os/cpu/i386/pmem.c

#include <grub/multiboot2.h>
#include "../halt.h"
#include <stdint.h>
#include <stdlib.h>
#include <klog.h>

static char bmap[131072];

static char get_bmap_bit(int index) {
  int byte=index/8;
  int bit=index%8;
  char entry=bmap[byte];
  return (entry&(1<<bit))>0;
}

static void set_bmap_bit(int index) {
  int byte=index/8;
  int bit=index%8;
  bmap[byte]=bmap[byte]|(1<<bit);
}

static void clear_bmap_bit(int index) {
  int byte=index/8;
  int bit=index%8;
  bmap[byte]=bmap[byte]&(~(1<<bit));
}

void pmem_init(struct multiboot_boot_header_tag* tags) {
  for (int i=0;i<131072;i++) {
    bmap[i]=0xFF;
  }
  char found_mmap=0;
  struct multiboot_tag* tag=(struct multiboot_tag*)(tags+1);
  while (tag->type!=0) {
    switch (tag->type) {
      case MULTIBOOT_TAG_TYPE_MMAP: {
        found_mmap=1;
        struct multiboot_mmap_entry* orig_ptr=(struct multiboot_mmap_entry*)(((char*)tag)+16);
        for (struct multiboot_mmap_entry* ptr=orig_ptr;(char*)ptr<((char*)orig_ptr)+tag->size;ptr++) {
          if (ptr->type!=MULTIBOOT_MEMORY_AVAILABLE) continue;
          uint32_t size=ptr->len;
          uint32_t start=ptr->addr;
          if (start<0x100000) continue;
          uint32_t end=start+ptr->len-1;
          if (start&0xFFF) {
            start+=0x1000;
          }
          start=start>>12;
          end=end>>12;
          for (uint32_t i=start;i<end;i++) {
            clear_bmap_bit(i);
          }
        }
        char str[256];
        break;
      }
    }
    tag=(struct multiboot_tag*)((char*)tag+((tag->size+7)&0xFFFFFFF8));
  }
  if (!found_mmap) {
    vga_write_string("[PANIC] No memory map supplied by bootloader!");
    halt();
  }
  for (uint32_t i=0;i<2048;i++) {
    set_bmap_bit(i);
  }
}

void* pmem_alloc(int num_pages) {
  uint32_t bmap_index;
  uint32_t remaining_blks;
  for(uint32_t i=0;i<131072;i++) {
    if (bmap[i]!=0xFF) {
      char got_0=0;
      remaining_blks=num_pages;
      uint32_t old_j;
      for (uint32_t j=i*8;;j++) {
        char bit=get_bmap_bit(j);
        if (got_0) {
          if (bit) {
            if (remaining_blks==0) {
                bmap_index=old_j;
                break;
            } else {
              i+=j/8;
              i--;
              break;
            }
          } else {
            remaining_blks--;
          }
        } else {
          if (!bit) {
            got_0=1;
            old_j=j;
            remaining_blks--;
          }
        }
        if (remaining_blks==0) {
          bmap_index=old_j;
          break;
        }
      }
    }
    if (remaining_blks==0) {
      break;
    }
  }
  if (remaining_blks!=0) {
    return NULL;
  }
  for (int i=0;i<num_pages;i++) {
    set_bmap_bit(bmap_index+i);
  }
  void* addr=(void*)(bmap_index<<12);
  return addr;
}

void pmem_free(int start_page,int num_pages) {
  for (int i=start_page;i<num_pages;i++) {
    set_bmap_bit(i);
  }
}