amiga-bootcamp/06_exec_os/memory_management.md

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Memory Management — AllocMem, FreeMem, MemHeader

Overview

AmigaOS memory management is built directly into exec.library. There is no malloc/free in the OS itself — applications call AllocMem and FreeMem which operate on a linked list of MemHeader regions representing physical RAM.

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MemHeader — Memory Region Descriptor

/* exec/memory.h — NDK39 */
struct MemHeader {
    struct Node  mh_Node;       /* ln_Type=NT_MEMORY, ln_Pri=region priority */
                                /* ln_Name = e.g. "chip memory" */
    UWORD        mh_Attributes; /* MEMF_* flags describing this region */
    struct MemChunk *mh_First;  /* pointer to first free chunk in this region */
    APTR         mh_Lower;      /* lowest byte address of region */
    APTR         mh_Upper;      /* highest byte address + 1 */
    ULONG        mh_Free;       /* total free bytes currently */
};

struct MemChunk {
    struct MemChunk *mc_Next;   /* next free chunk (NULL = end of list) */
    ULONG            mc_Bytes;  /* size of this free chunk in bytes */
};

The OS maintains a doubly-linked list of MemHeader regions at SysBase→MemList. On a stock A1200:

• "chip memory" covering $000000–$1FFFFF (2 MB Chip RAM)
• "fast memory" covering $200000–$9FFFFF (up to 8 MB Fast RAM if fitted)

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MEMF_ Flag Constants

/* exec/memory.h — NDK39 */
#define MEMF_ANY      0L          /* no placement preference */
#define MEMF_PUBLIC   (1L<<0)     /* accessible by all hardware/software */
#define MEMF_CHIP     (1L<<1)     /* must be in Chip RAM (DMA-reachable) */
#define MEMF_FAST     (1L<<2)     /* prefer Fast RAM (CPU-only, faster) */
#define MEMF_CLEAR    (1L<<16)    /* zero-fill the allocation */
#define MEMF_LARGEST  (1L<<17)    /* return single largest free block */
#define MEMF_REVERSE  (1L<<18)    /* allocate from top of list */
#define MEMF_TOTAL    (1L<<19)    /* AvailMem: report total, not largest free */

Chip RAM is required for anything the custom chips DMA from — bitmaps, audio samples, Copper lists, blitter sources/destinations, sprite data. The custom chip DMA controllers cannot reach Fast RAM.

Fast RAM has no DMA contention with the custom chips, making it faster for pure CPU use.

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AllocMem / FreeMem

/* exec/execbase.h — LVO -198 */
APTR AllocMem(ULONG byteSize, ULONG requirements);
/* Returns: pointer to allocated block, or NULL on failure */

/* LVO -210 */
void FreeMem(APTR memoryBlock, ULONG byteSize);

Usage

/* Allocate 512 bytes of Chip RAM, zero-filled: */
UBYTE *buf = AllocMem(512, MEMF_CHIP | MEMF_CLEAR);
if (!buf) { /* handle out-of-memory */ }

/* Free it: */
FreeMem(buf, 512);

Important

FreeMem requires the exact same size as AllocMem. The OS does not store the size internally — you must track it yourself.

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AllocVec / FreeVec (OS 2.0+)

/* LVO -684 (exec.library 36+) */
APTR AllocVec(ULONG byteSize, ULONG requirements);
void FreeVec(APTR memoryBlock);   /* LVO -690 */

AllocVec stores the size in the 4 bytes immediately before the returned pointer, allowing FreeVec to work without a size argument. Prefer this in new code.

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AvailMem — Query Free Memory

/* LVO -216 */
ULONG AvailMem(ULONG requirements);

ULONG chip_free = AvailMem(MEMF_CHIP);
ULONG fast_free = AvailMem(MEMF_FAST);
ULONG total_chip = AvailMem(MEMF_CHIP | MEMF_TOTAL);

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Pool Allocator (OS 3.0+)

For many small allocations, use the pool API which reduces fragmentation:

/* LVO -696 */
APTR pool = CreatePool(MEMF_ANY, 4096, 1024);
/* puddleSize=4096, threshSize=1024 */

APTR p1 = AllocPooled(pool, 32);   /* LVO -702 */
FreePooled(pool, p1, 32);           /* LVO -708 */

DeletePool(pool);                   /* LVO -714 */

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Memory Map (A1200 Example)

Range	Type	Used for
$000000–$000400	Chip	68k exception vectors
$000400–$000BFF	Chip	exec library, SysBase
$000C00–$1FFFFF	Chip	Application allocations, DMA buffers
$200000–$9FFFFF	Fast	Fast RAM expansion (if present)
$A00000–$BFFFFF	Slow/Ranger	A500 slow RAM (not on A1200)
$BFD000–$BFDFFF	CIA	CIA-B registers
$BFE001–$BFEFFF	CIA	CIA-A registers
$C00000–$D7FFFF	Slow	A500 slow RAM expansion
$D80000–$DFFFFF	Custom	Custom chip registers ($DFF000)
$E00000–$E7FFFF	ROM mirror	(A500)
$F80000–$FFFFFF	ROM	Kickstart 3.1 (512 KB)

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References

• NDK39: exec/memory.h, exec/execbase.h
• ADCD 2.1: AllocMem, FreeMem, AllocVec, FreeVec, CreatePool
• 01_hardware/common/address_space.md — full address map
• Amiga ROM Kernel Reference Manual: Exec — memory management chapter