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Microsoft KB Archive/34221

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IBM PC/XT Motherboard Switch Settings: System Memory PSS ID Number: Q34221 Article last modified on 09-23-1991 PSS database name: PSS

Summary:

The IBM PC/XT series of computers have switches on the motherboard to determine the system configuration. If these switches are set incorrectly, erratic errors can result.

Switches 3 and 4 of Switch Block 1 determine how many banks of motherboard memory are enabled. The exact amount of memory enabled in each case will depend on the maximum motherboard memory.

More Information:

There are three types of motherboards: 16/64K, 64/256K, and 256/640K.

The following are the correct settings:

Memory Banks Active Switch Block 1 Memory Enabled

                 -------------------     16/64K 64/256K 256/640K
                 : 1 2 3 4 5 6 7 8 :
                 :     x x         : on

Bank 1 only : : 16K 64K 256K : : : : off ——————-

                 -------------------
                 : 1 2 3 4 5 6 7 8 :
                 :       x         : on

Banks 1-2 : : 32K 128K 512K : : : x : off ——————-

                 -------------------
                 : 1 2 3 4 5 6 7 8 :
                 :     x           : on

Banks 1-3 : : 48K 192K 576K : : : x : off ——————-

                 -------------------
                 : 1 2 3 4 5 6 7 8 :
                 :                 : on

Banks 1-4 : : 64K 256K 640K : : : x x : off ——————-

On only the PC series (not the XT), there is a second block of eight switches. These must be set to indicate the total amount of memory in the system, including all memory-expansion boards (up to 640K).

Total Memory: SW2-1 SW2-2 SW2-3 SW2-4 SW2-5* 16K ON ON ON ON ON 32K ON ON ON ON ON 48K ON ON ON ON ON 64K ON ON ON ON ON 96K OFF ON ON ON ON 128K ON OFF ON ON ON 160K OFF OFF ON ON ON 192K ON ON OFF ON ON 224K OFF ON OFF ON ON 256K ON OFF OFF ON ON 288K OFF OFF OFF ON ON 320K ON ON ON OFF ON 352K OFF ON ON OFF ON 384K ON OFF ON OFF ON 416K OFF OFF ON OFF ON 448K ON ON OFF OFF ON 480k OFF ON OFF OFF ON 512K ON OFF OFF OFF ON 544K OFF OFF OFF OFF ON 576K ON ON ON ON OFF 608K OFF ON ON ON OFF 640K ON OFF ON ON OFF

  • Some motherboards disregard SW2-5 and thus are limited to 544K.

Switches SW-6, SW-7, and SW-8 always are left in the OFF position.

Copyright Microsoft Corporation 1991.