Sigma Designs Port Devices Driver

Posted By admin On 12/10/21

The Color 400 is a graphics card that was designed to compete with the EGA. It provides the features of a CGA card, with theaddition of three new video modes: 80×30 text, 80×50 text and 640×400 graphics in 16 colours.

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Table of contents

  • Memory


The card seems to have gone through various iterations of hardware design. Pictures of early versions show a full-length ISA card, whileTotal Hardware 99 shows a more compact half-length design.


According to TH99, the address ranges used by the card can be configuredwith DIP switches. However the BIOS in mine is hardcoded to use one specificset of addresses, so that's what I will use in the remainder of this document.

Along with the DIP switches, the card is equipped with two ROMs (one for the text-mode font, one for the BIOS), a 9-pin video connector, and a Bergconnector for a light pen.

The card has two 8k ROM chips; one contains 8×8 and 8×16 fontsfor text modes, while the other contains 6k of BIOS code and an 8×16 font for the 640×400 graphics mode.

Text-mode font ROMGraphics-mode font in BIOS


Where a normal CGA would have its I/O ports in the 3D0-3DF range, the Color 400 uses the 2D0-2DF range. I/O accesses to 3D0-3DF can trigger a non-maskable interrupt; this is handled by the card's BIOS and translated toaccesses in the 2D0-2DF range.

I/O ports are:

The BIOS does not appear to attempt to emulate the CGA 'blink' / 'brightbackground' toggle, so I don't know if the card supports this function.


The video RAM is arranged as four 32k planes at 0B8000h. In text modes, plane 0 contains the characters and attributes in their usual layout. The way the graphics modes work is slightly more complex.

320×200 mode

This is notionally a 4-colour mode. In fact it's a 16-colour mode, drawingits data from planes 2 and 3:

When selecting the mode, the BIOS writes all ones to plane 3, and sendsmemory accesses to plane 2. The palette registers are programmed so that the pixel patterns map to CGA-compatible colours as they should.

640×200 mode

On a CGA, this is a monochrome mode. On the Sigma 400, it's actually a 16-colour mode; each pixel draws its colour from all four planes(bit 0 from plane 0, bit 1 from plane 1 etc.)

When the BIOS selects this mode, it clears all four planes and sendsmemory accesses to plane 2. Set pixels will therefore display in colour 4,so the palette is programmed to make colour 4 the foreground colour.

A program aware of the Sigma 400 can use this mode as a 16-colour mode byprogramming a suitable palette and accessing the plane select register.

Sigma Designs Port Devices Driver

640×400 mode

The memory layout of this mode is identical to the 'AT&T' mode found on some CGA cards, or in Hercules monochrome mode:

Sigma Designs Port Devices Driver Downloads

  • Lines 0,4,8,12... are at B800:0000
  • Lines 1,5,9,13... are at B800:2000
  • Lines 2,6,10,14... are at B800:4000
  • Lines 3,7,11,15... are at B800:6000

The arrangements for memory planes are identical to the 640×200 mode. By default the BIOS selects plane 1 (green) so that set pixels display in green on black. As with the 640×200 mode, full colour support ispossible by programming a suitable palette and using the plane select register; this is what the Windows 1.03 drivers SIGMA4.DRV and SIGMA8.DRV do.


The card BIOS supports the following interrupts:


This checks to see whether the NMI was triggered by a write to ports03D0-03DF; if so, it performs the appropriate translation to the card's actual video hardware.

INT 8 (timer)

This is hooked if the processor is 286 or higher. If so, it will pollthe I/O channel check bit (port 61h bit 6 on ISA systems, port 62h bit 6on PC/XT type systems) and if it's set simulate an NMI.

INT 9 (keyboard)

This resets bit 5 of port 02D9h (so the card will not trigger a NMI), calls the original INT 9 handler, and restores port 02D9h to its originalvalue.

INT 10h (video)

The card's video BIOS provides INT 10h functions 00h-13h; some are implemented by passing the call on to the motherboard's CGA BIOS. Functionsworth noting are:

INT 10h/AH=0 (set mode)

Three additional modes are supported: 40h (80×30 text), 41h (80×50 text) and 42h (640×400 graphics).

INT 10h/AH=10h (palette functions)

Subfunctions supported are 0 (set single palette register) and 2 (set all palette registers). There is no support for setting the bordercolour.

INT 10h/AH=8Fh (installation check)

This returns:

INT 1Fh (graphics table):

This is expanded. Where a normal CGA BIOS would expect 1k of data for a 128-character 8×8 font, the Sigma 400 expects 3k of data: 128 8×8 characters, followed by the same characters at 8×16 resolution.

BIOS parameter table

At C000:000C is a pointer to a 5-word table:

The tables are:

Each mode setup table is 32 bytes long:

For text modes the palette table is 64 bytes, containing four sets of identical palette register values.

For 4-colour CGA mode the palette table is 128 bytes, corresponding tothe possible values of the CGA mode control and colour select registers:

  • Black / Red / Green / Brown
  • Black / Red / Green / Yellow (high intensity)
  • Black / Magenta / Cyan / Grey
  • Black / Magenta / Cyan / White (high intensity)
  • Black / Red / Cyan / Grey
  • Black / Red / Cyan / White (high intensity)
  • Black / Red / Cyan / Grey
  • Black / Red / Cyan / White (high intensity)


Sigma Designs Port Devices Driver Download

The Windows 1.03 driver pack (V1DSK.ZIP / V1DSK2.ZIP at includestwo drivers for this card: 8-colour and 4-colour. The latter is, as far asI know, the only 4-colour driver for any version of Windows 1.x:

For GEM / ViewMAX, the latest FreeGEM driver pack (version 3.14.12) contains three suitable drivers:

Sigma Designs Port Devices Driver Update

  • SDSIG9.EGA: 640×400 in 16 colours.
  • SDS2G9.EGA: 640×200 in 16 colours.
  • SDCAT9.EGA: 640×400 in monochrome.

Sigma Designs Port Devices Drivers

John Elliott16 October 2018