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.

01_hardware/aga_a1200_a4000/aga_display_modes.md

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+[← Home](../../README.md) · [Hardware](../README.md) · [AGA](README.md)
+
+# AGA Display Modes
+
+## Overview
+
+AGA introduces several new display modes and extends existing ones, enabled via the OS **screenmode** system (`graphics/modeid.h`) and `OpenScreenTags()`.
+
+## Mode ID System
+
+AmigaOS uses 32-bit **ModeIDs** to identify display modes. The format is:
+```
+[Monitor ID (16 bits)] | [Mode flags (16 bits)]
+```
+
+Key AGA monitor IDs:
+```c
+#define PAL_MONITOR_ID      0x00000000
+#define NTSC_MONITOR_ID     0x00010000
+#define A2024_MONITOR_ID    0x00020000
+#define VGA_MONITOR_ID      0x00031000  /* ECS productivity */
+#define A2024_10HZ_ID       0x00040000
+#define DBLPAL_MONITOR_ID   0x00420000  /* AGA double PAL */
+#define DBLNTSC_MONITOR_ID  0x00400000  /* AGA double NTSC */
+#define SUPER72_MONITOR_ID  0x00080000
+```
+
+Mode flags (lower 16 bits):
+```c
+#define LORES_KEY    0x0000  /* 320 wide (PAL) */
+#define HIRES_KEY    0x8000  /* 640 wide */
+#define HAM_KEY      0x0800  /* HAM mode */
+#define EXTRAHALFBRITE_KEY 0x0080 /* EHB */
+#define LACE_KEY     0x0004  /* interlace */
+```
+
+## Standard AGA Modes
+
+| Mode ID | Resolution | Colours | H rate |
+|---|---|---|---|
+| `PAL_MONITOR_ID \| LORES_KEY` | 320×256 | 256 | 15.6 kHz |
+| `PAL_MONITOR_ID \| HIRES_KEY` | 640×256 | 256 | 15.6 kHz |
+| `PAL_MONITOR_ID \| HIRES_KEY \| LACE_KEY` | 640×512 | 256 | 15.6 kHz (interlace) |
+| `DBLPAL_MONITOR_ID \| LORES_KEY` | 320×512 | 256 | 31.25 kHz |
+| `DBLPAL_MONITOR_ID \| HIRES_KEY` | 640×512 | 256 | 31.25 kHz |
+| `PAL_MONITOR_ID \| LORES_KEY \| HAM_KEY` | 320×256 | 262,144 (HAM8) | 15.6 kHz |
+| `SUPER72_MONITOR_ID \| HIRES_KEY` | 800×600 (approx) | 256 | 28+ kHz |
+
+## Opening an AGA Screen (OS 3.1)
+
+```c
+#include <intuition/screens.h>
+#include <graphics/modeid.h>
+#include <proto/intuition.h>
+
+struct Screen *scr;
+
+/* 256-colour AGA screen, PAL, 320×256 */
+scr = OpenScreenTags(NULL,
+    SA_DisplayID,  PAL_MONITOR_ID | LORES_KEY,
+    SA_Width,      320,
+    SA_Height,     256,
+    SA_Depth,      8,           /* 8 bitplanes = 256 colours */
+    SA_Colors32,   (ULONG)colour_table,  /* LoadRGB32 format */
+    SA_Title,      (ULONG)"My AGA Screen",
+    SA_Quiet,      TRUE,
+    TAG_DONE);
+```
+
+## HAM8 Screen
+
+```c
+scr = OpenScreenTags(NULL,
+    SA_DisplayID,  PAL_MONITOR_ID | LORES_KEY | HAM_KEY,
+    SA_Width,      320,
+    SA_Height,     256,
+    SA_Depth,      8,
+    TAG_DONE);
+```
+
+> [!NOTE]
+> HAM8 screens require 8 bitplanes. The display system automatically programmes BPLCON0 with HAM=1. The first 64 colour registers are used as the HAM8 index palette.
+
+## BestModeID() — Querying Available Modes
+
+```c
+#include <proto/graphics.h>
+
+ULONG modeid = BestModeID(
+    BIDTAG_NominalWidth,  640,
+    BIDTAG_NominalHeight, 512,
+    BIDTAG_Depth,         8,
+    BIDTAG_MonitorID,     DBLPAL_MONITOR_ID,
+    TAG_DONE);
+
+if (modeid == INVALID_ID) {
+    /* Requested mode not available */
+}
+```
+
+## Double Scan Modes (DblPAL / DblNTSC)
+
+`DBLPAL` and `DBLNTSC` modes use AGA's scan doubler to produce non-interlaced 31 kHz output from PAL/NTSC timing:
+
+- DblPAL: 320×512 or 640×512, 31.25 kHz — compatible with VGA monitors
+- DblNTSC: 320×400 or 640×400, 31.47 kHz
+
+These require a multisync monitor and AGA chipset. The A1200 can drive a 1084S in scan-doubled mode.
+
+## Super72 Mode
+
+Super72 provides approximately 800×600 resolution at ~28 kHz horizontal:
+- Used by some Workbench productivity configurations
+- Requires multisync monitor
+- Available via `SUPER72_MONITOR_ID`
+
+## References
+
+- NDK39: `graphics/modeid.h` — all monitor and mode ID definitions
+- ADCD 2.1: `Libraries_Manual_guide/` — graphics.library OpenScreen
+- Autodocs: `BestModeID`, `OpenScreenTags`
+- AmigaMail Vol. 2 — AGA display mode programming