ECS Productivity & Multiscan Display Modes

Overview

ECS introduces BEAMCON0 which allows the Amiga to produce non-standard display timings. The most useful are productivity mode (~28 kHz / 31 kHz horizontal) and multiscan mode, which provide flicker-free, high-resolution displays compatible with standard SVGA monitors.

These modes are available on A3000 and some A2000/A600 configurations with a multisync monitor.

Standard OCS Timings (for reference)

Mode	H rate	V rate	Resolution
PAL LORES	15.625 kHz	50 Hz	320×256
PAL HIRES	15.625 kHz	50 Hz	640×256
NTSC LORES	15.720 kHz	60 Hz	320×200
NTSC HIRES	15.720 kHz	60 Hz	640×200
PAL interlace	15.625 kHz	50 Hz (25 Hz/field)	320×512 / 640×512

ECS Multiscan Modes

Mode	H rate	V rate	Resolution	BEAMCON0
Productivity	~28.6 kHz	57 Hz	640×480	$0A00
Super72	~28.6 kHz	72 Hz	800×600 (approx)	varies
DblPAL	31.25 kHz	50 Hz	640×512	custom
DblNTSC	31.47 kHz	60 Hz	640×400	custom
VGA-like	31.47 kHz	60 Hz	640×480	custom

Programming Productivity Mode

Productivity mode on the A3000 (PAL, 640×480):

; Set BEAMCON0 for variable beam timing
move.w  #$0A00, $DFF1DC    ; VARBEAMEN | VARVSYEN

; Program horizontal total, sync, blank (custom timing)
move.w  #$71, $DFF1C0      ; HTOTAL (horizontal total - 1)
move.w  #$0F, $DFF1C4      ; HSSTRT (H sync start)
move.w  #$19, $DFF1C6      ; HSSTOP (H sync stop)
move.w  #$09, $DFF1C8      ; HBSTRT (H blank start)
move.w  #$71, $DFF1CA      ; HBSTOP (H blank stop)

; Vertical timing
move.w  #$0242, $DFF1E0    ; VTOTAL
move.w  #$0015, $DFF1E6    ; VSSTRT
move.w  #$001D, $DFF1E8    ; VSSTOP

[!NOTE] The exact register values depend on the target monitor's sync requirements. The A3000's monitor (1084S or Commodore 1950) has a specific timing window. Always consult the monitor's datasheet.

OS Support: ScreenModes

AmigaOS 3.1 integrates ECS productivity modes through the screenmode system:

#include <graphics/modeid.h>
#include <libraries/asl.h>

/* Open a 640×480 productivity screen */
struct Screen *scr = OpenScreenTags(NULL,
    SA_DisplayID,  PRODDBL_MONITOR_ID | HIRES_KEY,
    SA_Width,      640,
    SA_Height,     480,
    SA_Depth,      4,
    TAG_DONE);

Key mode IDs for ECS:

#define MULTISCAN_MONITOR_ID  0x00041000  /* multiscan / productivity */
#define SUPER72_MONITOR_ID    0x00081000
#define DBLNTSC_MONITOR_ID    0x00401000
#define DBLPAL_MONITOR_ID     0x00421000

These IDs are returned by BestModeID() and accepted by OpenScreen() / OpenScreenTags().

Hardware Requirements

ECS chipset (Super Agnus + ECS Denise) — required for BEAMCON0
Multisync monitor — standard 15 kHz PAL/NTSC monitors do not support 31 kHz
A3000 — has built-in multiscan support; A2000 requires a separate scan doubler card

Flicker Fixer (A2000/A500)

Some Zorro II cards (e.g., Flicker Fixer by MicroWay, Indivision ECS) scan-double the 15 kHz signal to 31 kHz for use with VGA monitors. These operate transparently — no BEAMCON0 programming needed.

References

ADCD 2.1 Hardware Manual — ECS display modes
AmigaMail Vol. 2 — ECS multiscan articles
NDK39: graphics/modeid.h — monitor mode IDs
A3000 Technical Reference Manual — display timing chapter