Programming Forums > Application Development > Assembly

oxyi · Jan 26th, 2005, 4:17 PM

Im trying to find the GCD of two int,
below is an alog. that I tried to make it into assembly..
but it looks like im missing , can anyone help out ??

Gcd(int a, int b, int *result)
{ while(a!=b)
{
if(a > b)
a = a - b;
else
b = b - a;
}
*result = a;
}

(Toggle Plain Text)

void lab1( int a, int b, int *result )
{
__asm
{
mov esi, result
//save the current status of the registers
push eax
push ebx
push edx

//put the variables into the registers
mov eax, a
mov ebx, b


START_WHILE:
cmp eax, ebx
je END_WHILE

cmp eax,ebx
jle Else_Block

sub eax, ebx
jmp START_WHILE

Else_Block:
sub ebx, eax
jmp START_WHILE

END_WHILE:
move edx



//put the answer to result
mov [esi][0], edx

//restore the original status of registers.
pop edx
pop ebx
pop eax
}
return;

}

lostcauz · Jan 27th, 2005, 12:50 AM

I didn't test this or your algorithm. The extra compare is unnecessary in your code.

(Toggle Plain Text)

; assumes eax contains 'a' variable and ecx contains 'b' variable
begin:  cmp eax, ecx
        je  getout
        ja  higher
        sub ecx, eax
        jmp begin
higher: sub eax, ecx
        jmp begin
getout: ; result returned in eax

This might be better.

(Toggle Plain Text)

; gcd - greatest common divisor
;       by Paul Hsieh
;
; input:
;   eax = a
;   edx = b
;
; output:
;   eax = gcd
;
; destroys:
;   edx
;   flags
;
gcd:    neg     eax
        je      L3
L1:     neg     eax
        xchg    eax,edx
L2:     sub     eax,edx
        jg      L2
        jne     L1
L3:     add     eax,edx
        jne     L4
        inc     eax
L4:     ret

Lance · Jan 27th, 2005, 11:14 AM

$ gcc -S gcd.c
$ cat gcd.s

(Toggle Plain Text)

        .file   "gcd.c"
        .text
.globl Gcd
        .type   Gcd, @function
Gcd:
        pushl   %ebp
        movl    %esp, %ebp
        nop
.L2:
        movl    8(%ebp), %eax
        cmpl    12(%ebp), %eax
        jne     .L4
        jmp     .L3
.L4:
        movl    8(%ebp), %eax
        cmpl    12(%ebp), %eax
        jle     .L5
        movl    12(%ebp), %eax
        subl    %eax, 8(%ebp)
        jmp     .L2
.L5:
        movl    8(%ebp), %edx
        leal    12(%ebp), %eax
        subl    %edx, (%eax)
        jmp     .L2
.L3:
        movl    16(%ebp), %eax
        movl    8(%ebp), %edx
        movl    %edx, (%eax)
        popl    %ebp
        ret
        .size   Gcd, .-Gcd
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.3.4 20040623 (Gentoo Linux 3.3.4-r1, ssp-3.3.2-2, pie-8.7.6)"

oxyi · Jan 31st, 2005, 1:53 PM

wow.. thanks for all the replys ...

im just a beiginner in assembly in x86.

Still try to figure out why you guys did it in the format of L1: L2: .. etc
is that how everyone do it ?

and wow you guys's code are soooo complicated...

Mjordan2nd · Jan 31st, 2005, 3:16 PM

The GNU C compiler will convert your C code to assembly for you. I believe they used that to convert. Lance gives you an example of how to do it, if you have a *nix system.

Lance · Jan 31st, 2005, 3:35 PM

Indeed. GCC is definitely the best compiler in the world.

You don't even have to type out fullly accurate code to convert it. I simple copied and pasted his code into vim, saved it as gcd.c, then used -S to pre-process/convert to assembly. It's really nice, and fast!

Either way, it helps a lot to convert some C code to Assembly if you're trying to learn Assembly, or already know the basics, but want to see how a compiler does it. You eventually grow much respect for compiler coders, especially those of GCC. You can even compile Java to Assembly if you wish!

Again... GCC is the lord almighty. It is sad that DJGPP isn't as good as pure GCC, so Windows is such a crippling system for coders. :/ Unless you're doing those graphical applications in Visual Studio... then you're stuck with MS' compiler. Oh welll...

