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Posted: **Mon Oct 06, 2014 1:36 pm**

Edit: Found a solution, I set the ram disk only as readable and kept the other information at a lower location. I still would like clarification if anyone is willing to say where I went wrong.

If someone could give me advice on the ram disk, I've run into a page fault when trying to init the ram disk but it comes from init paging due to the high placement of the ram disk. (http://www.jamesmolloy.co.uk/tutorial_h ... 0Heap.html)

I did some research because I was confused why GRUB was loading the image at a high address and found that GRUB2 unlike legacy does this. The line that errors...

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heap_t *create_heap(u32int start, u32int end_addr, u32int max, u8int supervisor, u8int readonly)
{
	heap_t *heap = (heap_t*)kmalloc(sizeof(heap_t));
	
	kout("HEAP IDX\n");
	kohex(&heap);
	kochr('\n');
	
    //S&E MUST BE PAGE ALLIGNED
    ASSERT(start%0x1000 == 0);
    ASSERT(end_addr%0x1000 == 0);
    
    //Index init - The error spot <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
    heap->index = place_ordered_array( (void*)start, HEAP_INDEX_SIZE, &header_t_less_than);

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ordered_array_t place_ordered_array(void *addr, u32int max_size, lessthan_predicate_t less_than)
{
    ordered_array_t to_ret;
    to_ret.array = (type_t*)addr;
    memset(to_ret.array, 0, max_size*sizeof(type_t));
    to_ret.size = 0;
    to_ret.max_size = max_size;
    to_ret.less_than = less_than;
    return to_ret;
}

I had tested without the ramdisk at lower locations in memory it was fine. This made me conclude it was mapping physical->virtual and found the algorithm was mapping incorrectly at higher addresses 0x1FFxxxxx+. His expects to initialize the heap at a low placement address. I went back and checked and noticed that when mapping phys to virtual, some page frames are not zeroed when they have not yet been modified with the exception of memset 0.. This prevents the tables from being mapped as write and read. This is where I am stuck. Maybe I have completely missed a concept here but I thought I understood the code. The page fault gives a read only indication which has pointed me towards this as being the issue.

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//16MB per 1 - F *16 physical memory - the issue
    u32int mem_end_page = 0x1000000; //FIX ONE
	int i = 0;
	
    nframes = mem_end_page / 0x1000;
    frames = (u32int*)kmalloc(INDEX_FROM_BIT(nframes));
    memset(frames, 0, INDEX_FROM_BIT(nframes));
	
    kernel_directory = (page_directory_t*)kmalloc_a(sizeof(page_directory_t));
    memset(kernel_directory, 0, sizeof(page_directory_t));
    current_directory = kernel_directory;
	
	//map pages in kernel heap area
    for (i = KHEAP_START; i < KHEAP_START+KHEAP_INITIAL_SIZE; i += 0x1000)
        get_page(i, 1, kernel_directory);

	//DEBUG ============================================
	
    //phys->virt
    for(i=0; i<placement_address+0x1000; i+=0x1000)
        alloc_frame( get_page(i, 1, kernel_directory), 0, 0);
	
	//DEBUG ============================================
	
	//Allocate previously mapped pages
    for (i = KHEAP_START; i < KHEAP_START+KHEAP_INITIAL_SIZE; i += 0x1000)
        alloc_frame( get_page(i, 1, kernel_directory), 0, 0);
		
    //handler
    register_interrupt_handler(14, page_fault);

	kout("Kernel directory: ");
	kohex(kernel_directory);
	kochr('\n');
	
    //paging setup
    switch_page_directory(kernel_directory);

	kout("Entering problem state - kheap.c\n");
	
	//Kheap init - THE PROBLEM
    kheap = create_heap(KHEAP_START, KHEAP_START+KHEAP_INITIAL_SIZE, 0xCFFFF000, 0, 0);

Posted: **Tue Oct 07, 2014 8:16 am**

Hey Zz,

I couldn't exactly tell from your description what your exact problem is, but looking at the last code snippet you posted:
You first create a page directory. Then you zero each entry, that is okay.
Then you try to "map pages in kernel heap area" right after that.
Where are your page tables?

The structure must be like this (on 32bits, no PAE):
- a page directory contains 1024 entries of 4 bytes, each containing the phys. address of a page table + flags (or 0 if not existing)
- a page table contains 1024 entries of 4 bytes, each containing the phys. address of a page in memory + flags (or 0 if not existing)

Greets, Max

EDIT: By the way, on your project page I read you're using clang, have you made an actual LLVM cross compiler for this? Does it work out well?

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Trying to implement to ram disk

Trying to implement to ram disk

Re: Trying to implement to ram disk