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docs(amiga): complete AmigaOS 3.1/3.2 developer reference — 172 files across 17 sections
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.
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Ilia Sharin
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06_exec_os/tasks_processes.md
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06_exec_os/tasks_processes.md
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[← Home](../README.md) · [Exec Kernel](README.md)
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# Tasks and Processes — Structures, States, Scheduling
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## Overview
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AmigaOS uses **cooperative/preemptive** scheduling. Tasks are the fundamental unit of execution; Processes are Tasks with an additional DOS environment (message port, CLI, segment list). The scheduler runs at each quantum (50 Hz VBL interrupt) and after any `Signal()` or `Wait()` call.
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---
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## struct Task
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```c
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/* exec/tasks.h */
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struct Task {
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struct Node tc_Node; /* ln_Type=NT_TASK or NT_PROCESS */
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/* ln_Pri = scheduling priority */
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/* ln_Name = task name string */
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UBYTE tc_Flags; /* TF_LAUNCH, TF_STRIKE, TF_EXCEPT */
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UBYTE tc_State; /* TS_RUN, TS_READY, TS_WAIT, TS_EXCEPT */
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BYTE tc_IDNestCnt; /* interrupt disable nesting */
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BYTE tc_TDNestCnt; /* task disable (Forbid) nesting */
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ULONG tc_SigAlloc; /* allocated signal bits mask */
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ULONG tc_SigWait; /* signals task is waiting for */
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ULONG tc_SigRecvd; /* signals received */
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ULONG tc_SigExcept; /* exception signals */
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/* ... stack bounds, context, exception handler ... */
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APTR tc_SPLower; /* lowest valid stack address */
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APTR tc_SPUpper; /* highest valid stack address + 2 */
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APTR tc_SPReg; /* saved stack pointer (when not running) */
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};
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```
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---
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## struct Process (extends Task)
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```c
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/* dos/dosextens.h */
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struct Process {
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struct Task pr_Task; /* embedded Task */
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struct MsgPort pr_MsgPort; /* I/O message port */
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UWORD pr_Pad;
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BPTR pr_SegList; /* BPTR to segment list */
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LONG pr_StackSize;
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APTR pr_GlobVec; /* BCPL global vector */
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LONG pr_TaskNum; /* CLI task number */
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BPTR pr_StackBase; /* base of stack (BPTR) */
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LONG pr_Result2; /* secondary result */
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BPTR pr_CurrentDir; /* current directory lock */
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BPTR pr_CIS; /* current input stream */
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BPTR pr_COS; /* current output stream */
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APTR pr_ConsoleTask;
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APTR pr_FileSystemTask;
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BPTR pr_CLI; /* CLI structure (NULL if WB) */
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...
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struct MsgPort *pr_ReturnAddr; /* return address for CLI tasks */
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APTR pr_PktWait;
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struct SaveMsg pr_ExitData;
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UBYTE *pr_Arguments; /* argument string */
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struct MinList pr_LocalVars; /* local shell variables */
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ULONG pr_ShellPrivate;
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BPTR pr_CES; /* current error stream */
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};
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```
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---
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## Task States
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| State | Value | Meaning |
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|---|---|---|
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| `TS_INVALID` | 0 | Not a valid task |
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| `TS_ADDED` | 1 | Just added, not yet scheduled |
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| `TS_RUN` | 2 | Currently running (only one task) |
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| `TS_READY` | 3 | On the `TaskReady` list, waiting for CPU |
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| `TS_WAIT` | 4 | Blocked on `Wait()` — on `TaskWait` list |
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| `TS_EXCEPT` | 5 | Handling an exception |
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| `TS_REMOVED` | 6 | Removed from scheduling |
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---
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## Scheduling: Priority-Based Round Robin
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The scheduler (`exec.library` internal) picks the highest-priority task from `SysBase→TaskReady`. Among equal-priority tasks, they get equal time slices (round-robin).
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- Default priority: 0
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- Range: −128 to +127 (higher = more CPU)
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- OS tasks run at priority 10–20
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- Input handler: priority 20
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- Disk tasks: priority 10
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```c
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SetTaskPri(FindTask(NULL), 5); /* raise current task to priority 5 */
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```
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---
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## Creating Tasks and Processes
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```c
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/* Simple task (exec level): */
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struct Task *t = AllocMem(sizeof(struct Task), MEMF_PUBLIC|MEMF_CLEAR);
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t->tc_Node.ln_Name = "MyTask";
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t->tc_Node.ln_Pri = 0;
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t->tc_SPLower = stack;
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t->tc_SPUpper = stack + stacksize;
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t->tc_SPReg = (APTR)((ULONG)stack + stacksize);
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AddTask(t, myTaskFunc, NULL);
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/* DOS Process (with message port, filesystem access): */
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struct Process *p = CreateNewProcTags(
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NP_Entry, myFunc,
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NP_Name, "MyProcess",
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NP_StackSize, 8192,
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NP_Priority, 0,
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TAG_DONE);
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```
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---
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## Task State Machine
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```mermaid
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stateDiagram-v2
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[*] --> READY : AddTask()
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READY --> RUN : Scheduler picks task
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RUN --> READY : Quantum expired or higher-priority task
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RUN --> WAIT : Wait(signal_mask)
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WAIT --> READY : Signal() delivers awaited signals
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RUN --> EXCEPT : Exception signal received
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EXCEPT --> RUN : Exception handler returns
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RUN --> [*] : Task function returns / RemTask()
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```
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---
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## FindTask and Task Identity
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```c
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struct Task *me = FindTask(NULL); /* NULL = current task */
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printf("Running as: %s\n", me->tc_Node.ln_Name);
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/* Check if we are a Process (vs plain Task): */
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if (me->tc_Node.ln_Type == NT_PROCESS) {
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struct Process *pr = (struct Process *)me;
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/* access pr_CLI, pr_MsgPort, etc. */
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}
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```
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---
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## References
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- NDK39: `exec/tasks.h`, `dos/dosextens.h`
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- ADCD 2.1: `AddTask`, `RemTask`, `FindTask`, `SetTaskPri`, `CreateNewProc`
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- *Amiga ROM Kernel Reference Manual: Exec* — tasks and scheduling chapter
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