And sorry if I sound somewhat matter-of-fact-ly; I'm absolutely decaffeinated and sober. That should explain enough.

lostcauz · Feb 1st, 2005, 6:40 AM

I simply looked at your C code and wrote it in assembler. I offered some other code by a more experienced assembler programmer for comparison also. While compiler output is a good way to learn and see a conversion the goal, for me at least, is to write tighter, faster code than the compiler. Either way it's awesome to see some others interested in learning the language. I commented the code I wrote.

(Toggle Plain Text)

; assumes eax contains 'a' variable and ecx contains 'b' variable

begin:  cmp eax, ecx   ; compare the 2 variables
        je  getout     ; if equal, we are done
        ja  higher     ; if a is greater than b jump to higher
        sub ecx, eax   ; otherwise a<b so subtract a from b
        jmp begin      ; loop until equal
higher: sub eax, ecx   ; subtract b from a
        jmp begin      ; loop
getout:                ; result returned in eax

oxyi · Feb 1st, 2005, 3:21 PM

Wow thanks a lot such a clear code

I thought my way was the only way to do this gcd problem

Yap I am in the process of learning assemlby, but my professor so far has
done a terrible job on teaching it, or you can say he is not teaching it,
we just got a sheet where it explain some loop control and if /else statement
then we are off on our own, and he is off in the silicon world...

I have programming experience in java and c++, i heard about the converting process, but the result is too weird to for me to understand it.
And im currently using Visual .Net to do my assemblly work...

now i have another question..

implement a function that counts the number of vowels in a given string

edi is input string's memory address.
i know i would use a for loop to loop through the string
and check each position for A E I O U and a e i o u ,

I kinda know the how the loop control work in assembly, but I am not sure
how do I check into the string for each of the vowel, as oppose in java, i can just use a for loop and check each position within the string till the end.
Is that how I suppose to do it in assembly as well ? and is string is the same thing as array ?

a simple java code would take care of this, but should i try to write it in java first and try to convert to assembly to see how is it done ?

Im sorry that I asked a lot questions, but i just want to know this stuffs, thanks a lot for any replys

Mjordan2nd · Feb 1st, 2005, 6:24 PM

Probably not the best way, but this is how I'd do it:

(Toggle Plain Text)

findVowels:
    cmp [di], BYTE 'a'
    je increment
    cmp [di], BYTE 'e'
    je increment
    cmp [di], BYTE 'i'
    je increment
    cmp [di], BYTE 'o'
    je increment
    cmp [di], BYTE 'u'
    je increment
    cmp [di], BYTE 0
    je end
    inc di
    jmp findVowels
increment:
    inc di
    mov bx, counter
    inc WORD [bx]
    jmp findVowels

Oh, and remember, no problem with asking questions.

oxyi · Feb 1st, 2005, 7:45 PM

so no for loop control is required in it ?

what is [di] ? and what does WORD[bx] do ?

hehe thanks

Jan 26th, 2005, 4:17 PM	#1
oxyi Newbie Join Date: Jan 2005 Posts: 17 Rep Power: 0	How do i convert this code into assembly ?? Im trying to find the GCD of two int, below is an alog. that I tried to make it into assembly.. but it looks like im missing , can anyone help out ?? Gcd(int a, int b, int result) { while(a!=b) { if(a > b) a = a - b; else b = b - a; } result = a; } (Toggle Plain Text) void lab1( int a, int b, int result ) { __asm { mov esi, result //save the current status of the registers push eax push ebx push edx //put the variables into the registers mov eax, a mov ebx, b START_WHILE: cmp eax, ebx je END_WHILE cmp eax,ebx jle Else_Block sub eax, ebx jmp START_WHILE Else_Block: sub ebx, eax jmp START_WHILE END_WHILE: move edx //put the answer to result mov [esi][0], edx //restore the original status of registers. pop edx pop ebx pop eax } return; } void lab1( int a, int b, int result ) { __asm { mov esi, result //save the current status of the registers push eax push ebx push edx //put the variables into the registers mov eax, a mov ebx, b START_WHILE: cmp eax, ebx je END_WHILE cmp eax,ebx jle Else_Block sub eax, ebx jmp START_WHILE Else_Block: sub ebx, eax jmp START_WHILE END_WHILE: move edx //put the answer to result mov [esi][0], edx //restore the original status of registers. pop edx pop ebx pop eax } return; }

