amiga-bootcamp/01_hardware/aga_a1200_a4000/aga_display_modes.md

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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:

#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):

#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)

#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

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

#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