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IBM 64/256KB 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).  See note 1 below.
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).  See note 1 below.
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).  See note 1 below.


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 banks of RAM.
In other words, how much of the card's RAM is enabled.

Enabled banks Switch settings     Comment
Bank 0 only 5=on, 6=off, 7=off, 8=off  One bank enabled = 64 KB of RAM
Banks 0/1 only 5=off, 6=on, 7=off, 8=off  Two banks enabled = 128 KB of RAM
Banks 0/1/2 only 5=off, 6=off, 7=on, 8=off  Three banks enabled = 192 KB of RAM
All 4 banks 5=off, 6=off, 7=off, 8=on  Four banks enabled = 256 KB of RAM





Note 1 Putting RAM into the 640 KB to 1 MB address space is an advanced subject.
Do the research.
E.g. Will the RAM conflict with something else?
E.g. Do the parity bits need to be set?
E.g. xxxxxxxxxxxx
E.g. xxxxxxxxxxxx