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Thread: Multiple Graphics Cards in IBM PC

  1. #1

    Default Multiple Graphics Cards in IBM PC

    I recently bought an IBM PC AT 5170 from eBay along with a 5153 CGA monitor. The PC only came with a VGA card, so I have been using a modern LCD with it. However, I just got a Color Graphics Adapter to use the 5153 with the PC. It works great, but when I have both the VGA and the CGA cards in the PC only the CGA card outputs a blinking cursor, and the PC never boots. It will boot with only one card at a time. How could I have both cards in the PC at once and have it still boot? I would like to be able to switch between CGA and VGA by unplugging and plugging in the cables for each. Is this possible? I could install a new BIOS if that is necessary.

  2. #2


    I think you can use a VGA card and a CGA card in the same machine if the VGA card is configured for monochrome output. That causes it to use the memory range 0xa0000-0xb7fff and ports 0x3b0-0x3cf, avoiding the CGA memory range 0xb8000-0xbffff and ports 0x3d0-0x3df. If the VGA card is configured for colour output then the addresses will collide and no software or BIOS change will be able to fix that. The exact details of how the VGA card is configured for mono or colour depends on the specific card. There may be a DIP switch or it may automatically sense based on the current drawn on the lines of the VGA connector.

    Unfortunately most VGA software (and games in particular) will only support VGA when configured for colour output, so (depending on what you want to do with the VGA card) you may be out of luck. To enable/disable an ISA card like you really want to do would probably require a somewhat invasive motherboard modification or moving the cards to some kind of external enclosure.

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