power.s

#PURPOSE:	Program to illustrate how functions work
#			This program will compute the value of
#			2^3 + 5^2
#

#Everything in the main program is stored in registers,
#so the data section doesn't have anything.
.section .data

.section .text

.globl _start
_start:
	pushl $3			#push second argument
	pushl $2			#push first argument
	call power			#call the function
	addl $8, %esp		#move the stack pointer back
	pushl %eax			#save the first answer before
						#calling the next function

	pushl $2			#push second argument
	pushl $5			#push first argument
	call power			#call the function
	addl $8, %esp		#move the stack pointer back

	popl %ebx			#The second answer is already
						#in %eax. We saved the
						#first answer onto the stack,
						#so now we can just pop it
						#out into $ebx

	addl %eax, %ebx		#add them together
						#the result is in %ebx

	movl $1, %eax		#exit (%ebx is returned)
	int $0x80


#PURPOSE:	This function is used to compute
#			the value of a number raised to
#			a power.
#
#INPUT:		First argument - the base number
#			Second argument - the power to
#			raise it to
#
#OUTPUT:	Will give the result as a return value
#
#NOTES:		The power must be 1 or qreater
#
#VARIABLES:
#			%ebx - holds the base number
#			%ecx - holds the power
#
#			-4(%ebp) - holds the current result
#
#			%eax is used for temporary storage
#
.type power, @function
power:
	pushl %ebp				#save old base pointer
	movl %esp, %ebp			#make stack pointer the base pointer
	subl $4, %esp			#get room for our local storage
	
	movl 8(%ebp), %ebx		#put first argument in %eax
	movl 12(%ebp), %ecx		#put second argument in %ecx
	
	movl %ebx, -4(%ebp)		#store current result
	
power_loop_start:
	cmpl $1, %ecx			#if the power is 1, we are done
	je end_power
	movl -4(%ebp), %eax		#move the current result into %eax
	imull %ebx, %eax		#multiply the current result by
							#the base number
	movl %eax, -4(%ebp)		#store the current result
	
	decl %ecx				#decrease the power
	jmp power_loop_start	#run for the next power
	
end_power:
	movl -4(%ebp), %eax		#return value goes in %eax
	movl %ebp, %esp			#restore the stack pointer
	popl %ebp				#restore the base pointer
	ret