Memory Management

NØNOS implements a complete memory management subsystem.

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Memory Layout

Virtual Address Space (x86_64)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
0xFFFFFFFF_FFFFFFFF ┐
                    │ Kernel Space
0xFFFFFFFF_80000000 ┤ (Higher Half)
                    │
                    ├─ .text    (RX)  Code
                    ├─ .rodata  (RO)  Constants
                    ├─ .data    (RW)  Initialized data
                    ├─ .bss     (RW)  Zero-initialized
                    ├─ Heap     (RW)  Dynamic
                    └─ Stacks   (RW)  Per-task

0x00007FFF_FFFFFFFF ┐
                    │ User Space
0x00000000_00000000 ┘ (Reserved for future)

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Physical Memory

Frame Allocator

• Manages 4KB physical frames

• Bitmap tracks allocation state

• Initialized from UEFI memory map

Memory Regions

Types from UEFI memory map:

Type	Usage
Conventional	Available for allocation
Reserved	Hardware reserved
ACPI	ACPI tables
Framebuffer	GPU memory

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Virtual Memory

Paging Structure

x86_64 4-level paging:

CR3 → PML4 → PDPT → PD → PT → Physical Frame
       │       │     │    │
      512    512   512  512 entries each

Page Sizes

Size	Use Case
4KB	Default
2MB	Large mappings
1GB	Huge mappings

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Page Flags

Flag	Meaning
Present	Page is mapped
Writable	Write access allowed
User	User-mode accessible
No-Execute	Code execution forbidden

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Heap Allocator

Implementation

• Linked-list allocator

• #[global_allocator] attribute

• Thread-safe with spinlock

Allocation Sizes

Request	Strategy
< 4KB	Heap allocator
≥ 4KB	Direct frame allocation

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Stack Management

Kernel Stacks

• 64KB per task

• Guard pages for overflow detection

• Grows downward

Stack Layout

┌─────────────────┐ High address
│   Guard Page    │ (Unmapped)
├─────────────────┤
│                 │
│   Stack Space   │
│                 │
├─────────────────┤
│   Guard Page    │ (Unmapped)
└─────────────────┘ Low address

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Security Features

W^X Enforcement

No page is both writable and executable:

• Code pages: RX (read + execute)

• Data pages: RW (read + write)

• Rodata: RO (read only)

KASLR

Kernel Address Space Layout Randomization:

• Randomized kernel base address

• Randomized stack locations

• Uses hardware entropy (RDRAND)

Guard Pages

Unmapped pages to catch:

• Stack overflow

• Buffer overflows

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Memory APIs

Kernel Allocation

// Allocate memory
let ptr = alloc::alloc::alloc(layout);

// Deallocate
alloc::alloc::dealloc(ptr, layout);

Page Mapping

// Map a virtual address to physical
memory::map_page(virt_addr, phys_addr, flags);

// Unmap
memory::unmap_page(virt_addr);

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Performance

Operation	Time
Page fault	~1μs
Allocation (small)	~100ns
TLB flush	~50 cycles