Need to bring this function up to scratch

**cboard_member** · 03-10-2006

I've just spent ~20 mins hacking together this function as part of my VM. It does it's job ok so far (I haven't extensively tested it yet) but it is very, very badly done - I get the feeling I've overdone it. Could somebody help me clean it up?

Code:

// FindEmptyRAMHunk
//
// Searches in RAM for a hunk of free space
// big enough to hold data of size (sz).
//
// Returns: A pointer to the start of the
//          free space, 0 if enough space
//          was not available
//
byte* CVirtualMachine::FindEmptyRAMHunk(size_t sz)
{
    byte* search = &RAM[0];
    byte* lookAhead = 0;
    byte* grabbed = new byte[sz];
    bool found = false;
    unsigned lim = 0;

    // Find first free cell
    if (*search != 0xFF)
        while (*search != 0xFF) search++;

    lookAhead = search;

    while (! found) {
reGrab:
        if (lim++ == ByteRAM) return 0;

        // Grab a chunk of RAM of size sz to examine
        for (size_t i = 0; i < sz; i++)
            *(grabbed + i) = ++*lookAhead;

        // Check grabbed chunk
        for (i = 0; i < sz; i++)
            if (*(grabbed + i) != 0xFF)
                goto reGrab;

        found = true;
    }

    delete[] grabbed;
    return lookAhead;
}

NOTE: I just added lim in as I was posting this because I realised it actually wasn't returning 0 if it couldn't find a hunk big enough. Yet to test it so slap me if it's not going to work.

**major_small** · 03-10-2006

...why not just try to allocate it and let the OS tell you if there isn't enough memory?

**Daved** · 03-10-2006

To get rid of the goto and actually use the while loop you could put found = true above the "Check grabbed chunk" for loop and then replace goto reGrab; with

Code:

{
  found = false;
  break;
}

**cboard_member** · 03-10-2006

Originally Posted by major_small

...why not just try to allocate it and let the OS tell you if there isn't enough memory?

I'm not sure what you mean.
I'm working on a Virtual Machine, so...

**MadCow257** · 03-10-2006

Maybe kind of like this

Code:

#include <iostream>
char * allocate(int size)
{
	return new char[size];
}

int main()
{
	char * test = allocate(6);
	test = "sweet";
	std::cout << test;
}

**major_small** · 03-10-2006

Originally Posted by MadCow257

Maybe kind of like this

Code:

#include <iostream>
char * allocate(int size)
{
	return new char[size];
}

int main()
{
	char * test = allocate(6);
	test = "sweet";
	std::cout << test;
}

that's pretty much what I had in mind

**Tonto** · 03-10-2006

Code:

	char * test = allocate(6);
	test = "sweet";
	std::cout << test;

I'm a ..........

Code:

	char * test = allocate(6);
	strcpy(test, "sweet");
	std::cout << test;
	delete[] test;

**anonytmouse** · 03-10-2006

I think this is what you are trying to do. Although as mentioned, writing a virtual maching does not strictly require you to write your own memory allocator. This is also a very inefficient allocator. You might wish to do a search for 'memory allocation algorithm'.

Code:

byte* CVirtualMachine::FindEmptyRAMHunk(size_t sz)
{
    size_t start         = 0;
    size_t count         = 0;
    const byte FREE_BYTE = 0xFF;

    while (i < RAM_SIZE)
    {
        if (RAM[i] == FREE_BYTE)
        {
            if (++count == sz)
            {
                return &RAM[start];
            }
        }
        else
        {
            count = 0;
            start = i + 1;
        }

        i++;
    }

    return NULL; /* Large enough block not found or sz is zero. */
}

**cboard_member** · 03-11-2006

Yeah I thought it wasn't very effective, I've got no experience writing this sort of thing (VM's) so I just wanted to do it for good practice.

I'll do some searching later on when the caffeine kicks in

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