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AST RESEARCH TECHNICAL BULLETIN TB-0003A 4-30-84
MEMORY SWITCH ERROR IN IBM GUIDE
TO OPERATIONS FOR IBM PORTABLE PC
When extending the memory size of the IBM Portable PC beyond the
128K minimum capacity of the system motherboard, the procedure is
essentially the same as that for the XT.
Additional memory may be installed on the I/O bus without first
fully populating the motherboard. However, it is necessary to
indicate the amount of motherboard memory installed via switches
3 and 4 of the system motherboard switch block.
Unfortunately, page 4-81 of the IBM Guide to Operations Manual
erroneously shows that the switch position for 256K on the
motherboard is switch 3 off and switch 4 on. This is actually
the setting for 128K of RAM.
The following is the proper settings for switches 3 and 4 of the
system motherboard switch block.
AMOUNT OF MEMORY SWITCH SWITCH
INSTALLED ON MOTHERBOARD 3 4
128K OFF ON
192K ON OFF
256K (fully populated) OFF OFF