Multi-Color Graphics Array

Model 30 Video Interface Schematic (page 7 physical)
Model 25 Technical Reference (Video Subsystem - page 49 physical)

MCGA Components
MCGA Registers


The MCGA "Multi-Color Graphics Array" subsystem is present on the planar board of the original Models 30 and 25. It was never made available in form of an adapter card. The only known "clone" that implemented MCGA was Epson Equity Ie.

Feature-wise it's a superset of the older CGA "Graphics Adapter" standard, but it uses analog video output (VGA compatible) and adds two new video modes:

  • 320 x 200 graphics (APA) with 256 colors
  • 640 x 480 graphics (APA) with 2 colors

Note: MCGA does NOT provide compatibility with EGA "Enhanced Graphics Adapter".

MCGA Components

The MCGA subsystem consists of:

  • Video Memory Controller gate array (72X8300)
  • Video Formatter gate array (72X8205)
  • 64 KB of multi-port video DRAM
  • 8 KB of SRAM for character generator
  • 256 x 18-bit color palette with three 6-bit DACs (Inmos IMSG171 RAMDAC)
  • Support ("glue") logic

MCGA Block Diagram

Video Memory Controller

P/N: 72X8300 (old) or 11F8028 (new)
Gate Array: Seiko-Epson SLA6430J

Die Photo (21808x21778px! 154 MB) online zoomable version Courtesy of John McMaster.

Video Formatter

P/N: 72X8205
Gate Array: Seiko-Epson SLA6330J or IBM GA (early samples only?)

MCGA Registers

The Video Memory Controller gate array responds to I/O addresses 304h and 305h.
The Video Formatter gate array responds to I/O addresses 308h through 30Fh.
The color palette is programmed through the video formatter at addresses 3C6h through 3C9h.

See the Technical Reference (page 57 physical) for more information.

MCGA and VGA Design Teams

niksgarage said:
  I was working in Boca in 1985-1987 .. happy times. ... MCGA and VGA were two design teams working in different buildings, separated by a parking lot but shared a cafeteria. One interesting feature was MCGA used a VRAM design, which was a little more sophisticated than VGA's plain ol' DRAM design.

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