Untitled_Kernel/src/physmem.c

#include "physmem.h"
#include <stdint.h>
#include <io.h>

uint8_t* pmem_bitmap;
uint32_t bitmap_size;

void free_page(void* addr) {

             //Available. Calculate the offset of the bit on the bitmap based on the page address
            unsigned int pagenumber = (unsigned int)addr / PAGE_SIZE;
            uint8_t* bitmap_byte = pmem_bitmap + (pagenumber / 8); 
            int bitmap_bit = pagenumber % 8;
            
            //ok now set the bit to unused
            *bitmap_byte &= ~(0x80 >> bitmap_bit);

}

unsigned int build_bitmap(mb_mmap_entry_t* mmap, int mmap_size) {
    //rather important global pointer to the bitmap
    pmem_bitmap = (&_kernel_end);
    
    //some variables
    int mmap_end = (uint32_t)mmap + mmap_size;
    unsigned int pages_allocated = 0;    
    uint32_t max_memory = 0;

    //How much memory do we even have? Find the highest usable mmap region, and that'll be our bitmap size.
    for(mb_mmap_entry_t* mmap_walk = mmap;
        (int)mmap_walk < mmap_end;
        mmap_walk = (mb_mmap_entry_t*)((int)mmap_walk + mmap_walk->entry_size + sizeof(mmap_walk->entry_size))) {
       
        if(mmap_walk->type != 1)
            //lol lmao
            continue;

        uint32_t section_upper = (uint32_t)mmap_walk->addr + (uint32_t)mmap_walk->mem_size;
        if(section_upper > max_memory) {
            max_memory = section_upper;
        } 
    }

    //Now that we know the bitmap size, we can define it a bit
    bitmap_size = max_memory / (PAGE_SIZE*8);
    uint32_t bitmap_end = (uint32_t)pmem_bitmap + bitmap_size;
    
    //Now we'll Fill out our bitmap with 1s.
    for(unsigned int i = 0; i < bitmap_size; i++) {
        *(pmem_bitmap + i) = 0xFF;
    }

    //Time to actually fill it out. Iterate the memory map, and for every VALID memory region, set their bits to 0.

    for(;
        (int)mmap < mmap_end;
        mmap = (mb_mmap_entry_t*)((int)mmap + mmap->entry_size + sizeof(mmap->entry_size))) {
        
        //The bottom of our memory, rounded up to the nearest page boundary.
        uint8_t* mem_bottom =(uint8_t*) (((uint32_t)mmap->addr + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1));

        //Is it usable mem?
        if(mmap->type != 1) {
            //nah...
            continue;
        }
   
        //Top of our memory, rounded UP this time.
        uint8_t* mem_top = (uint8_t*) (((uint32_t)mmap->addr + (uint32_t)mmap->mem_size) & ~(PAGE_SIZE - 1));

        //Iterating through our pages in the memory region. Free the ones that meet our criteria.
        int page_count = (mem_top - mem_bottom) / PAGE_SIZE;
        for(int i = 0; i < page_count; i++){
            
            //OK which page is this
            uint8_t* page_addr = mem_bottom + (PAGE_SIZE * i);
           
            //Is it in an invalid region? skip this page if so.
            if((uint32_t)page_addr < bitmap_end) { 
                continue;
            }
            //Looks good! Go ahead and free it.
            free_page((void*)page_addr);
            pages_allocated++;   
        }
    } 
    return pages_allocated;
}