Jan 27th, 2005, 12:50 AM	#2
lostcauz Hobbyist Programmer Join Date: Nov 2004 Location: 1691 miles East of L.A. Posts: 159 Rep Power: 4	I didn't test this or your algorithm. The extra compare is unnecessary in your code. (Toggle Plain Text) ; assumes eax contains 'a' variable and ecx contains 'b' variable begin: cmp eax, ecx je getout ja higher sub ecx, eax jmp begin higher: sub eax, ecx jmp begin getout: ; result returned in eax ; assumes eax contains 'a' variable and ecx contains 'b' variable begin: cmp eax, ecx je getout ja higher sub ecx, eax jmp begin higher: sub eax, ecx jmp begin getout: ; result returned in eax This might be better. (Toggle Plain Text) ; gcd - greatest common divisor ; by Paul Hsieh ; ; input: ; eax = a ; edx = b ; ; output: ; eax = gcd ; ; destroys: ; edx ; flags ; gcd: neg eax je L3 L1: neg eax xchg eax,edx L2: sub eax,edx jg L2 jne L1 L3: add eax,edx jne L4 inc eax L4: ret ; gcd - greatest common divisor ; by Paul Hsieh ; ; input: ; eax = a ; edx = b ; ; output: ; eax = gcd ; ; destroys: ; edx ; flags ; gcd: neg eax je L3 L1: neg eax xchg eax,edx L2: sub eax,edx jg L2 jne L1 L3: add eax,edx jne L4 inc eax L4: ret __________________ -- lostcauz Stepped in what?... Behind whose barn?... I didn't even know they had a cow!

Jan 27th, 2005, 11:14 AM	#3
Lance Programmer Join Date: Oct 2004 Location: Chicago, IL USA Posts: 74 Rep Power: 4	$ gcc -S gcd.c $ cat gcd.s (Toggle Plain Text) .file "gcd.c" .text .globl Gcd .type Gcd, @function Gcd: pushl %ebp movl %esp, %ebp nop .L2: movl 8(%ebp), %eax cmpl 12(%ebp), %eax jne .L4 jmp .L3 .L4: movl 8(%ebp), %eax cmpl 12(%ebp), %eax jle .L5 movl 12(%ebp), %eax subl %eax, 8(%ebp) jmp .L2 .L5: movl 8(%ebp), %edx leal 12(%ebp), %eax subl %edx, (%eax) jmp .L2 .L3: movl 16(%ebp), %eax movl 8(%ebp), %edx movl %edx, (%eax) popl %ebp ret .size Gcd, .-Gcd .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.3.4 20040623 (Gentoo Linux 3.3.4-r1, ssp-3.3.2-2, pie-8.7.6)" .file "gcd.c" .text .globl Gcd .type Gcd, @function Gcd: pushl %ebp movl %esp, %ebp nop .L2: movl 8(%ebp), %eax cmpl 12(%ebp), %eax jne .L4 jmp .L3 .L4: movl 8(%ebp), %eax cmpl 12(%ebp), %eax jle .L5 movl 12(%ebp), %eax subl %eax, 8(%ebp) jmp .L2 .L5: movl 8(%ebp), %edx leal 12(%ebp), %eax subl %edx, (%eax) jmp .L2 .L3: movl 16(%ebp), %eax movl 8(%ebp), %edx movl %edx, (%eax) popl %ebp ret .size Gcd, .-Gcd .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.3.4 20040623 (Gentoo Linux 3.3.4-r1, ssp-3.3.2-2, pie-8.7.6)" __________________ (Toggle Plain Text) /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise"; /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise";

Jan 31st, 2005, 3:16 PM	#5
Mjordan2nd The Supreme Ruler Join Date: May 2004 Location: Houston Posts: 1,476 Rep Power: 6	The GNU C compiler will convert your C code to assembly for you. I believe they used that to convert. Lance gives you an example of how to do it, if you have a *nix system. __________________ "Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower

Jan 31st, 2005, 3:35 PM	#6
Lance Programmer Join Date: Oct 2004 Location: Chicago, IL USA Posts: 74 Rep Power: 4	Indeed. GCC is definitely the best compiler in the world. You don't even have to type out fullly accurate code to convert it. I simple copied and pasted his code into vim, saved it as gcd.c, then used -S to pre-process/convert to assembly. It's really nice, and fast! Either way, it helps a lot to convert some C code to Assembly if you're trying to learn Assembly, or already know the basics, but want to see how a compiler does it. You eventually grow much respect for compiler coders, especially those of GCC. You can even compile Java to Assembly if you wish! Again... GCC is the lord almighty. It is sad that DJGPP isn't as good as pure GCC, so Windows is such a crippling system for coders. :/ Unless you're doing those graphical applications in Visual Studio... then you're stuck with MS' compiler. Oh welll... And sorry if I sound somewhat matter-of-fact-ly; I'm absolutely decaffeinated and sober. That should explain enough. __________________ (Toggle Plain Text) /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise"; /* LANCE / C++; / this makes C bigger but returns the old value / char site = "slackwise.net", home = "lance.slackwise.net", pics = "flickr.com/photos/slackwise";

Jan 31st, 2005, 1:53 PM	#4
oxyi Newbie Join Date: Jan 2005 Posts: 17 Rep Power: 0	wow.. thanks for all the replys ... im just a beiginner in assembly in x86. Still try to figure out why you guys did it in the format of L1: L2: .. etc is that how everyone do it ? and wow you guys's code are soooo complicated...

