View Full Version : ISA slot memory mapping

December 1st, 2008, 11:40 AM
I just decided to put in and try a VGA adapter in my PC-XT.

However i have no idea about if each ISA slot have dedicated memory space. If the VGA card require i use special memory adresses (ISA slot).
If the OS DOS 6.22 look for VGA card in a special memory space etc.

My computer have a vertical add onboard mounted on motherboard with 4 8-bit isa slot. I did put the VGA board in the second slot from top and put on the pc, the CGA/MGA 9-pin adapter still show picture. Is this normal or will it switch to VGA once VGA drivers detected in DOS.

I do not dare to put my LCD into the socket quite yet i will try get hold of an older monitor.

But if anyone have configuration tips regarding ISA slot, motherboard jumpers, VGA-card switches and eventual DOS configuration of VGA space, i am all open ears.


Terry Yager
December 1st, 2008, 11:56 AM
ISA slots are all created equal...except for slot #8, which has a bug, but that's a whole 'nother thread. It's just a plain-jane backplane, a VGA card should work fine in any slot 1 - 7. Just be sure the mainboard switch(es) are set for color.


December 1st, 2008, 12:25 PM
ISA slots are all created equal...except for slot #8, which has a bug, but that's a whole 'nother thread.


Wouldn't call it a BUG, but... That's for another thread.

For the VGA, at boot, it will most of the time emulate the CGA 80*25 Texmode (nearly all BIOSes boots in this mode unless set to other). However, DOS doesn't do autodetect of display device, and it'll just keep using the mode BIOS left controll in.

So as of the XT:
1.If you set the switches to MDA, you will have to boot on the MDA monitor and then use the "MODE" command to swich to CGA 80*25 textmode.
2.If you set the switches to CGA 40*25 textmode, you will problably not get too good results with a VGA (IDK if any VGA cards support this mode, however, if it doesn't work, use the "MDA" technique in step 1 to swich to VGA).
3.If you set the switches to CGA 80*25 Texmode, you will be just fine with a VGA. No need for other CGA or MDA cards then. since this mode is emulated by most, if not all VGA cards.

Make sure to DISCONNECT the MDA monitor before booting in a CGA mode. It's for the monitor's safety, as of it really doesn't handle CGA scanrates. If you use a CGA monitor and a VGA monitor on the same time, you will problably get the same output on them.