5  Memory Allocator: Implementation & Algorithms

This tour examines z_zone.c, Doom’s zone memory allocator. The tour covers the heap container definition, zone initialization, memory allocation and deallocation operations, block splitting and merging, and heap management utilities for debugging and maintenance.

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5.0.1 📄 File: z_zone.c (lines 46–60)

memzone_t tracks memory allocation with a size counter, block list, and search pointer. mainzone is the global instance.

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5.0.2 📄 File: z_zone.h (lines 58–66)

Z_ClearZone resets a zone to contain a single free block, with rover pointing to its start.

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5.0.3 📄 File: z_zone.c (lines 67–86)

Z_Init obtains heap memory from I_ZoneBase and sets up a single free block spanning the entire space.

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5.0.4 📄 File: z_zone.c (lines 93–116)

Tags control how long blocks stay allocated.

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Next we’ll look at Z_Malloc, which handles memory allocation requests in Doom’s zone allocator.

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5.0.5 📄 File: z_zone.h (line 36)

PU_STATIC marks a block as static for the program’s duration — never purged.

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5.0.6 📄 File: z_zone.h (line 43)

Z_Malloc searches from mainzone->rover for a free block, purging blocks with tag ≥ PU_PURGELEVEL or skipping others.

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When a block has enough extra space (>64 bytes), Z_Malloc splits it into an allocated block and a new free block.

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5.0.7 📄 File: z_zone.c (lines 195–203)

Z_Malloc marks the found block as in use, optionally sets a back-pointer, and moves rover for the next allocation.

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5.0.8 📄 File: z_zone.c (lines 205–244)

Z_Free returns memory by marking a block as free and merging with neighbors. Validates ZONEID, clears ownership, and ensures proper linkage.

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5.0.9 📄 File: z_zone.c (lines 247–265)

Frees all memory blocks whose tags fall within the given range.

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5.0.10 📄 File: z_zone.c (lines 267–286)

Prepares to inspect the heap and perform maintenance/debugging operations.

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5.0.11 📄 File: z_zone.c (lines 122–172)

Z_DumpHeap prints diagnostic info for memory blocks within a tag range.

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5.0.12 📄 File: z_zone.c (lines 296–317)

Traverses the block list to print details and verify integrity invariants.

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Z_FileDumpHeap writes the heap’s block list and integrity results to a file.

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5.0.13 📄 File: z_zone.c (lines 326–334)

Z_CheckHeap checks block list structure and usage correctness.

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5.0.14 📄 File: z_zone.c (lines 339–359)

Changes a block’s tag after confirming memory safety and purge eligibility.

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5.0.15 📄 File: z_zone.c (lines 366–392)

Z_FreeMemory sums all unused and purgeable memory blocks to estimate allocatable bytes.

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5.0.16 📄 File: z_zone.c (lines 399–419)

And that’s it! You’ve explored Doom’s memory management system from structures to strategies and the tools that keep it stable.