What is recursion in assembly language?

Recursion is a unique way of implementing a function and is commonly used in high-level programming languages. A recursive function typically calls itself within itself and returns only when the base case- a special condition- is met.

The time and space complexity of recursive functions may be greater than usual functions in some cases. However, recursive functions are typically much shorter in size.

In assembly language, we can implement recursive functions using procedures.

Multiple procedures can be defined in the text segment. Unless the base case is met, the same procedure may be called repeatedly. Upon meeting the base case, we can return from the procedure or terminate the program using the exit system call.

Example

The program below recursively prints the digits of an array in reverse order. First, we store an array with six elements alongside its size in a variable in the data segment. Subsequently, we use the print and check procedures to print array elements and check whether or not the base case has been met. If the base case is met, the exit system call is called to terminate the program gracefully.

section  .text
   global _start  
   
_start:  
          
   call check
   
print:
   mov  edi, arr        ;fetches pointer to array
   add  edi, [count]    ;updates and stores the adress of the array element in edi that is to be printed
   mov  ecx, [edi]      ;gets element in the adress that needs to be printed from the array
   add  ecx, 48         ;converts number in array to char
   push ecx             ;pushes to stack as we require an address
   mov ecx, esp         ;obtains the address of the number converted to char and saves it in ecx
   
   mov  edx, 1          ;message length, 1 byte stored in edx
   mov  ebx, 1          ;file descriptor number for stdout stream stored in ebx
   mov  eax, 4          ;system call number for the write system call stored in eax
   int  0x80            ;kernel interrupt to execute the write system call in kernel mode
   dec byte [count]     ;decrements count
   call check           ;calls check
   ret
check:
   cmp byte [count], -1
   jg  print  
   mov  eax, 1          ;system call number of the exit system call
   int  0x80            ;kernel interrupt to call the exit system call
section  .data
   arr db 1,2,3,4,5,6
   count db 5
section .bss

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