8088/80188 Address Pins to Memory Map/ Address help

[80186 Enthusiast]

Hello! I need some help. The screenshot below is from a YouTube Tutorial made by Slador. In this chart, when A19 goes low, that represents Addresses 0x7FFFF through 0x00000. However, I'm considering making my own Address Decoder that uses some of the Address Pins.

Is there a chart that shows all possible 20 address bits and all of the addresses they relate too?

Any help and advice will be thanked and appreciated in advance!

 

[daver2]

It would be a very long chart!

You just use the digits 0, 1 and X (meaning logic '0', logic '1' and 'don't care') and design your logic from that.

Or backwards, by specifying the address range you require and work out from that the logic.

For example, if you want to decode addresses 0x00000 to 0x03FFF. Convert from hex to binary:

00000 = 0000 0000 0000 0000 0000
03FFF = 0000 0011 1111 1111 1111

Anywhere there is a 0 and 1 - call it X:

0000 00XX XXXX XXXX XXXX

The right hand bit is address line A0 and the left hand bit is address line A19. So you need to decode A19 down to A14 being low.

You repeat this process for all of the address ranges required.

Once you have done that, you may be able to merge common terms together to minimise the logic required.

Dave