-4(%ebp) is a local variable that is constantly updated with the most recent result of the multiplication. If the 1st parameter to the function is not stored somewhere and left unchanged, think about how the function would know what needs to be multiplied.
On Sun, Mar 20, 2011 at 6:59 PM, 김태윤 <[email protected]> wrote: > 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 > > _______________________________________________ > Pgubook-readers mailing list > [email protected] > http://lists.nongnu.org/mailman/listinfo/pgubook-readers >
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