This topic discusses the switches as defined for IBM logoed PCs only.  On
PC-compatible computers, there may be more or fewer switches and they may
have different meanings.

On the original PC (64K byte motherboard; sometimes called the PC-1),
there are two DIP switch banks labeled SW1 and SW2.  This is a summary of
switch meanings:
SW1                                SW 2  (PC-1)
ON  1 2 3 4 5 6 7 8                    ON  1 2 3 4 5 6 7 8 
                                                          
                                                        
                 7-8: diskette drives                    5-8: always OFF
                 5-6: active display                     1-4: total RAM
                 3-4: system board RAM
                   2: 8087 coprocessor
                   1: diskette drives

A later version of the PC (sometimes         SW 2  (PC-2)
called the PC-2) has a different BIOS  ON  1 2 3 4 5 6 7 8 
which interprets the switches somewhat                     
differently.  SW1 is the same but SW2                      
differs as shown:                                           6-8: always OFF
              1-5: total RAM

On PCs and XTs, you can change the effect of the switch settings by
changing the Equipment List bytes in BIOS Data Area 0:0410, then rebooting
DOS.  First store address 0:0472 with the value 1234H, then do an INT 19H.

  PC and XT SW1: Diskette Drives  
This indicates how many diskette drives are attached to the system.
SW1
ON  1 2 3 4 5 6 7 8 
                    1 diskette drive  (Note: position 1 is ON if 0 drives)
                 
ON  1 2 3 4 5 6 7 8 
                    2 diskette drives
                 
ON  1 2 3 4 5 6 7 8 
                    3 diskette drives
                 
ON  1 2 3 4 5 6 7 8 
                    4 diskette drives
                 

  PC and XT SW1: Active Monitor  
These switches tell the system which display adapter is to be used when
the system is turned on.  If the PC displays nothing after power-up, it
may be because of an incorrect switch setting.
SW1
ON  1 2 3 4 5 6 7 8 
                    None or EGA
                 
ON  1 2 3 4 5 6 7 8 
                    40x25 Color/Graphics Adapter (CGA)
                 
ON  1 2 3 4 5 6 7 8 
                    80x25 Color/Graphics Adapter (CGA)
                 
ON  1 2 3 4 5 6 7 8 
                    Monochrome Adapter (MDA) or both MDA and CGA
                 

  PC and XT SW1: 8087  
This switch enables the 8087 Coprocessor to interrupt CPU (via INT 02H
Non-Maskable Interrupt) when it encounters an exception condition (as in
division by 0).  When OFF, software should be installed to trap INT 02H
and determine if the interrupt was caused by a memory parity error or an
8087 exception condition.
SW1
ON  1 2 3 4 5 6 7 8 
                    8087 is installed
                 
ON  1 2 3 4 5 6 7 8 
                    8087 is NOT installed
                 

  PC SW1: System board RAM  
These switches should both be OFF on systems that have 64K of memory or
more.
SW1
ON  1 2 3 4 5 6 7 8 
                    64K or more on the system board
                 

  PC-1 SW2: System board RAM  
These settings are for the original PC (64K maximum on the system board)
which does not have a later version of the ROM-BIOS; i.e., it does not
have ROM-Scan and does not connect to an expansion unit or have a
self-booting hard disk.  The maximum settings for this system is 544K.

SW2  (PC-1)
ON  1 2 3 4 5 6 7 8      Note: on PC-1, SW2 positions 5-8 are always OFF
                    64K
                 
ON  1 2 3 4 5 6 7 8                                                        
                    128K   The settings reflect a binary number of 32K    
                         blocks where an OFF is a 1-bit.  To set the    
ON  1 2 3 4 5 6 7 8         switches without using a chart, just remember: 
                    192K                                                  
                             Position 4 = 256Ks  (ON=0, OFF=1)          
ON  1 2 3 4 5 6 7 8             Position 3 = 128Ks                         
                    256K       Position 2 =  64Ks                         
                             Position 1 =  32Ks                         
ON  1 2 3 4 5 6 7 8                                                        
                    320K   Add them up, then add 64K (system board RAM)   
                         to get the total system memory.                
ON  1 2 3 4 5 6 7 8                                                        
                    384K
                 
ON  1 2 3 4 5 6 7 8 
                    448K
                 
ON  1 2 3 4 5 6 7 8 
                    512K
                 
ON  1 2 3 4 5 6 7 8 
                    544K
                 

  PC-2 SW2: System board RAM  
These settings are for a later version of the IBM-PC (sometimes called the
PC-2) which is capable of having 256K on the system board.  These setting
also apply to the PC-1 (64K system board) if a later version of the
ROM-BIOS has been installed.  If you have a self-booting hard disk or an
IBM expansion chassis, you have PC-2 ROMS.  The maximum memory for this
model is 640K.

SW2  (PC-2)
ON  1 2 3 4 5 6 7 8       Note: on PC-2, SW2 positions 6-8 are always OFF
                    64K
                 
ON  1 2 3 4 5 6 7 8                                                        
                    128K   The settings reflect a binary number of 32K    
                         blocks where an OFF is a 1-bit.  To set the    
ON  1 2 3 4 5 6 7 8         switches without using a chart, just remember: 
                    192K                                                  
                             Position 5 = 512Ks  (ON=0, OFF=1)          
ON  1 2 3 4 5 6 7 8             Position 4 = 256Ks                         
                    256K       Position 3 = 128Ks                         
                             Position 2 =  64Ks                         
ON  1 2 3 4 5 6 7 8             Position 1 =  32Ks                         
                    320K                                                  
                         Add them up, then add 64K to get the total RAM 
ON  1 2 3 4 5 6 7 8                                                        
                    384K
                 
ON  1 2 3 4 5 6 7 8 
                    448K
                 
ON  1 2 3 4 5 6 7 8 
                    512K
                 
ON  1 2 3 4 5 6 7 8 
                    576K
                 
ON  1 2 3 4 5 6 7 8 
                    640K
                 

See Also: Equipment List
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PC Switch Settings