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Problem with a Function to list the Free Memory
Posted: Fri Aug 24, 2007 12:19 am
by ChristianF
Hi
I have written a little function which writes all the free memory addresses above 16MB in an Array.
When i want to put the address on screen i got nothing there.
Here is my code for listing the free memory and getting a free block:
Code: Select all
unsigned long free_blocks_index = 0;
unsigned long free_blocks[1024];
void make_free_blocks_list()
{
unsigned int x;
int i=0;
for(x = 0x000000; x < (1024 * 1024 * 64); x += MEMORY_BLOCK)
{
// this is a page (considered a block)
// if block is above 16MB.. then it should have a great chance of not being used?
if(x > 0x100000 + (1024 * 1024 * 16))
{
free_blocks[free_blocks_index] = x;
free_blocks_index = free_blocks_index + 1;
}
}
}
unsigned long get_free_block()
{
free_blocks_index = free_blocks_index - 1;
return(free_blocks[free_blocks_index]);
}
And Then I have a second Problem: I have written a Function in ASM
which counts the memory, for example 512MB or so. When i use instead of the 64 in make_free_blocks_list the function
GetMemSize() then there is an endless loop.
This is the Sourcecode:
Code: Select all
; Function for Memorytesting
global _GetMemSize
_GetMemSize:
push ebp
mov ebp, esp
push eax
push edx
mov eax, 0
mov edx, 0x1EFFFC
.1:
inc eax
mov DWORD [edx], 0xAAAAAAAA
mov ebx, [edx]
add edx, 0x100000
cmp ebx, 0xAAAAAAAA
je .1
pop edx
pop eax
mov esp, ebp
pop ebp
ret
I call this like this (puts() prints a string on the display):
Code: Select all
puts("There is a free block od memory at this address:");
puts(get_free_block());
And what I see on the Display is like this:
Code: Select all
There is a free block od memory at this address:
What am I doing wrong? (My English isn't very good
)
Posted: Fri Aug 24, 2007 2:11 am
by JamesM
First: Does your number-printing function work? It doesn't look like you're using C++, so you can't have overloaded the puts function to take an integer parameter.
Code: Select all
puts("There is a free block od memory at this address:"); // Calling puts(char*)
puts(get_free_block()); // Calling puts(unsigned int)
I don't know how much C you know, but an integer can
not be implicitly cast to a string. In fact, in C, there is no such thing as a string type. It's just a collection of characters ending in a zero. You have to implement your own 'puthex' or 'puti' functions. At the moment I would
expect to see what you're seeing as output.
As for the second problem, that is not the correct way to query the memory size. Get it from the BIOS. Get into real mode and query it using a bios interrupt. Your address space is full of little bits of memory-mapped IO devices, like for example the VGA framebuffer. writing to that memory will, in some instances, destroy things. It's best never to do it.
On a side note, for future reference, you
don't need to save the eax, ecx or edx registers when using the cdecl (default gcc) calling convention. They are assumed to be trampled by the caller function. You
do, however, need to save ebx. (You have pushed eax, edx below but not ebx).
Plus, you only need to save the stack pointer into ebp and push ebp when you use the stack. Which you don't here.
JamesM
Posted: Fri Aug 24, 2007 3:51 am
by ChristianF
You do, however, need to save ebx. (You have pushed eax, edx below but not ebx).
After I have done this, I get something cryptic on the screen.
Now I have a question: Do somebody have a tutorial for implementing something like printf?
My Knowledge in C was better before 2 months.
Just when I started working, i programmed ASP Websites and no C, so my knowledge in C is at the moment not very good, but i work on it
.
Posted: Fri Aug 24, 2007 4:00 am
by JamesM
I wholeheartedly suggest that you program something smaller before trying an OS. Seriously, this is not the project to learn C with. It requires at least a sound, if not an inside-out knowledge of the language and compiler.
And check linux v0.01 for an implementation of vsprintf. writehex and writedec you can implement yourself if you think about it the right way.
And, by the by, if you can't implement functions to convert ascii to hex and ascii to decimal, you really should not be writing an operating system.
JamesM
Posted: Fri Aug 24, 2007 4:32 am
by ChristianF
I am programming C for about 5 years.
But if you are programming vbs so there are many differences between this and C.
I know the C language well.
And thank you for your reply.
Posted: Fri Aug 24, 2007 7:57 am
by JamesM
I know the C language well.
Yet you were trying to call a function taking a char* parameter with an integer argument and expecting it to work...
I am programming C for about 5 years.
And, this:
Code: Select all
free_blocks_index = free_blocks_index + 1;
Can be rewritten better as this:
Similarly:
Code: Select all
free_blocks_index = free_blocks_index - 1;
=>
free_blocks_index --;
You see why I questioned your C ability.
But if you are programming vbs so there are many differences between this and C.
I hope I am reading that incorrectly - it looks like you just said that visual basic script was similar to C......
JamesM
Posted: Sun Aug 26, 2007 11:33 am
by ChristianF
I know that i can write "free_blocks_index = free_blocks_index + 1;" as "free_blocks_index++;". But I think it is easier for people to understand which do not know the C language very well.
This was my reason to write it like this.
Posted: Sun Aug 26, 2007 11:45 am
by JamesM
You decided to write less readable code... so that people who don't know the language can understand it better?!
Seems a bit backwards.
Posted: Sun Aug 26, 2007 12:41 pm
by inflater
"free_blocks_index = free_blocks_index + 1;"
This is like from pascal syntax
Of course even there you can use inc(free_blocks_index);, like from "free_blocks_index++".
You decided to write less readable code... so that people who don't know the language can understand it better?!
Well don't know, what's worse?
If the code is very well commented and tabbed properly why not use the "pascalish" syntax (free_blocks_index = free_blocks_index + 1") ?
Regards
inflater