assembly code to find the max number of consecutive ones

Question

How can I find the max number of consecutive ones in a byte when I took a character from user and determine the number of max consecutive ones in the byte. I tried this code but it is not working, it is print all ones in the number not the consecutive ones.

.model small
.stack  100
.data
    message db "Enter one charachter $" 
    NewLine DB 0DH,0AH, "$"
    message db "Enter one charachter $" 
    NewLine DB 0DH,0AH, "$"
    message2 db "The maximum number of consecutive ones is $" 

.code
    mov ax, @data
    mov ds,ax
    push ax
    mov ah,09 
    lea dx,message 
    int 21h 
    pop ax
    MOV AH,09 
    MOV DX,OFFSET NewLine 
    INT 21H 
    mov ah,1
    int 21h
    mov si,0
    mov di,8
    l1:
        shl al,1
        jnc no_inc_count
            inc si
        no_inc_count:
        dec di
    jnz l1
    MOV AH,09h
    MOV DX,OFFSET NewLine 
    INT 21H 
    push ax
    mov ah,09 
    lea dx,message2 
    int 21h 
    pop ax
    MOV AH,09 
    MOV DX,OFFSET NewLine
    INT 21H 
    mov ah,2
    add si,30h
    mov dx,si
    int 21h
    mov ah,4ch 
    int 21h 
end

Answer 1

his block counts all ones:

mov si,0
mov di,8
l1:
shl al,1
jnc no_inc_count
inc si
no_inc_count:
dec di
jnz l1

SI is the "one"-counter which counts the ones.
DI is the "loop"-counter which counts (down) the shifts.

If want to store the maximum number of ones you need to add a variable "max" and two pieces of code:

• Code which resets the one-counter if the fetched bit is null.
  
• Code which compares the maximum with the one-counter and replace it eventually.

I hope the comments in the following code make it little bit clearer:

.MODEL small
.STACK                          ;  Default 1024

.DATA
message     db "Enter one character $"
message2    db "The maximum number of consecutive ones is $"
NewLine     db 0DH,0AH, "$"
max         dw 0            ; Holds the maximum number of consecutive ones

.CODE
start:                      ; Entry point needed for END
    mov ax, @data           ; Let DS point to .DATA
    mov ds,ax

    mov ah,09h              ; http://www.ctyme.com/intr/rb-2562.htm
    lea dx,message
    int 21h                 ; Call MS-DOS

    mov ah, 01h             ; http://www.ctyme.com/intr/rb-2552.htm
    int 21h                 ; Call MS-DOS

    mov si, 0               ; One-counter
    mov di, 8               ; Loop-counter: Shift 8 bits

    FOR:                    ; Loop 8 times
    shl al, 1               ; shift one bit into carry flag (CF)
    jnc IS_NULL             ; jump if no "one"

    IS_ONE:                 ; Code to increment one-counter and replace max
    inc si                  ; one-counter++
    cmp si, max             ; si > max ?
    jbe LOWER               ; No - comparation has set CF or ZF
        GREATER:            ; Yes, si > max (comparation has cleared CF and ZF)
        mov max, si         ; Replace max by SI
        LOWER:
    jmp CONTFOR             ; Decrement loop-counter and loop

    IS_NULL:
        mov si, 0           ; Reset one-counter

    CONTFOR:                ; Decrement loop-counter and continue looping
    dec di
    jnz FOR

    mov ah, 09h             ; http://www.ctyme.com/intr/rb-2562.htm
    lea dx, NewLine
    int 21h                 ; Call MS-DOS

    mov ah, 09h             ; http://www.ctyme.com/intr/rb-2562.htm
    lea dx, message2
    int 21h                 ; Call MS-DOS

    add max, 30h            ; To ASCII
    mov ah, 2               ; http://www.ctyme.com/intr/rb-2554.htm
    mov dx, max
    int 21h                 ; Call MS-DOS

    mov ah, 09h             ; http://www.ctyme.com/intr/rb-2562.htm
    lea dx, NewLine
    int 21h                 ; Call MS-DOS

    mov ax,4C00h            ; AH=4Ch (exit), exitcode AL=0 (ok)
    int 21h                 ; Call MS-DOS

END start                   ; END < entry point >

Notes:

• The program needs an entry point (label) which doesn't need to be at the beginning of the code. That entry point is the argument of the END statement.
• Don't use PUSH and POP when you don't know exactly what you're doing. Use variables in the .DATA segment instead.
• It doesn't matter, if you use capital letters (MOV AH,09h) or small letters (mov ah,09) characters, but you should keep to the method of your choice.
• Find a style for formatting your code e.g. with indenting and new lines, so that you can still understand it after a few days. Comments serve the same purpose.