Page tables

Memory management and isolation

Page tables are a core part of OS memory management. They provide a mapping from virtual addresses to physical memory while enforcing isolation and protection. They use a hierarchical multi-level structure (e.g., a four-level page table for 48-bit x86-64 addresses), so that only needed portions are allocated; at the lowest level of this hierarchy, each entry maps a single physical page. This design lets the kernel efficiently manage memory (mapping, swapping, or sharing pages) and gives each process a separate address space, isolating processes from each other.

Permission bits in page table entries (present, read/write, user/supervisor, execute-disable) ensure one process or ring cannot access or execute another’s memory; violations trigger page faults, preserving memory safety and containment.

Implementation differences and protections

Apple’s XNU and Microsoft Windows rely on page tables for these purposes, but they implement some protections differently. Each employs address space layout randomization (ASLR) to randomize where code and data are loaded in memory (Windows introduced ASLR in Vista, and macOS added full kernel ASLR by OS X 10.8). Windows uses a four-level page table on x86-64 and leverages hardware features like the NX bit (marking data pages as non-executable). It also enables Supervisor Mode Execution Prevention (SMEP) and Access Prevention (SMAP) to block the kernel from executing or even reading user-space memory unless explicitly allowed. macOS similarly marks writable pages as non-executable and, on Intel platforms, takes advantage of SMEP/SMAP for kernel-user isolation.

On Apple Silicon ( #ARM64e ), it uses analogous ARMv8 features (PXN to prevent privileged code execution of user pages, and PAN to prevent unintended access) and introduces pointer authentication codes (PACs) to detect and thwart unexpected pointer modifications. Additionally, XNU goes further by running critical page-table updates in a privileged Page Protection Layer; essentially a “kernel within the kernel” that prevents even a compromised kernel from arbitrarily modifying page tables or code.

Security implications and threat prevention

From a security engineering perspective, these page-table mechanisms underpin process isolation and memory safety on both platforms. A compromise in one process generally cannot directly affect others or the kernel without bypassing these memory protections. Memory corruption exploits are likewise constrained: an injected code payload will be stopped by non-executable memory (NX/DEP) enforcement, and a kernel exploit that attempts to execute user-space shellcode will be blocked by SMEP on Windows or the analogous PXN on ARM.

Windows, on the other hand, relies on ASLR and the above hardware policies to prevent straightforward memory corruption attacks, so focus unusual memory access or pages with changed permissions. In both cases, page-table enforcement helps in preventing process escapes and kernel compromises.