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151 lines
4.8 KiB
Markdown
151 lines
4.8 KiB
Markdown
[← Home](../README.md) · [Exec Kernel](README.md)
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# Memory Management — AllocMem, FreeMem, MemHeader
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## Overview
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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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---
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## MemHeader — Memory Region Descriptor
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```c
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/* exec/memory.h — NDK39 */
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struct MemHeader {
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struct Node mh_Node; /* ln_Type=NT_MEMORY, ln_Pri=region priority */
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/* ln_Name = e.g. "chip memory" */
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UWORD mh_Attributes; /* MEMF_* flags describing this region */
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struct MemChunk *mh_First; /* pointer to first free chunk in this region */
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APTR mh_Lower; /* lowest byte address of region */
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APTR mh_Upper; /* highest byte address + 1 */
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ULONG mh_Free; /* total free bytes currently */
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};
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struct MemChunk {
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struct MemChunk *mc_Next; /* next free chunk (NULL = end of list) */
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ULONG mc_Bytes; /* size of this free chunk in bytes */
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};
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```
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The OS maintains a doubly-linked list of `MemHeader` regions at `SysBase→MemList`. On a stock A1200:
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- `"chip memory"` covering `$000000–$1FFFFF` (2 MB Chip RAM)
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- `"fast memory"` covering `$200000–$9FFFFF` (up to 8 MB Fast RAM if fitted)
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---
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## MEMF_ Flag Constants
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```c
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/* exec/memory.h — NDK39 */
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#define MEMF_ANY 0L /* no placement preference */
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#define MEMF_PUBLIC (1L<<0) /* accessible by all hardware/software */
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#define MEMF_CHIP (1L<<1) /* must be in Chip RAM (DMA-reachable) */
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#define MEMF_FAST (1L<<2) /* prefer Fast RAM (CPU-only, faster) */
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#define MEMF_CLEAR (1L<<16) /* zero-fill the allocation */
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#define MEMF_LARGEST (1L<<17) /* return single largest free block */
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#define MEMF_REVERSE (1L<<18) /* allocate from top of list */
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#define MEMF_TOTAL (1L<<19) /* AvailMem: report total, not largest free */
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```
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**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.
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**Fast RAM** has no DMA contention with the custom chips, making it faster for pure CPU use.
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---
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## AllocMem / FreeMem
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```c
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/* exec/execbase.h — LVO -198 */
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APTR AllocMem(ULONG byteSize, ULONG requirements);
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/* Returns: pointer to allocated block, or NULL on failure */
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/* LVO -210 */
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void FreeMem(APTR memoryBlock, ULONG byteSize);
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```
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### Usage
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```c
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/* Allocate 512 bytes of Chip RAM, zero-filled: */
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UBYTE *buf = AllocMem(512, MEMF_CHIP | MEMF_CLEAR);
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if (!buf) { /* handle out-of-memory */ }
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/* Free it: */
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FreeMem(buf, 512);
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```
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> [!IMPORTANT]
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> `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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---
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## AllocVec / FreeVec (OS 2.0+)
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```c
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/* LVO -684 (exec.library 36+) */
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APTR AllocVec(ULONG byteSize, ULONG requirements);
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void FreeVec(APTR memoryBlock); /* LVO -690 */
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```
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`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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---
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## AvailMem — Query Free Memory
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```c
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/* LVO -216 */
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ULONG AvailMem(ULONG requirements);
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```
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```c
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ULONG chip_free = AvailMem(MEMF_CHIP);
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ULONG fast_free = AvailMem(MEMF_FAST);
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ULONG total_chip = AvailMem(MEMF_CHIP | MEMF_TOTAL);
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```
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---
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## Pool Allocator (OS 3.0+)
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For many small allocations, use the pool API which reduces fragmentation:
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```c
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/* LVO -696 */
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APTR pool = CreatePool(MEMF_ANY, 4096, 1024);
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/* puddleSize=4096, threshSize=1024 */
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APTR p1 = AllocPooled(pool, 32); /* LVO -702 */
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FreePooled(pool, p1, 32); /* LVO -708 */
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DeletePool(pool); /* LVO -714 */
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```
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---
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## Memory Map (A1200 Example)
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| Range | Type | Used for |
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|---|---|---|
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| `$000000–$000400` | Chip | 68k exception vectors |
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| `$000400–$000BFF` | Chip | exec library, SysBase |
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| `$000C00–$1FFFFF` | Chip | Application allocations, DMA buffers |
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| `$200000–$9FFFFF` | Fast | Fast RAM expansion (if present) |
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| `$A00000–$BFFFFF` | Slow/Ranger | A500 slow RAM (not on A1200) |
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| `$BFD000–$BFDFFF` | CIA | CIA-B registers |
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| `$BFE001–$BFEFFF` | CIA | CIA-A registers |
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| `$C00000–$D7FFFF` | Slow | A500 slow RAM expansion |
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| `$D80000–$DFFFFF` | Custom | Custom chip registers ($DFF000) |
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| `$E00000–$E7FFFF` | ROM mirror | (A500) |
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| `$F80000–$FFFFFF` | ROM | Kickstart 3.1 (512 KB) |
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---
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## References
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- NDK39: `exec/memory.h`, `exec/execbase.h`
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- ADCD 2.1: `AllocMem`, `FreeMem`, `AllocVec`, `FreeVec`, `CreatePool`
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- [address_space.md](../01_hardware/common/address_space.md) — full address map
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- *Amiga ROM Kernel Reference Manual: Exec* — memory management chapter
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