Parent

IBM 128KB/640KB Memory Expansion Option  -  Switches


Switch 8

Per the 'Memory Address Switches' section of IBM's technical document for the card, switch 8 must always be set to OFF.


Switches 6 and 7

Per the 'Memory Address Switches' section of IBM's technical document for the card.

Bank/s enabled	Switch settings	Comment
Bank 0 only	6=on, 7=off	Bank 0 is the 512 KB sized bank (single row at top of card)
Bank 1 only	6=off, 7=on	Bank 1 is the 128 KB sized bank
Both banks	6=on, 7=on

Switches 1 to 5

Switches 1 to 5 control the starting address of the 512 KB sized bank, bank 0.
They are only used if bank 0 is enabled.

Starting address	Switch settings	Comment
1 MB	1=on, 2=on, 3=on, 4=off, 5=on
1.5 MB	1=on, 2=on, 3=on, 4=off, 5=off
2 MB	1=on, 2=on, 3=off, 4=on, 5=on
2.5 MB	1=on, 2=on, 3=off, 4=on, 5=off
3 MB	1=on, 2=on, 3=off, 4=off, 5=on
3.5 MB	1=on, 2=on, 3=off, 4=off, 5=off
4 MB	1=on, 2=off, 3=on, 4=on, 5=on
4.5 MB	1=on, 2=off, 3=on, 4=on, 5=off
5 MB	1=on, 2=off, 3=on, 4=off, 5=on
5.5 MB	1=on, 2=off, 3=on, 4=off, 5=off
6 MB	1=on, 2=off, 3=off, 4=on, 5=on
6.5 MB	1=on, 2=off, 3=off, 4=on, 5=off
7 MB	1=on, 2=off, 3=off, 4=off, 5=on
7.5 MB	1=on, 2=off, 3=off, 4=off, 5=off
8 MB	1=off, 2=on, 3=on, 4=on, 5=on
8.5 MB	1=off, 2=on, 3=on, 4=on, 5=off
9 MB	1=off, 2=on, 3=on, 4=off, 5=on
9.5 MB	1=off, 2=on, 3=on, 4=off, 5=off
10 MB	1=off, 2=on, 3=off, 4=on, 5=on
10.5 MB	1=off, 2=on, 3=off, 4=on, 5=off
11 MB	1=off, 2=on, 3=off, 4=off, 5=on
11.5 MB	1=off, 2=on, 3=off, 4=off, 5=off
12 MB	1=off, 2=off, 3=on, 4=on, 5=on
12.5 MB	1=off, 2=off, 3=on, 4=on, 5=off
13 MB	1=off, 2=off, 3=on, 4=off, 5=on
13.5 MB	1=off, 2=off, 3=on, 4=off, 5=off
14 MB	1=off, 2=off, 3=off, 4=on, 5=on
14.5 MB	1=off, 2=off, 3=off, 4=on, 5=off
15 MB	1=off, 2=off, 3=off, 4=off, 5=on
15.5 MB	1=off, 2=off, 3=off, 4=off, 5=off	Invalid selection.  Card would conflict with motherboard's BIOS ROM's.

How were those calculated ?
It's an inverted binary sequence based on the fact that switches 1 to 5 monitor address lines A23 to A19 respectively.

1. Divide by 0.5 MB
2. Convert result to binary
3. Prefix binary with additional zeroes to make 5 digits
4. Invert it

Example: 6.5 MB --> 13 --> 1101 binary --> 01101 --> 10010 (on/off/off/on/off)