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Comprehensive technical documentation covering: - Hardware: OCS/ECS/AGA custom chip registers, Copper & Blitter deep dives - Boot sequence: cold boot through startup-sequence - Binary format: HUNK executable spec, relocation, debug info - Linking & ABI: .fd files, LVO tables, register calling conventions - Exec kernel: tasks, interrupts, memory, signals, semaphores - AmigaDOS: file I/O, FFS/OFS layout, CLI/Shell scripting - Graphics: planar bitmaps, Copper programming, HAM/EHB modes - Intuition: screens, windows, IDCMP, BOOPSI - Devices: trackdisk, SCSI, serial, timer, audio, keyboard - Libraries: utility, expansion, IFFParse, locale, ARexx - Networking: bsdsocket API, SANA-II, TCP/IP stack comparison - Toolchain: GCC, vasm/vlink, SAS/C, NDK, debugging - Reverse engineering: IDA/Ghidra setup, compiler fingerprints, case studies - CPU & MMU: 68040/060 emulation libs, PMMU, cache management - Driver development: SANA-II, Picasso96/RTG, AHI audio All files include breadcrumb navigation. No local paths or proprietary content.
113 lines
3.3 KiB
Markdown
113 lines
3.3 KiB
Markdown
[← Home](../README.md) · [Exec Kernel](README.md)
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# Semaphores — SignalSemaphore, ObtainSemaphore, Shared/Exclusive
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## Overview
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Semaphores are the AmigaOS mechanism for **mutual exclusion and shared-read access** to resources. Unlike `Forbid()` (which blocks all scheduling), semaphores allow other tasks to run while waiting — the waiting task simply sleeps until the resource is available.
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---
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## struct SignalSemaphore
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```c
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/* exec/semaphores.h — NDK39 */
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struct SignalSemaphore {
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struct Node ss_Link; /* ln_Type = NT_SIGNALSEM */
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/* ln_Name = semaphore name (public) */
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WORD ss_NestCount; /* how many times THIS task has obtained it */
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struct MinList ss_WaitQueue;/* tasks waiting for exclusive access */
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struct SemaphoreRequest ss_MultipleLink; /* shared-reader slot */
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struct Task *ss_Owner; /* task holding exclusive lock (or NULL) */
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WORD ss_QueueCount; /* number of waiters */
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};
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```
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---
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## Initialising a Semaphore
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```c
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/* Stack or AllocMem — always initialise before use: */
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struct SignalSemaphore sem;
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InitSemaphore(&sem); /* LVO -558 */
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/* Public (named) semaphore — so other tasks can find it: */
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sem.ss_Link.ln_Name = "myapp.lock";
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AddSemaphore(&sem); /* LVO -564 */
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/* Later: */
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RemSemaphore(&sem); /* LVO -570 */
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```
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---
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## Exclusive (Write) Lock
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```c
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/* Block until this task holds the semaphore exclusively: */
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ObtainSemaphore(&sem); /* LVO -534 */
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/* --- critical section: only one task in here at a time --- */
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ReleaseSemaphore(&sem); /* LVO -546 */
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```
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### Non-Blocking Try
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```c
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/* Returns TRUE if obtained, FALSE if someone else holds it: */
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if (AttemptSemaphore(&sem)) { /* LVO -540 */
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/* got it */
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ReleaseSemaphore(&sem);
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} else {
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/* resource busy — do something else */
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}
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```
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---
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## Shared (Read) Lock
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Multiple tasks may hold a shared lock simultaneously. An exclusive lock blocks until all shared holders release.
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```c
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ObtainSemaphoreShared(&sem); /* LVO -768 */
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/* --- read-only access: multiple tasks may be here at once --- */
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ReleaseSemaphore(&sem); /* same release for both modes */
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```
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---
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## Nesting
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Semaphores are **reentrant** — the same task can call `ObtainSemaphore` multiple times. The `ss_NestCount` tracks how many times the current owner has obtained it. `ReleaseSemaphore` must be called the same number of times.
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```c
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ObtainSemaphore(&sem); /* NestCount = 1 */
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ObtainSemaphore(&sem); /* NestCount = 2 — safe, same task */
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ReleaseSemaphore(&sem); /* NestCount = 1 */
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ReleaseSemaphore(&sem); /* NestCount = 0 — fully released, waiters wake */
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```
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---
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## Semaphore vs Forbid/Disable
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| Mechanism | Blocks | Other tasks run while waiting? | Interrupt safe? |
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|---|---|---|---|
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| `Forbid()` | All task switching | ❌ No | ✅ (interrupts still run) |
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| `Disable()` | All task switching + interrupts | ❌ No | ✅ |
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| `ObtainSemaphore()` | Only contending tasks | ✅ Yes | ❌ Not from interrupt context |
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Use semaphores for anything that may take more than a few microseconds. Use `Forbid()` only for very short list manipulations.
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---
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## References
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- NDK39: `exec/semaphores.h`
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- ADCD 2.1: `InitSemaphore`, `ObtainSemaphore`, `ObtainSemaphoreShared`, `ReleaseSemaphore`, `AttemptSemaphore`
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- *Amiga ROM Kernel Reference Manual: Exec* — semaphores chapter
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