[Pgubook-readers] [chapter 4] assembly function flow

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assembly function flow

hello

I am reading a "programming from the ground up"

if you don't know what this book is, you still can help me.

in this book(chapter 4) there are 2 things that I don't understand.


Q. I don't understand

1. what "movl %ebx, -4(%ebp) #store current result" for.

2. and what does "current result" means

in marked section in the code below

little upperside, there is

"movl 8(%ebp), %ebx" which means save 8(%ebp) to %ebx

but the reason why I don't understand is

if the programmer want 8(%ebp) to save to -4(%ebp),

why should 8(%ebp) be passed through %ebx?

is "movl 8(%ebp), -4(%ebp)" akward?

or is there any typo in "movl 8(%ebp), %ebx #put first argument in %eax"?
(I think %ebx should be %eax or vice versa)

Thanks in advanced

#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 greater

#

#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