Physical Memory Manager

The Physical Memory Manager (PMM) is one of the most foundational components is OS development – it manages actual RAM, not virtual abstractions.

What is the Physical Memory Management?

The Physical Memory Manager (PMM) is responsible for:

• Tracking which parts of RAM are free/used
• Allocating and freeing physical memory blocks (frames/pages)
• Providing memory to:
  • Kernel
  • Virtual Memory Manager (VMM)
  • Devices drivers (DMA, buffers)

The PMM doesn't manage individual bytes of memory, rather it keeps track of pages. A page is a fixed size determined by the MMU: in the case of x86 this is 4096 (0x1000) bytes.

Memory Layout at Boot

When the OS boots, RAM is NOT fully free.

Some regions are already occupied:

• Kernel code/data
• Bootloader
• BIOS/UEFI structures
• Reserved hardware regions

Example Memory Map

From BIOS (e.g., via E820):

0x00000000 - 0x0009FC00  → Usable
0x0009FC00 - 0x000A0000  → Reserved
0x00100000 - 0x7FFFFFFF  → Usable

PMM must:

• Parse this map
• Only manage usable regions

Basic Unit: Frames (Pages)

Physical memory is dividied into fixed-size blocks:

• Usually 4 KB pages
• Called:
  • Frames (physical)
  • Pages (virtual)

Example:

If RAM = 4 GB:

4GB / 4KB = ~1 million frames

Data Structures Used

There are different ways on how to handle a PMM.

This is where design choices matter:

1 Bitmap (Most Common)

Idea:

Each frame = 1 bit

This is the simplest approach: one bit per page frame, where 0 = free and 1 = allocated. For a 4GiB machine (1,048,576 pages), this costs just 128 KB of RAM – negligible.

0 → free
1 → used (allocated)

Example:

Frame:  0 1 2 3 4 5 6 7
Bitmap: 1 0 0 1 1 0 0 0

Allocations:

Allocation is O(n) in the worst case – you must scan until you find a zero bit.

Freeing is O(1): just clear the bit.

• Scan for first 0
• Mark it 1

Pros:

• Very space efficient
• Simple

Cons:

• Slow for large memory (linear scan)

Why Do We Need a Physical Memory Manager?

When your OS boots, memory is:

• Partially used (kernel, BIOS, bootloader)
• Fragmented
• Unstructured

The bootloader gives us a memory map, but that's just information, not a management system.

So, PMM must answer:

• Which pages are free?
• Which pages are reserved?
• How do I allocate contiguous memory?
• How do I avoid overwriting critical regions?

Core Design: Bitmap Allocator

We would use a bitmap-based allocator.

Concept:

Divide memory into fixed-size pages (usually 4 KB):

Physical Memory → [Page0][Page1][Page2]...[PageN]

Then track each page with a bit:

Bitmap → 1 0 1 0 0 1 ...
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         Used/Free