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

Easter date - C

scvalex (Alexandru Scvortov) — Wed, 09 May 2007 09:36:22 GMT

Easter_c computes the easter date for year Y. It uses the standard (as used by most western churches) algorithm developed by the Napolitan astronomer Aloysius Lilius and the Jesuit mathematician Christopher Clavius. It works for any year after 1582.



typedef struct {

  unsigned char d;

  unsigned char m;

} Date;



void easter_c(unsigned short Y, Date *d) {

  int G = (Y % 19) + 1;

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

  int X = 3 * C / 4 - 12;

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

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

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

  if (((E == 25) && (G > 11)) || (E == 24))

    ++E;

  int N = 44 - E;

  if (N < 21)

    N += 30;

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



  if (N > 31) {

    d->d = N - 31;

    d->m = 4;

  } else {

    d->d = N;

    d->m = 3;

  }

}

DZone Snippets: easter code

Easter date - NASM

Easter date - C