CODEWITHSUNDEEP
assemblyx86nasmintel
Diego Hernández
Diego Hernández

Reputation: 792

How to declare an array with size indicated by the user in nasm?

wanted to make an example with nasm using the x86 architecture, that can create an array with size "n", where "n" will be the number that the user wants to have the size of the array at run time

extern _printf
extern _scanf
extern _scanf
global _main

section .data
    array:  10,5,4
    msg:    db  "enter the size of the array: ",10,0
    size:   db 10
    format: db "%i",0
section .text
    _main:
          push msg
          call _printf
          add esp, 4

          push size
          push format
          call _scanf
          add esp, 8

    ret

Upvotes: 1

Views: 1701

Answers (2)

vitsoft
vitsoft

Reputation: 5775

If you'll satisfy with an example for Windows, see the following program in EuroAssembler. It creates 3128 bytes long AllocArray.exe which will reserve and initialize the requested size in BSS section.

AllocArray PROGRAM Format=PE, IconFile=, Entry=Main:
           INCLUDE winapi.htm, cpuext32.htm
%MaxPossibleLength %SETA 1_000_000 ; Specify the maximal acceptable allocation here.
[.text]
Main: StdOutput ="Enter the size of the array (1..%MaxPossibleLength): "
      StdInput EnterredLength    ; Let the user to set array length.
      LodD EnterredLength        ; Convert the enterred decimal number to integer in EAX.
      CMP EAX,%MaxPossibleLength
      JA Main:
      MOV [LengthOfTheArray],EAX
      StoD AllocatedLength       ; Convert the integer EAX to decimal.
      XOR EAX,EAX
      STOSB                      ; Zero-terminate the decimal number.
      MOV EDI,TheArray:
      MOV ECX,[LengthOfTheArray]
      MOV EAX,0x5A5A5A5A
      REP STOSD                  ; Initialize the array with 5A.
      StdOutput ="The array of ", AllocatedLength, =" DWORDs is now allocated statically."
      TerminateProgram
[.bss]
EnterredLength:   DB 16 * BYTE  
AllocatedLength:  DB 16 * BYTE  
LengthOfTheArray: DD DWORD
TheArray:         DD %MaxPossibleLength * DWORD
      ENDPROGRAM AllocArray

Upvotes: 0

Peter Cordes
Peter Cordes

Reputation: 363980

Do you mean like resd n in the BSS, and the user can build the program with nasm -felf -Dn=1024 to set the NASM macro as a constant? You could provide a default with %ifdef.

If you want a runtime-variable array size, it obviously can't be in static storage (unless you massively over-allocate and only use whatever part of the array is needed. That's fine on systems that do lazy allocation for the BSS.)

Upvotes: 1

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