Feb 1st, 2005, 6:40 AM	#7
lostcauz Hobbyist Programmer Join Date: Nov 2004 Location: 1691 miles East of L.A. Posts: 159 Rep Power: 4	I simply looked at your C code and wrote it in assembler. I offered some other code by a more experienced assembler programmer for comparison also. While compiler output is a good way to learn and see a conversion the goal, for me at least, is to write tighter, faster code than the compiler. Either way it's awesome to see some others interested in learning the language. I commented the code I wrote. (Toggle Plain Text) ; assumes eax contains 'a' variable and ecx contains 'b' variable begin: cmp eax, ecx ; compare the 2 variables je getout ; if equal, we are done ja higher ; if a is greater than b jump to higher sub ecx, eax ; otherwise a<b so subtract a from b jmp begin ; loop until equal higher: sub eax, ecx ; subtract b from a jmp begin ; loop getout: ; result returned in eax ; assumes eax contains 'a' variable and ecx contains 'b' variable begin: cmp eax, ecx ; compare the 2 variables je getout ; if equal, we are done ja higher ; if a is greater than b jump to higher sub ecx, eax ; otherwise a<b so subtract a from b jmp begin ; loop until equal higher: sub eax, ecx ; subtract b from a jmp begin ; loop getout: ; result returned in eax __________________ -- lostcauz Stepped in what?... Behind whose barn?... I didn't even know they had a cow!

Feb 1st, 2005, 3:21 PM	#8
oxyi Newbie Join Date: Jan 2005 Posts: 17 Rep Power: 0	Wow thanks a lot such a clear code I thought my way was the only way to do this gcd problem Yap I am in the process of learning assemlby, but my professor so far has done a terrible job on teaching it, or you can say he is not teaching it, we just got a sheet where it explain some loop control and if /else statement then we are off on our own, and he is off in the silicon world... I have programming experience in java and c++, i heard about the converting process, but the result is too weird to for me to understand it. And im currently using Visual .Net to do my assemblly work... now i have another question.. implement a function that counts the number of vowels in a given string edi is input string's memory address. i know i would use a for loop to loop through the string and check each position for A E I O U and a e i o u , I kinda know the how the loop control work in assembly, but I am not sure how do I check into the string for each of the vowel, as oppose in java, i can just use a for loop and check each position within the string till the end. Is that how I suppose to do it in assembly as well ? and is string is the same thing as array ? a simple java code would take care of this, but should i try to write it in java first and try to convert to assembly to see how is it done ? Im sorry that I asked a lot questions, but i just want to know this stuffs, thanks a lot for any replys

Feb 1st, 2005, 6:24 PM	#9
Mjordan2nd The Supreme Ruler Join Date: May 2004 Location: Houston Posts: 1,476 Rep Power: 6	Probably not the best way, but this is how I'd do it: (Toggle Plain Text) findVowels: cmp [di], BYTE 'a' je increment cmp [di], BYTE 'e' je increment cmp [di], BYTE 'i' je increment cmp [di], BYTE 'o' je increment cmp [di], BYTE 'u' je increment cmp [di], BYTE 0 je end inc di jmp findVowels increment: inc di mov bx, counter inc WORD [bx] jmp findVowels findVowels: cmp [di], BYTE 'a' je increment cmp [di], BYTE 'e' je increment cmp [di], BYTE 'i' je increment cmp [di], BYTE 'o' je increment cmp [di], BYTE 'u' je increment cmp [di], BYTE 0 je end inc di jmp findVowels increment: inc di mov bx, counter inc WORD [bx] jmp findVowels Oh, and remember, no problem with asking questions. __________________ "Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower

Feb 1st, 2005, 7:45 PM	#10
oxyi Newbie Join Date: Jan 2005 Posts: 17 Rep Power: 0	so no for loop control is required in it ? what is [di] ? and what does WORD[bx] do ? hehe thanks

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Programming Forums > Application Development > Assembly

How do i convert this code into assembly ??