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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.
134 lines
3.8 KiB
Markdown
134 lines
3.8 KiB
Markdown
[← Home](../README.md) · [Graphics](README.md)
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# HAM and EHB — Special Display Modes
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## Overview
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The Amiga offers two unique display modes that squeeze many more colours from limited bitplane hardware: **EHB** (Extra Half-Brite) and **HAM** (Hold-And-Modify). These modes have no direct equivalent on other platforms and are critical for understanding Amiga graphics capability.
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---
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## EHB — Extra Half-Brite (OCS/ECS/AGA)
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### How It Works
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Uses **6 bitplanes** (64 possible values):
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- Bitplane values 0–31: index into the 32-colour palette normally
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- Bitplane values 32–63: display the colour from register (value − 32) at **half brightness** (all RGB components halved)
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```
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Bitplanes 0–4 → 5-bit colour index (0–31)
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Bitplane 5 → "half brightness" flag
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```
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### Effective Result
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- 32 programmer-defined colours + 32 fixed half-brightness versions = **64 colours**
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- Zero additional palette RAM needed
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- Useful for shadows and smooth gradients
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### Enabling EHB
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```c
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/* In ViewPort ColorMap: */
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/* Simply use 6 bitplanes with no HAM flag */
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struct BitMap bm;
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InitBitMap(&bm, 6, 320, 256); /* 6 planes */
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/* No EXTRA_HALFBRITE flag needed — it's automatic when depth=6 */
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```
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---
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## HAM — Hold-And-Modify (OCS/ECS)
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### How It Works
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Uses **6 bitplanes**. Each pixel's 6 bits are interpreted as:
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| Bits 5–4 | Meaning | Bits 3–0 |
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|---|---|---|
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| `00` | **SET** — index colour register | 4-bit palette index (0–15) |
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| `01` | **MODIFY BLUE** — hold R,G; set B | New blue nibble |
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| `10` | **MODIFY RED** — hold G,B; set R | New red nibble |
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| `11` | **MODIFY GREEN** — hold R,B; set G | New green nibble |
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### Effective Result
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- Each pixel can set one of 16 base colours, OR modify one component of the previous pixel's colour
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- Theoretical maximum: **4,096 colours** on screen simultaneously
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- Practical result: colour fringing at sharp edges (each pixel depends on its left neighbour)
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### OCS/ECS Limitations
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- Only 16 base colours (SET mode uses 4 bits → 16 palette entries)
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- Only 4-bit component modification → 16 levels per channel
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- Total colour space: 12-bit (4096 colours)
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---
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## HAM8 — AGA Enhanced HAM
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### How It Works
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Uses **8 bitplanes**:
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| Bits 7–6 | Meaning | Bits 5–0 |
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|---|---|---|
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| `00` | **SET** — index colour register | 6-bit palette index (0–63) |
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| `01` | **MODIFY BLUE** | 6-bit blue value |
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| `10` | **MODIFY RED** | 6-bit red value |
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| `11` | **MODIFY GREEN** | 6-bit green value |
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### Effective Result
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- 64 base colours from the 256-entry palette
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- 6-bit component modification → 64 levels per channel
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- Total colour space: **18-bit** (262,144 colours)
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- Significantly reduced fringing compared to HAM6
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### HAM8 Memory Layout
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```
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8 bitplanes × 320 pixels × 256 lines = 81,920 bytes per plane × 8
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= 655,360 bytes (640 KB) for a single HAM8 320×256 display
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```
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---
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## Enabling HAM
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```c
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/* Via ViewPort: */
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vp->Modes |= HAM; /* HAMF flag in modes */
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/* Via SA_ tags (Intuition screen): */
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struct TagItem scrTags[] = {
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{ SA_Width, 320 },
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{ SA_Height, 256 },
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{ SA_Depth, 6 }, /* 6 for HAM6, 8 for HAM8 */
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{ SA_DisplayID, HAM_KEY }, /* or SUPER_KEY|HAM for Super-HiRes HAM */
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{ TAG_DONE, 0 }
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};
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```
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---
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## Comparison Table
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| Feature | EHB | HAM6 | HAM8 |
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|---|---|---|---|
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| Bitplanes | 6 | 6 | 8 |
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| Chipset | OCS/ECS/AGA | OCS/ECS/AGA | AGA only |
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| Base palette | 32 | 16 | 64 |
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| Max on-screen colours | 64 | 4,096 | 262,144 |
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| Colour depth | 12-bit | 12-bit | 24-bit (via 18-bit HAM) |
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| Fringing | None | Significant | Mild |
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| Good for | GUI, sprites | Photos, static art | Photos, video stills |
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| Bad for | — | Animation, scrolling | Memory-hungry |
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---
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## References
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- HRM: *Display modes* chapter
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- NDK39: `graphics/displayinfo.h` — `HAM_KEY`, `EXTRAHALFBRITE_KEY`
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