用汇编语言实现递归 - 爱吱声

LEN_OF_OUTPUT equ 10
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section .data ;
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userMsg db 'Please enter a single-digit number: ' ;" d' r- Z* q, u
lenUserMsg equ $-userMsg ;7 Y- F) D, E& t: u: h7 K' @, a
disp1Msg db 'The factorial of '
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lenDisp1Msg equ $-disp1Msg
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disp2Msg db ' is '2 c" c7 K R' f9 P9 n4 W
lenDisp2Msg equ $-disp2Msg
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errMsg db 'Wrong input. Only one-digit number can be accepted.',0x0a, K v) j5 Z a E# @
lenErrMsg equ $-errMsg # E, j* I9 r9 d3 ~0 B8 Y
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section .bss ;
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num resb 1# s/ s1 @' a6 O' ?6 z1 D
result resd 1
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buffer resb LEN_OF_OUTPUT+1
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section .text ;/ W8 L* w8 c$ c' Q' g* u
global _start
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_start: ;
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mov eax, 4: G! q' U' b6 c. w8 E! l$ _
mov ebx, 15 ~( v9 p3 h) j0 n7 @; H" @" F) s5 k
mov ecx, userMsg/ j3 z+ M. C$ d2 w' k3 z7 h% f
mov edx, lenUserMsg
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int 80h. H b7 t& ?( J9 A& i
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; read user's input! C9 V4 P! c1 C8 _
mov eax, 3* e) ]' Y, a. V3 `; d" u |, H
mov ebx, 0
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mov ecx, num $ H6 { C7 c3 H3 I% x
mov edx, 1 ; length of input buffer
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int 80h
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mov eax, [num]
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and eax, 0xFF
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sub eax, 0x30
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jl ERROR
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cmp eax, 102 J/ S' Y6 V7 c6 i; U+ [$ n
jl DO_CALC8 n; J. _& q. y; r- ?, M: v
ERROR:
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mov eax, 4
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mov ebx, 1' x- d# d8 p# |6 a: b
mov ecx, errMsg
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mov edx, lenErrMsg
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int 80h
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jmp EXIT
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DO_CALC:0 o ]0 }) }5 A
call factorial
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; The parameter is in eax. After calling, the result will be stored in eax.
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mov [result], eax ; Save the result) w5 _* l- u& c) `
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; print result
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mov eax, 4 u8 p0 T* Y3 t4 h
mov ebx, 17 g1 x/ V# V+ U) ~
mov ecx, disp1Msg$ n; Q+ c. n4 C' D% p
mov edx, lenDisp1Msg
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int 80h
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mov eax, 4
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mov ebx, 1
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mov ecx, num
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mov edx, 1
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int 80h 2 t0 c) B) x) |
mov eax, 4
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mov ebx, 1
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mov ecx, disp2Msg
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mov edx, lenDisp2Msg4 G8 p% W6 [* z7 I d# `
int 80h
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; output number
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mov eax, [result] ; VALUE THAT I WANT TO CONVERT AND DISPLAY; ]( ?3 ^% g# X; V
mov byte [buffer+LEN_OF_OUTPUT],0x0a
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lea esi,[buffer+LEN_OF_OUTPUT]
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mov ebx,10 / e2 P/ N* M! ^* a# Y
ASC_LOOP:
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mov edx,0 ; clear dx prior to dividing edx:eax by bx! {8 L. Z, ^% H2 G
div ebx ;DIV EAX/105 a. b" g7 V, C: | @* ?
add edx,0x30 ;ADD 48 TO REMAINDER TO GET ASCII CHARACTER OF NUMBER : |1 j4 x- b4 g5 ?' C* d8 u. f/ e
dec esi ; store characters in reverse order5 L$ a% m" H3 O& C$ f
mov [esi],dl
cmp eax,0 8 W- F: q( T) J9 C9 p
jz END_ASC ;IF AX=0, END THE PROCEDURE OF NUMBER TO ASCII3 E8 h8 A6 ?) z) Z9 i5 `! I
jmp ASC_LOOP ;ELSE REPEAT
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END_ASC:+ D7 q+ c* E3 A4 S6 g
mov eax, 4) r/ |3 X1 H8 T3 l4 p0 {- H
mov ebx, 16 [/ M( c' L& s/ s
mov ecx, esi/ y6 {. q5 a- w
lea edx,[buffer+LEN_OF_OUTPUT+1]* X( B5 O9 L" z( I! u1 s: _# L4 o
sub edx, ecx. V( ^2 a* p1 w2 j$ K* O6 X
int 80h . ^2 g: L7 O( @ V [
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; Exit code
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EXIT:# \5 m4 z" a; `
mov eax, 11 `6 V. I% T, L, l$ x
mov ebx, 0- b6 V, ?& R2 c# b/ B! S
int 80h' e8 s- F. F% U
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; Factorial function using recursion0 C$ B P% C# g. f3 B
factorial:2 r! L+ Q+ K7 _
; Check if eax is 0 (base condition)
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cmp eax, 0. e+ C) _- ]0 S# I* q
jz end_recursion
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; Save the current value of eax8 a5 a& c- Z, x- B
push eax) S0 m4 t0 J1 e
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; Decrement eax and call factorial function recursively% b6 n/ t# a. p
dec eax5 t! h" k% H4 K7 ~% a
call factorial
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; Multiply the result returned in eax with the saved value of eax
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pop ebx7 ?$ x7 R ?6 d
imul eax, ebx R; c2 A% I$ ? u* z+ p
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ret: i0 \7 k$ K8 \3 Z; K
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end_recursion:2 |+ n! C! K5 p- R+ _
; Return 1 when eax is 0 (base condition)
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mov eax, 1
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ret

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nasm -f elf factorial.asm% s1 T* Q: r8 N' J5 R
ld -m elf_i386 -s -o factorial factorial.o
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./factorial$ R3 M& N- U* [7 a8 b, J

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; Factorial function using recursion$ u( U6 r; f$ Z* _
factorial:
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; Check if eax is 0 (base condition)
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cmp eax, 0
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jz end_recursion
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; Save the current value of eax
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push eax
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; Decrement eax and call factorial function recursively2 t7 m# N. z ?, v
dec eax; Q$ T0 ]: j& X' _" k2 d
call factorial
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; Multiply the result returned in eax with the saved value of eax0 @* o0 c+ A, O9 @) F2 A
pop ebx- I1 l% _# z- T
imul eax, ebx
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ret
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end_recursion:
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; Return 1 when eax is 0 (base condition)
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mov eax, 1' B$ {: \7 ?7 d; C, i7 [7 m
ret

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