Write x86 ALP to find the factorial of a given integer number on a command line by using recursion.

Second Year Computer Engineering Microprocessor Programs:

Microprocessor Lab:

Practical 9:

Write x86 ALP to find the factorial of a given integer number on a command line by using recursion. Explicit stack manipulation is expected in the code.

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%macro dispmsg 2
mov rax,1
mov rdi, 1
mov rsi, %1
mov rdx, %2
syscall
%endmacro


%macro exitprog 0
mov rax, 60
mov edi,0
syscall
%endmacro

%macro gtch 1
mov rax, 0
mov rdi, 0
mov rsi, %1
mov rdx, 1
syscall
%endmacro

section .data
nwline db 10
m0 db 10,10,"Program to calculate factorial of a given number.",10,10
l0 equ $-m0

m2 db 10,"Enter Number (2 digit HEX no) : "
l2 equ $-m2

m4 db 10,"The factorial is : "
l4 equ $-m4
factorial  dq 1

section .bss
no1 resq 1
input resb 1
output resb 1


section .text
global _start
_start :

dispmsg m0,l0


dispmsg m2,l2    ; Display message
call getnum

mov [no1],rax     ; Accept number
gtch input ; To read and discard ENTER key pressed.

mov rcx,[no1]

call facto
mov rax,00



dispmsg m4,l4
mov rax,qword[factorial]

call disphx16    ; displays a 8 digit hex number  in rax


exitprog



facto:
push rcx
cmp rcx,01
jne ahead
jmp exit2

ahead:dec rcx
call facto
exit2:pop rcx
mov rax,rcx
mul qword[factorial]
mov qword[factorial],rax
ret


;; Procedure to get a 2 digit hex no from user; number returned in rax
getnum:
mov cx,0204h
mov rbx,0
ll2:
push rcx ; syscall destroys rcx. Rest all regs are preserved
gtch input
pop rcx
mov rax,0
mov al,byte[input]
sub rax,30h
cmp rax,09h
jbe skip1
sub rax,7
skip1:
shl rbx,cl
add rbx,rax
dec ch
jnz ll2
mov rax,rbx
ret


disphx16:   ; displays a 16 digit hex number passed in rax
mov rbx,rax
mov cx,1004h ;16 digits to display and 04 count to rotate
ll6:
rol rbx,cl
mov rdx,rbx
and rdx,0fh
add rdx,30h
cmp rdx,039h
jbe skip4
add rdx,7
skip4:
mov byte[output],dl
push rcx
dispmsg output,1
pop rcx
dec ch
jnz ll6
ret

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