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: ' ;
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lenUserMsg equ $-userMsg ;3 r" F6 [5 y) j n
disp1Msg db 'The factorial of '
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lenDisp1Msg equ $-disp1Msg ; \1 [0 H2 P6 ]
disp2Msg db ' is ': B% Y U9 w9 M3 w( f2 I
lenDisp2Msg equ $-disp2Msg 2 _% T8 r* I9 z& b% m( {
errMsg db 'Wrong input. Only one-digit number can be accepted.',0x0a' w1 f9 X, G/ f1 C1 p5 N2 e
lenErrMsg equ $-errMsg
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section .bss ;
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num resb 1- o% R9 o/ r7 Z& j6 l7 W8 K# V( V
result resd 1* y: ?# ]2 N; B( G* V+ j7 {
buffer resb LEN_OF_OUTPUT+1
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section .text ;1 _5 M3 L/ B* d' s: M
global _start4 z8 b4 ? m0 r7 H! R
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_start: ;
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mov eax, 4# }# h- U4 D& m, |" ?: ?
mov ebx, 1
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mov ecx, userMsg
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mov edx, lenUserMsg
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int 80h$ N, R$ p4 {$ c3 A; i0 p
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; read user's input
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mov eax, 32 p- I( e1 ]$ @: |+ O
mov ebx, 0; }. I6 N1 h5 U* c& S" u1 ^
mov ecx, num
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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$ |. W+ m0 U, m% k( u" M1 | b
sub eax, 0x30
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jl ERROR
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cmp eax, 10- ?+ V( [: v5 ~& J: \. K
jl DO_CALC& K% f/ E, v {1 s `
ERROR:
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mov eax, 4
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mov ebx, 17 u- }0 B' W) ~5 R/ `! I# Y
mov ecx, errMsg8 P0 b: _0 X3 d" ?, I: u
mov edx, lenErrMsg' [, q* {8 w0 y# Y) @& E- v2 e
int 80h 8 Q3 _# @! d9 l) M; T6 V( c0 Y4 `
jmp EXIT! X9 J x- ?' w9 \9 w* T
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DO_CALC:
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call factorial$ w* P/ x& i6 ]' ^" Z
; The parameter is in eax. After calling, the result will be stored in eax.
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mov [result], eax ; Save the result" _% ]- B& b: M! Y( u' n, Y P; \
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; print result1 E; s3 Y' g! T8 @
mov eax, 4
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mov ebx, 1 ~0 [1 t: Y) f5 I2 Y
mov ecx, disp1Msg/ q# R( w" z' b5 [) H+ \
mov edx, lenDisp1Msg% e* g) i* m$ T* w7 ]& z
int 80h
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mov eax, 4' Q& U0 h/ r4 @9 Z% [; ^
mov ebx, 1" ~$ ^1 ~" ^' n8 X# x; j
mov ecx, num
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mov edx, 19 y, a) ?3 V7 N1 Q0 k
int 80h 1 |8 R! _, T: B7 w/ O
mov eax, 4: b+ P8 _. [ ~9 X# {, a$ y
mov ebx, 11 J- P! a% J9 U% n5 k
mov ecx, disp2Msg
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mov edx, lenDisp2Msg
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int 80h
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; output number
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mov eax, [result] ; VALUE THAT I WANT TO CONVERT AND DISPLAY3 F f# j( h, L( ~, H6 [( \
mov byte [buffer+LEN_OF_OUTPUT],0x0a; q1 R" h+ }- t+ g
lea esi,[buffer+LEN_OF_OUTPUT]) \5 p" x+ ~% g( c) L
mov ebx,10 & t' @- m& H/ N
ASC_LOOP:
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mov edx,0 ; clear dx prior to dividing edx:eax by bx) s: h2 V% c" {* Q: P
div ebx ;DIV EAX/10
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add edx,0x30 ;ADD 48 TO REMAINDER TO GET ASCII CHARACTER OF NUMBER
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dec esi ; store characters in reverse order" r0 X4 D K' b, ?3 @0 O5 i
mov [esi],dl9 M% N+ [3 R5 y M/ F
cmp eax,0
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jz END_ASC ;IF AX=0, END THE PROCEDURE OF NUMBER TO ASCII) ^: c3 ~7 B7 Q. w) N
jmp ASC_LOOP ;ELSE REPEAT/ m1 w& h) c6 O* `; R( [
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mov eax, 40 q J4 H+ n2 ]( I9 F3 x6 ^# z! J
mov ebx, 1
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mov ecx, esi
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lea edx,[buffer+LEN_OF_OUTPUT+1]7 P( a/ q5 {. |; h4 \
sub edx, ecx$ J& s7 `: }/ g8 F8 e3 y/ _5 K
int 80h ! J9 X8 ?/ {" _
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; Exit code
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EXIT:
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mov eax, 19 Y m. C7 M' f6 |
mov ebx, 0
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int 80h% z5 w$ ?% y( v. S
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; Factorial function using recursion
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factorial:
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; Check if eax is 0 (base condition)8 W3 U9 s8 C% ]* w
cmp eax, 06 M& s1 @" h8 k. W" p
jz end_recursion
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; Save the current value of eax
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push eax% G/ C5 P* F) I( g
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; Decrement eax and call factorial function recursively
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dec eax9 ?- A7 Y, Y5 C& \% g3 a
call factorial$ L: r, S1 `# Y- n$ j- u) j6 z2 P7 ^
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; Multiply the result returned in eax with the saved value of eax$ I2 V7 a, K) s
pop ebx
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imul eax, ebx' h8 k% T( t1 e0 g4 b
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ret9 I; X! X% A* ?6 ]6 W
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end_recursion:
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; Return 1 when eax is 0 (base condition)# Z2 o8 C6 E+ Z7 g- a
mov eax, 1
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ret

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nasm -f elf factorial.asm9 z0 S; M3 v1 m
ld -m elf_i386 -s -o factorial factorial.o% a3 v* ^, T5 q, N0 ]
./factorial+ e6 a8 a' P2 N' T& `5 n, X0 _, |

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; Factorial function using recursion
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factorial:
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; Check if eax is 0 (base condition)' J9 |9 `$ X% W$ W c8 U
cmp eax, 0! x! }" ~6 `- \. E1 k) p2 O
jz end_recursion" R2 n' O3 p t" R4 n8 I
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; Save the current value of eax5 K2 ~9 P4 Y+ a+ u7 S, O& c8 Q
push eax
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; Decrement eax and call factorial function recursively
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dec eax
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call factorial
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; Multiply the result returned in eax with the saved value of eax" Q6 v% {) s- x% t) W
pop ebx
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imul eax, ebx0 l- J, `+ |5 i
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ret, l, I& f2 D) t0 q; h! ?
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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; P8 ?# N( p# ~% L* q6 ~
ret

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