Recursion Definition, Syntax and Examples
In assembly language, when any code block or instruction group has to be used repeatedly, then recursion is used. Recursion occurs when a function or instruction group repeatedly calls itself, directly or indirectly.
There are two types of recycling: direct and indirect.
- direct recursion - In direct recursion, the process calls itself.
- indirect recursion - in indirect recursion, the first process calls the second process, which in turn calls the first process.
Recursion can be seen in many mathematical algorithms. For example, consider the case of computing the factorial of a number. The factorization equation of a number is given by
Fact (n) = n * fact (n-1) for n > 0Recursion Exampal
For example: The factorial of 5 is the factorial of 1 x 2 x 3 x 4 x 5 = 5 x 4 and this can be a good example to show a recursive process. Every recursive algorithm must have a termination condition, that is, the recursive calling of the program must stop when a condition is met. In the case of the factorial algorithm, the final condition is reached when n is 0.
Example The following program shows how to implement factorial n in assembly language. To keep the program simple, we'll calculate the factorial of 3.
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
mov bx, 3 ;for calculating factorial 3
call proc_fact
add ax, 30h
mov [fact], ax
mov edx,len ;message length
mov ecx,msg ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov edx,1 ;message length
mov ecx,fact ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
proc_fact:
cmp bl, 1
jg do_calculation
mov ax, 1
ret
do_calculation:
dec bl
call proc_fact
inc bl
mul bl ;ax = al * bl
ret
section .data
msg db 'Factorial 3 is:',0xa
len equ $ - msg
section .bss
fact resb 1result −
Factorial 3 is: 6 
