Segment Heap Internals is a deep technical course that takes you through every layer of the Segment Heap, from the root _SEGMENT_HEAP structure to the individual allocation paths that serve requests of different sizes. You will learn how the heap routes allocations, how its four subsystems work internally, and how to examine all of this in a debugger.

What You Will Learn:

• Introduction: What the Segment Heap is, why it replaced the NT Heap, and a high-level overview of its four-component architecture with size-based allocation routing.
• Core Structures: The root _SEGMENT_HEAP structure, heap creation and initialization (including the 0xDDEEDDEE signature), and the encoding and security mechanisms that protect allocation metadata.
• Backend Allocator: The foundational segment allocator that manages large virtual memory regions. Covers the two _HEAP_SEG_CONTEXT entries (small and large segments), _HEAP_PAGE_SEGMENT layout, page range descriptors, and free/coalescing behavior.
• Variable Size (VS) Allocator: How allocations up to 128 KB are served using VS subsegments carved from the backend. Covers VS chunk headers, the balanced free-tree for best-fit allocation, and splitting/coalescing operations.
• Low Fragmentation Heap (LFH): The bucket-based allocator for high-frequency small allocations. Covers activation on the 17th allocation, size-class buckets, bitmap-based block tracking, headerless allocations, and per-CPU affinity slots.
• Large Allocations: How allocations exceeding approximately 8 MB bypass all subsystems and go directly to NtAllocateVirtualMemory, tracked in a dedicated metadata tree.