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Miscellaneous Video Cards

Notes on various vintage video cards, where I don't have enough information for a full writeup (perhaps because I don't have the card, or I don't have the right monitor).

Orchid Graphics Adapter

A very early monochrome graphics card for the PC. This information is derived from the GSX-86 1.1 driver for the card.

Tecmar Graphics Master

The Graphics Master is a card that attempts to combine CGA and MDA functionality. It can either be installed as the only card in a system, or alongside an existing CGA or MDA card (or both).

DIP switch settings are:

Configuration JP7JP1/AJP1/BJP1/CJP1/D JP1/EJP1/FSW1
Graphics Master is primary / only display, CGA monitor OpenClosedClosedClosedOpen OpenOpenDown
Graphics Master is primary / only display, MDA monitor ClosedClosedClosedClosedOpen OpenOpenUp
CGA is primary display, Graphics Master uses MDA monitor ClosedOpenClosedClosedOpen OpenOpenUp
MDA is primary display, Graphics Master uses CGA monitor OpenClosedOpenClosedOpen OpenOpenDown
Graphics Master (using CGA monitor) installed alongside CGA and MDA ClosedOpenOpenClosedOpen OpenOpenDown
Graphics Master (using MDA monitor) installed alongside CGA and MDA ClosedOpenOpenClosedOpen OpenOpenUp

Writing to port 03DAh controls the card's extra features:

Bit 7: Reverse the meaning of the 'bright' bit (in text & graphics modes)
Bit 6: Unknown
Bit 5: Set for 400-scanline modes?
Bit 4: Moves screen left (used for modes that are 720 rather than 640 pixels
Bit 3: Enable 128k framebuffer, 4-way interleave
         (eg: With framebuffer at 0xA000, first line comes from 0xA000, 
              second from 0xA800, third from 0xB000, fourth from 0xB800). 
         Done in text mode, this gets you something resembling the 160x100x16
         mode, except it's 160x200.

Bits 2-0: Framebuffer base address. In text modes bit 0 appears to be ignored.
            111 => 0xA4000            
            110 => 0xA0000
            101 => 0xAC000
            100 => 0xA8000
            011 => 0xB4000
            010 => 0xB0000
            001 => 0xBC000
            000 => 0xB8000

The file tecmar.cfg at gives the register values to write to the card for a 16-colour 640×400 video mode.

Vermont Microsystems IM-1024

The IM-1024 operates in the same way as the IBM PGC, but with a 1024×800 resolution and an extended command set. What makes it, as far as I know, almost unique is that there is a Windows 1.0x driver for it that supports 256 colours — see V1DSK.ZIP.

There appears to be an alternative method of sending data to the card, selected by setting the byte at 0xC6330 is set to 1. In this mode, the host calculates the maximum number of bytes it can write as (0xFF - [byte at 0xC6331]). It then writes up to that number of bytes to 0xC6000.

The IM-1024 is able to draw polygons with more than 255 vertices. The Windows driver does this by creating a command list containing two POLY commands in succession. When the command list is executed, the two lists are combined and a single polygon is drawn.

The Windows driver uses some standard PGC commands (such as MOVE, COLOR and DRAW) but coordinates are passed as 2-byte integers rather than the PGC's usual 4-byte fixed-point fractions. It also sends a number of commands not in the PGC's repertoire. The extended command set includes:

Command (ASCII mode)Command byte (HEX mode) Parameters Notes
BLKMOV (BK)DF(6 words) A screen-to-screen blit. The first four words define the rectangle to copy; the last two give its new location.
BLKRD (BKR)DA(4 words) Screen-to-memory blit. The four words give the opposite corners of a rectangle; all bytes in that rectangle are returned to the caller. Unlike IMAGER, the result is not RLE-compressed.
IMGSIZ (IS)4Ewidth,height, unknown (3 words)Sent at initialisation. Followed by three words; the first two look like they define the framebuffer width and height, but I don't know what the third does.
LINFUN (LF)EBmode (1 byte) LINFUN is present on the PGC but only supports modes 0 (paint) or 1 (invert). The IM-1024 also accepts 2 (XOR) and 3 (AND).
LUT8 (L8)E6 ink, r, g, b (4 bytes) Define an ink in the palette using 8-bit RGB (the PGC uses 4-bit RGB). This is quite a common extension among PGC clones.
LUT8RD (L8RD)53ink (1 byte) Read back a palette register and return its 8-bit R/G/B values.
PAN (PA)B7x,y (words) Sets the origin of the viewable part of the screen. For a 1024×800 display with a 1024×1024 framebuffer, the values passed are (0,-112). This seems to cause the bottom 800 lines to be displayed; that is, -112 corresponds to an offset of 224 lines.
PLINE (PL)361 byte count, then 2 * count wordsDraw a polyline linking the specified points.
PLINER (PLR)37 Presumably as PLINE, but with coordinates relative to the current drawing position.
READP (RP)55noneRead the pixel at the current location. Returns a single byte giving the colour.

There is a similar card, the IM-640, which does not appear to have a Windows 1.x driver.

John Elliott 27 January 2023