    call showstr 
    jmp num2
checknum2 endp