I am writing a lot to help clarify what I need, but my actual question is very s

Question

I am writing a lot to help clarify what I need, but my actual question is very simple.

I need a portion of NASM assembly code please (using I/O asm_io.asm routines). Nothing too complicated, just very simple, beginner code (like the Hello World stuff used in someone's first course). I already have my assembly code, makefile, cdecl.h, driver.c etc etc. All I am missing is the few lines of code to modify it that will do what is specifically described below.

For example: make argv, then argv goodbye banana.

The loop would display the 2nd line argument (goodbye), go through the 3rd line argument (banana) one letter at a time counting the number of letters. If the number of letters in the 2nd string is greater than 4, then an error message would show. ie: argv goodbye banana would display:

goodbye

banana

more than 4

If the number of letters is equal to or less than 4, argv goodbye banana would display

goodbye

bana

Again, I do not need anything complicated, just a few basic lines using simple language to perform these loops using asm_io.asm routines (ie: mov ebx, dword [ebp+12] or call print_nl etc etc, this is the type of code I am missing in my .asm). I am new to this and appreciate help.

Can you please give a brief outline of each line so I understand and can write my own final code, thanks. (ie: mov [str1+1] ; means this and that will happen) Thank you!

Explanation / Answer

I hope, the below assembly code does what you are looking for.

First let me write some functions for doing basic tasks that we are going to perform in a separate file called func.asm

Func.asm
;------------------------------------------
; int slen(String input)
; String length calculation function
slen:
    push    ebx      ; move the address of input string into EBX
    mov     ebx, eax ; move the address in EBX into EAX as well (Both now point to the same segment in memory)

nextchar:
   cmp     byte [eax], 0 ; compare the byte pointed to by EAX at this address against zero (Zero is an end of string delimiter)
         jz      finished       ; jump (if the zero flagged has been set) to the point in the code labeled 'finished'
       inc     eax            ; increment the address in EAX by one byte (if the zero flagged has NOT been set)
       jmp     nextchar       ; jump to the point in the code labeled 'nextchar'

finished:
    sub     eax, ebx       ; subtract the address in EBX from the address in EAX
                     ; remember both registers started pointing to the same address (see line 15)
                     ; but EAX has been incremented one byte for each character in the message string
                     ; when you subtract one memory address from another of the same type
                     ; the result is number of segments between them - in this case the number of bytes
   
    pop     ebx
    ret


;------------------------------------------
; void sprint(String input)
; String printing function
sprint:
    push    edx   ; Save all current register values to the stack
    push    ecx
    push    ebx
    push    eax

    call    slen ; calculate the length of the string

    mov     edx, eax ; move number of bytes to write – length of input string
    pop     eax   ; fetch the input string

    mov     ecx, eax ; move the memory address of input string into ecx
    mov     ebx, 1   ; write to STDOUT file
    mov     eax, 4   ; invoke SYS_WRITE (kernel opcode 4)
    int     80h

    pop     ebx
    pop     ecx
    pop     edx
    ret

;------------------------------------------
; void exit()
; Exit program and restore resources
quit:
    mov     ebx, 0
    mov     eax, 1
    int     80h
    ret

This is the main file with the assembly instructions.
Include the file func.asm with the basic function definitions

Main.asm
;---------------------------------------------------------
%include        'func.asm'

SECTION .data
errmsg    db      'more than 4', 0Ah, 0h

SECTION .text
global _start

_start:
    pop    ecx              ; first value on the stack is the number of arguments
    pop eax       ; next value on the stack will be the name of the program

    pop eax       ; This will be the first argument
    call sprint    ; This will print the string on the screen
    pop eax       ; fetch the second argument from the stack
    call sprint

    call slen      ; call the string length calculation function
    mov edx, eax ; store the value i.e. length in ecx
    cmp edx, 4   ; compare the length with value 4
    jle end     ; jump to end if less than or equal to 4
    mov eax, errmsg ; if string length is not less than 4, display the errmsg
    call sprint

end:
    call    quit

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