amiga-bootcamp/08_graphics/ham_ehb_modes.md

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HAM and EHB — Special Display Modes

Overview

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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EHB — Extra Half-Brite (OCS/ECS/AGA)

How It Works

Uses 6 bitplanes (64 possible values):

• Bitplane values 0–31: index into the 32-colour palette normally
• Bitplane values 32–63: display the colour from register (value − 32) at half brightness (all RGB components halved)

Bitplanes 0–4 → 5-bit colour index (0–31)
Bitplane 5    → "half brightness" flag

Effective Result

• 32 programmer-defined colours + 32 fixed half-brightness versions = 64 colours
• Zero additional palette RAM needed
• Useful for shadows and smooth gradients

Enabling EHB

/* In ViewPort ColorMap: */
/* Simply use 6 bitplanes with no HAM flag */
struct BitMap bm;
InitBitMap(&bm, 6, 320, 256);  /* 6 planes */
/* No EXTRA_HALFBRITE flag needed — it's automatic when depth=6 */

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HAM — Hold-And-Modify (OCS/ECS)

How It Works

Uses 6 bitplanes. Each pixel's 6 bits are interpreted as:

Bits 5–4	Meaning	Bits 3–0
00	SET — index colour register	4-bit palette index (0–15)
01	MODIFY BLUE — hold R,G; set B	New blue nibble
10	MODIFY RED — hold G,B; set R	New red nibble
11	MODIFY GREEN — hold R,B; set G	New green nibble

Effective Result

• Each pixel can set one of 16 base colours, OR modify one component of the previous pixel's colour
• Theoretical maximum: 4,096 colours on screen simultaneously
• Practical result: colour fringing at sharp edges (each pixel depends on its left neighbour)

OCS/ECS Limitations

• Only 16 base colours (SET mode uses 4 bits → 16 palette entries)
• Only 4-bit component modification → 16 levels per channel
• Total colour space: 12-bit (4096 colours)

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HAM8 — AGA Enhanced HAM

How It Works

Uses 8 bitplanes:

Bits 7–6	Meaning	Bits 5–0
00	SET — index colour register	6-bit palette index (0–63)
01	MODIFY BLUE	6-bit blue value
10	MODIFY RED	6-bit red value
11	MODIFY GREEN	6-bit green value

Effective Result

• 64 base colours from the 256-entry palette
• 6-bit component modification → 64 levels per channel
• Total colour space: 18-bit (262,144 colours)
• Significantly reduced fringing compared to HAM6

HAM8 Memory Layout

8 bitplanes × 320 pixels × 256 lines = 81,920 bytes per plane × 8
= 655,360 bytes (640 KB) for a single HAM8 320×256 display

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Enabling HAM

/* Via ViewPort: */
vp->Modes |= HAM;  /* HAMF flag in modes */

/* Via SA_ tags (Intuition screen): */
struct TagItem scrTags[] = {
    { SA_Width,      320 },
    { SA_Height,     256 },
    { SA_Depth,      6 },         /* 6 for HAM6, 8 for HAM8 */
    { SA_DisplayID,  HAM_KEY },   /* or SUPER_KEY|HAM for Super-HiRes HAM */
    { TAG_DONE, 0 }
};

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Comparison Table

Feature	EHB	HAM6	HAM8
Bitplanes	6	6	8
Chipset	OCS/ECS/AGA	OCS/ECS/AGA	AGA only
Base palette	32	16	64
Max on-screen colours	64	4,096	262,144
Colour depth	12-bit	12-bit	24-bit (via 18-bit HAM)
Fringing	None	Significant	Mild
Good for	GUI, sprites	Photos, static art	Photos, video stills
Bad for	—	Animation, scrolling	Memory-hungry

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References

• HRM: Display modes chapter
• NDK39: graphics/displayinfo.h — HAM_KEY, EXTRAHALFBRITE_KEY