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IBM 256KB Memory Expansion Option  -  Switches


Switches 1 to 4

Switches 1 to 4 control the starting address of the RAM.

Starting address	Switch settings	Comment
64 KB	1=on, 2=on, 3=on, 4=off
128 KB	1=on, 2=on, 3=off, 4=on
192 KB	1=on, 2=on, 3=off, 4=off
256 KB	1=on, 2=off, 3=on, 4=on
320 KB	1=on, 2=off, 3=on, 4=off
384 KB	1=on, 2=off, 3=off, 4=on
448 KB	1=on, 2=off, 3=off, 4=off	Not applicable if more than 192 KB is enabled on the card  (because conventional memory past 640K would be created).
512 KB	1=off, 2=on, 3=on, 4=on	Not applicable if more than 128 KB is enabled on the card  (because conventional memory past 640K would be created).
576 KB	1=off, 2=on, 3=on, 4=off	Not applicable if more than 64 KB is enabled on the card  (because conventional memory past 640K would be created).

How were those calculated ?
It's an inverted binary sequence based on the fact that switches 1 to 4 monitor address lines A19 to A16 respectively.

1. Divide by 64 KB
2. Convert result to binary
3. Prefix binary with additional zeroes to make 4 digits
4. Invert it

Example: 192 KB --> 3 --> 11 binary --> 0011 --> 1100 (on/on/off/off)


Switches 5 to 8

Switches 5 to 8 selectively disable/enable the four modules of RAM.
In other words, how much of the card's RAM is enabled.

Enabled modules	Switch settings	Comment
Module 0 only	5=on, 6=off, 7=off, 8=off	One module enabled = 64 KB of RAM
Modules 0/1 only	5=off, 6=on, 7=off, 8=off	Two modules enabled = 128 KB of RAM
Modules 0/1/2 only	5=off, 6=off, 7=on, 8=off	Three modules enabled = 192 KB of RAM
All 4 modules	5=off, 6=off, 7=off, 8=on	Four modules enabled = 256 KB of RAM