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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.
142 lines
3.5 KiB
Markdown
142 lines
3.5 KiB
Markdown
[← Home](../README.md) · [Exec Kernel](README.md)
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# Message Ports — MsgPort, Message, PutMsg, GetMsg, WaitPort
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## Overview
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AmigaOS inter-task communication uses a **message passing** system. Tasks send `Message` structures to `MsgPort` queues. The receiving task either polls (`GetMsg`) or blocks (`WaitPort`) for incoming messages. No shared memory is touched without the message handshake.
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---
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## Core Structures
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```c
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/* exec/ports.h — NDK39 */
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struct MsgPort {
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struct Node mp_Node; /* ln_Name = port name (for public ports) */
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UBYTE mp_Flags; /* PA_SIGNAL, PA_SOFTINT, PA_IGNORE */
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UBYTE mp_SigBit; /* signal bit used for PA_SIGNAL ports */
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APTR mp_SigTask; /* task to signal on message arrival */
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struct List mp_MsgList; /* queue of pending messages */
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};
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struct Message {
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struct Node mn_Node; /* ln_Type = NT_MESSAGE */
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struct MsgPort *mn_ReplyPort; /* port to send reply to (or NULL) */
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UWORD mn_Length; /* total size of message including header */
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};
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```
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`mp_Flags` values:
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| Value | Constant | Meaning |
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|---|---|---|
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| 0 | `PA_SIGNAL` | Signal `mp_SigTask` when message arrives |
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| 1 | `PA_SOFTINT` | Trigger software interrupt |
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| 2 | `PA_IGNORE` | Do not wake the task (polling only) |
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---
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## Creating a Message Port
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```c
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struct MsgPort *port = CreateMsgPort(); /* exec.library LVO -732 (OS 2.0+) */
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/* or manually for OS 1.x compatibility: */
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struct MsgPort *port = AllocMem(sizeof(struct MsgPort), MEMF_PUBLIC|MEMF_CLEAR);
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port->mp_Node.ln_Type = NT_MSGPORT;
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port->mp_Flags = PA_SIGNAL;
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port->mp_SigBit = AllocSignal(-1); /* any free signal bit */
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port->mp_SigTask = FindTask(NULL); /* signal current task */
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NewList(&port->mp_MsgList);
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```
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---
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## Sending a Message
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```c
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/* PutMsg: add message to queue, signal receiver */
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PutMsg(target_port, (struct Message *)my_msg);
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/* Non-blocking — returns immediately */
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```
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PutMsg can be called from interrupt context.
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---
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## Receiving Messages
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```c
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/* Block until at least one message arrives: */
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WaitPort(my_port);
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/* Then drain the queue: */
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struct MyMsg *msg;
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while ((msg = (struct MyMsg *)GetMsg(my_port)) != NULL) {
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/* process msg */
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ReplyMsg((struct Message *)msg); /* send reply if mn_ReplyPort != NULL */
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}
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```
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### GetMsg (non-blocking poll)
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```c
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struct Message *msg = GetMsg(my_port);
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/* Returns NULL if queue is empty */
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```
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---
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## Public Named Ports
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```c
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/* Register a port so others can find it by name: */
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port->mp_Node.ln_Name = "myapp.port";
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Forbid();
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AddPort(port);
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Permit();
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/* From another task: */
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Forbid();
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struct MsgPort *remote = FindPort("myapp.port");
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Permit();
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if (remote) PutMsg(remote, my_msg);
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/* Cleanup: */
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Forbid();
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RemPort(port);
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Permit();
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```
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`Forbid()` is required around `FindPort`/`AddPort`/`RemPort` to prevent the task list from changing mid-operation.
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---
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## Reply Pattern
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The standard request-reply idiom:
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```c
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/* Sender: */
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my_msg->mn_ReplyPort = reply_port;
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PutMsg(server_port, &my_msg->mn_Message);
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WaitPort(reply_port);
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struct MyMsg *reply = (struct MyMsg *)GetMsg(reply_port);
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/* reply now contains the server's response */
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/* Server: */
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WaitPort(server_port);
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struct MyMsg *req = (struct MyMsg *)GetMsg(server_port);
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/* process req... */
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req->result = 42;
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ReplyMsg(&req->mn_Message); /* sends back to req->mn_ReplyPort */
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```
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---
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## References
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- NDK39: `exec/ports.h`, `exec/messages.h`
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- ADCD 2.1: `CreateMsgPort`, `PutMsg`, `GetMsg`, `WaitPort`, `ReplyMsg`
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- *Amiga ROM Kernel Reference Manual: Exec* — messages and ports chapter
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