pmm - Physical memory manager

Header: kernel/include/kernel/pmm.h
Source: kernel/arch/i386/pmm.c

Manages the pool of available physical RAM using a flat bitmap. Each bit represents one 4 KiB frame: 1 = used, 0 = free.

Constants

Constant Value Description
PMM_FRAME_SIZE 0x1000 (4096) Size of one physical frame in bytes.
PMM_ALLOC_ERROR 0xFFFFFFFF Sentinel returned by pmm_alloc_frame() when no free frame is available.

Initialisation (pmm_init) 

void pmm_init(uint32_t magic, multiboot2_info_t *mbi);

Build the frame bitmap from the Multiboot 2 memory map tag.

Parameter Description
magic Multiboot 2 magic value passed by the bootloader (0x36D76289). If wrong, no memory is freed.
mbi Pointer to the Multiboot 2 information structure.

Algorithm:

  1. Mark every frame as used (memset(bitmap, 0xFF, ...)).
  2. Validate the Multiboot 2 magic; bail out if incorrect.
  3. Walk the tag list to find the memory map tag (type 6).
  4. For each MULTIBOOT2_MEMORY_AVAILABLE region, round the region boundaries to 4 KiB and clear the corresponding bitmap bits (free).
  5. Re-mark frame 0 (physical address 0x0000–0x0FFF) as used - guards against null-pointer dereferences.
  6. Re-mark all frames occupied by the kernel image (loaded at 1 MiB) as used, using the _kernel_end symbol from the linker script as the upper bound.
  7. Print the free frame count and total free MiB to both the VGA terminal and the serial port.

Functions

pmm_alloc_frame 

uint32_t pmm_alloc_frame(void);

Allocate one physical frame using a linear scan of the bitmap. Returns the physical address of the allocated frame (always a multiple of PMM_FRAME_SIZE), or PMM_ALLOC_ERROR if all frames are in use.

The frame is marked used before the function returns, so concurrent callers (once interrupts are involved) must ensure mutual exclusion at a higher level.

pmm_free_frame 

void pmm_free_frame(uint32_t addr);

Return the frame at physical address addr to the free pool. addr must be a value previously returned by pmm_alloc_frame(); passing an arbitrary address will silently corrupt the bitmap.

pmm_free_count 

uint32_t pmm_free_count(void);

Count and return the number of frames currently in the free pool, using Brian Kernighan’s bit-counting trick on each 32-bit bitmap word.

Bitmap internals

The bitmap covers the entire 32-bit physical address space:

  • 4 GiB / 4 KiB = 1 048 576 frames
  • 1 048 576 / 32 = 32 768 uint32_t words → 128 KiB of BSS

The bitmap is statically allocated in BSS and is therefore zeroed by the runtime startup before kernel_main runs. pmm_init immediately fills it with 0xFF (all used) before selectively freeing usable regions.

Future work

  • Break the 4GB barrier! This isn’t the 2000’s with a Pentium 4 anymore
  • Add a multi-frame contiguous allocator (pmm_alloc_contiguous(n)) for DMA buffers and page-table pools.
  • Expose NUMA zone information if a multi-processor platform is targeted.
  • Track total/used/free frame statistics without a full scan in pmm_free_count.