DZone Snippets: assembler code

Easter date - NASM

scvalex (Alexandru Scvortov) — Thu, 10 May 2007 08:17:06 GMT

Compute Easter date in NASM.



segment .text

  global easter_asm



;typedef struct {

;  unsigned short d;

;  unsigned short m;

;} Date;



;void easter_asm(unsigned short Y, Date *d)

;   computes the Easter date for the year Y and stores it in d

;

;Parameters

;   Y - the year

;   d - the Date struct in which to store the result



%define d [ebp + 12]

%define Y [ebp + 8]

easter_asm:

  enter 0, 0

  sub esp, 28

  %define G [ebp - 4]

  %define C [ebp - 8]

  %define X [ebp - 12]

  %define Z [ebp - 16]

  %define D [ebp - 20]

  %define E [ebp - 24]

  %define N [ebp - 28]



  ; int G = (Y % 19) + 1;

  xor edx, edx

  mov eax, Y

  mov ebx, 19

  div ebx

  inc edx

  mov G, edx



  ; int C = (int)(Y / 100) + 1;

  xor edx, edx

  mov eax, Y

  mov ebx, 100

  div ebx

  inc eax

  mov C, eax



  ; int X = 3 * C / 4 - 12;

  xor edx, edx

  mov eax, C

  mov ebx, 4

  div ebx

  mov ebx, 3

  mul ebx

  sub eax, 12

  mov X, eax



  ; int Z = (8 * C + 5) / 25 - 5;

  xor edx, edx

  mov eax, C

  mov ebx, 8

  mul ebx

  add eax, 5

  mov ebx, 25

  div ebx

  sub eax, 5

  mov Z, eax



  ; int D = 5 * Y / 4 - X - 10;

  xor edx, edx

  mov eax, Y

  mov ebx, 5

  mul ebx,

  mov ebx, 4

  div ebx

  sub eax, X

  sub eax, 10

  mov D, eax



  ; int E = (11 * G + 20 + Z - X) % 30;

  mov eax, G

  mov ebx, 11

  mul ebx

  add eax, 20

  add eax, Z

  sub eax, X

  xor edx, edx

  mov ebx, 30

  div ebx

  mov E, edx



  cmp dword E, 24

  jne after_e1

  inc dword E

  jmp after_e2

after_e1:

  cmp dword E, 25

  jne after_e2

  cmp dword G, 11

  jle after_e2

  inc dword E

after_e2:



  ; int N = 44 - E;

  mov eax, 44

  sub eax, E

  mov N, eax



  cmp dword N, 21

  jge after_n

  add dword N, 30

after_n:

  ; N = N + 7 - ((D + N) % 7);

  mov eax, N

  add eax, D

  xor edx, edx

  mov ebx, 7

  div ebx

  add dword N, 7

  sub N, edx



  mov eax, d



  ; N is the nuber of day from March 1 to easter. Check if easter is in April.

  cmp dword N, 31

  jle in_march

  mov bl, N

  sub bl, 31

  mov [eax], bl

  mov word [eax + 2], 4

  jmp quit

in_march:

  mov bl, N

  mov [eax], bl

  mov word [eax + 2], 3

quit:

  leave

  ret

Assembler - Insert sort

scvalex (Alexandru Scvortov) — Tue, 08 May 2007 19:32:03 GMT

This is a straightforward implementation of the insert sort algorithm in assembler (NASM).

Compile and link it with:
nasm -f elf isort.s
ld -s -o isort isort.o

Insert sort: http://en.wikipedia.org/wiki/Insert_sort



; void isort(int *a, int n)

;   sorts the first n elements of a

;

; Parameters

;   a - pointer to the array

;   n - number of elements to sorts



%define a [ebp + 8]

%define n [ebp + 12]

isort:

  enter 0, 0

  pusha



  mov ecx, 1

  for:

    mov ebx, ecx

    imul ebx, 4

    add ebx, a

    mov ebx, [ebx]



    mov edx, ecx

    dec edx



    while:

      cmp edx, 0

      jl while_quit



      mov eax, edx

      imul eax, 4

      add eax, a



      cmp ebx, [eax]

      jge while_quit



      mov esi, [eax]



      mov dword [eax + 4], esi



      dec edx

      jmp while

    while_quit:



    mov [eax], ebx



    inc ecx

    cmp ecx, n

    jl for



  popa

  leave

  ret

Converting diskette LBA to CHS

darktemplar (Piotr) — Tue, 12 Dec 2006 02:05:46 GMT

// description of your code here

Put LBA address in AX and get your results in CL, CH and DH.



;-----------------------------------------------------------------------------;

;		DISKETTE_LBA_TO_CHS(AX LBA) CL Head, CH Cylinder, DH Sector   ;

;-----------------------------------------------------------------------------;

diskette_lba_to_chs:

	xor dx, dx

	mov bx, 18

        div bx

	push dx				; = Stack(1)			

	mov dx, ax

        shr ax, 2

	mov cl, al

	mov ch, dl

	pop dx				; = Stack(0)

	inc dl

	mov dh, dl

	ret

[ASM]Write String only using BIOS

darktemplar (Piotr) — Tue, 12 Dec 2006 02:02:47 GMT

First, set starting address of text into DS:SI registers. Procedure writes text until null-character is met.



;-----------------------------------------------------------------------------;

;		WRITE_STRING(DS:SI Text)                                      ;

;-----------------------------------------------------------------------------;

;	Writes string from DS:SI until character #0 is met		      ;

Write_String:		

	mov ah, 0xE	

	xor bh, bh	

	mov bl, 0x7

.nextchar	

	lodsb		

	or al,al

	jz .return

	int 10h	

	jmp .nextchar

.return		

	ret