用汇编语言实现递归 - 爱吱声

LEN_OF_OUTPUT equ 10 j% l/ ^$ R) ^. c7 F1 f3 c
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section .data ;
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userMsg db 'Please enter a single-digit number: ' ;7 o- M+ a% v* _+ W
lenUserMsg equ $-userMsg ;
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disp1Msg db 'The factorial of '' U9 Q' G4 ^3 Y( P1 k4 O* o7 s) Z
lenDisp1Msg equ $-disp1Msg
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disp2Msg db ' is '4 C2 }5 u/ I: n; U
lenDisp2Msg equ $-disp2Msg
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errMsg db 'Wrong input. Only one-digit number can be accepted.',0x0a
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lenErrMsg equ $-errMsg - g5 h( }, A7 t* {* F. n6 n7 |* s( r
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section .bss ;. F2 r1 J1 W6 a# Q: j
num resb 1& n7 D8 [0 v5 M. M
result resd 1& [7 R4 D1 |( ~( K- `
buffer resb LEN_OF_OUTPUT+1
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section .text ;
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global _start
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_start: ;
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mov eax, 48 g! I! T j0 K! }/ @5 t
mov ebx, 1
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mov ecx, userMsg
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mov edx, lenUserMsg
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int 80h; j a4 M* L$ ]9 b8 U
; read user's input
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mov eax, 32 R) J; t+ m6 A+ d3 y4 h ^" a
mov ebx, 0( G9 _# A6 B& Q9 ~9 a1 [2 j3 q
mov ecx, num / j3 ~0 V. ?7 g# k
mov edx, 1 ; length of input buffer
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int 80h! h, G: c/ p }: Y; W6 `7 v r1 ~
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mov eax, [num]/ t) s1 \+ T0 Z
and eax, 0xFF
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sub eax, 0x30
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jl ERROR+ F- W5 Q$ z( N+ {
cmp eax, 10# N' W2 O$ y8 k+ D1 ^0 Y
jl DO_CALC
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ERROR:
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mov eax, 4
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mov ebx, 1
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mov ecx, errMsg: a) N. H2 z1 D0 Y) b: {9 J& ^. v
mov edx, lenErrMsg
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int 80h
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jmp EXIT( J3 _; `! Q: q/ k
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DO_CALC:
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call factorial
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; The parameter is in eax. After calling, the result will be stored in eax.' a& z L+ ^. m: m, G
mov [result], eax ; Save the result
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; print result( C6 a& f+ q2 T
mov eax, 45 e+ ]2 s0 K" S. p& `2 B! h0 y" o
mov ebx, 12 \1 Y9 ^2 |2 F! ] w0 t
mov ecx, disp1Msg2 Y% \; H+ m, L0 E/ M. w2 B
mov edx, lenDisp1Msg$ O2 r Q! [/ v# p7 D5 F
int 80h , p2 J3 g2 z6 b# N, B& y
mov eax, 4# H# O( h. N: [0 ~; o$ u. d6 k8 N2 o
mov ebx, 1% z* h# c g( W; b
mov ecx, num
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mov edx, 1
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int 80h - W1 U9 m9 F% _7 A$ X' Q; a
mov eax, 4
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mov ebx, 1+ L' S# a9 d: _
mov ecx, disp2Msg) T- G* _5 ~: {! s# v! u
mov edx, lenDisp2Msg
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int 80h / G' Q' {1 R5 S! E; [1 l
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; output number
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mov eax, [result] ; VALUE THAT I WANT TO CONVERT AND DISPLAY2 r9 k6 M2 @& U/ T0 C W
mov byte [buffer+LEN_OF_OUTPUT],0x0a, c- @7 ?: j. Y0 f# A/ F% i% J
lea esi,[buffer+LEN_OF_OUTPUT]% k* c2 y/ V# x2 P' P3 W+ `% p
mov ebx,10
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ASC_LOOP:4 z1 j; k4 x8 E" o5 T3 b% }) x
mov edx,0 ; clear dx prior to dividing edx:eax by bx
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div ebx ;DIV EAX/10
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add edx,0x30 ;ADD 48 TO REMAINDER TO GET ASCII CHARACTER OF NUMBER 7 h7 L! s; [8 }- Y( a4 p. t
dec esi ; store characters in reverse order
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mov [esi],dl
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cmp eax,0 - b# _3 F0 d0 K% i% s* u1 K
jz END_ASC ;IF AX=0, END THE PROCEDURE OF NUMBER TO ASCII E. |+ c0 Q' O3 C( M: }
jmp ASC_LOOP ;ELSE REPEAT
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END_ASC:$ F# b1 X& f3 K) h9 _. j! i
mov eax, 4
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mov ebx, 13 T) S% W0 U Q2 L( r
mov ecx, esi! B8 E: G# W8 I4 p7 x6 T
lea edx,[buffer+LEN_OF_OUTPUT+1]& A4 W X, G6 L: |# |+ L
sub edx, ecx
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int 80h
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; Exit code
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EXIT:. L) |4 e {) x; z% ?+ C
mov eax, 15 e4 S+ C1 z# o% j" g# f
mov ebx, 0
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int 80h" n- N+ n; I# V p/ b
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; Factorial function using recursion
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factorial:: e$ _5 N3 W6 b& |1 m5 T
; Check if eax is 0 (base condition)
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cmp eax, 0- R v" L! X y+ k
jz end_recursion/ Y; g+ N" G. l5 o! i4 m& g
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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 recursively( G2 l7 O1 |: b! V1 C5 [- h) I
dec eax
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call factorial
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; Multiply the result returned in eax with the saved value of eax% Z8 |& ]4 U! c1 H! } F2 l
pop ebx
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imul eax, ebx
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ret4 ^/ F% K0 g. ~* u& a% ~
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end_recursion:
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; Return 1 when eax is 0 (base condition)9 z" }- T. ?+ k9 M+ B
mov eax, 1- a) B7 s& M0 |+ D
ret

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nasm -f elf factorial.asm
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ld -m elf_i386 -s -o factorial factorial.o( w& {1 r# s: Z! ]* _ L9 A. l
./factorial
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; Factorial function using recursion9 O, A" h% o/ z3 E9 V9 |2 r
factorial:7 n# R% n; p8 }& m) m+ \
; Check if eax is 0 (base condition)
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cmp eax, 0" x: l2 i3 }! P2 t% Y
jz end_recursion
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; Save the current value of eax
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push eax0 m; x* O. ~/ Y7 ]
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; Decrement eax and call factorial function recursively
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dec eax, G: ]/ n4 a5 `; [6 p4 W
call factorial) {) z V' b; F% U( [
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; Multiply the result returned in eax with the saved value of eax7 Q# t$ y: x# W/ _7 i' `$ Z
pop ebx
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imul eax, ebx
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ret1 i0 t6 r7 ^; g1 M) h1 O; O* A
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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
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ret

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