PC Switch Settings

 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 12345678                   ON 12345678
 ^                                ^        ܳܳܳܺ
   Ľ                      Ľ
      ˼ ˼ >7-8: diskette drives  ͼ > 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 12345678
 which interprets the switches somewhat  ^          ܳܳܺ
 differently.  SW1 is the same but SW2     Ľ
 differs as shown:                         ͼ > 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 12345678
  ^ ܳ     ߳ߺ 1 diskette drive  (Note: position 1 is ON if 0 drives)
    Ľ
 ON 12345678
  ^ ܳ     ܳߺ 2 diskette drives
    Ľ
 ON 12345678
  ^ ܳ     ߳ܺ 3 diskette drives
    Ľ
 ON 12345678
  ^ ܳ     ܳܺ 4 diskette drives
    Ľ

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 12345678
  ^     ߳߳   None or EGA
    Ľ
 ON 12345678
  ^     ܳ߳   40x25 Color/Graphics Adapter (CGA)
    Ľ
 ON 12345678
  ^     ߳ܳ   80x25 Color/Graphics Adapter (CGA)
    Ľ
 ON 12345678
  ^     ܳܳ   Monochrome Adapter (MDA) or both MDA and CGA
    Ľ

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 12345678
  ^  ܳ       8087 is installed
    Ľ
 ON 12345678
  ^  ߳       8087 is NOT installed
    Ľ

PC SW1: System board RAM
  These switches should both be OFF on systems that have 64K of memory or
  more.
           SW1
 ON 12345678
  ^   ܳܳ     64K or more on the system board
    Ľ

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 12345678     Note: on PC-1, SW2 positions 5-8 are always OFF
  ^ ߳߳߳߳     64K
    Ľ
 ON 12345678      Ŀ
  ^ ߳ܳ߳߳     128K  The settings reflect a binary number of 32K   
    Ľ       blocks where an OFF is a 1-bit.  To set the   
 ON 12345678       switches without using a chart, just remember:
  ^ ߳߳ܳ߳     192K                                                
    Ľ           Position 4 = 256Ks  (ON=0, OFF=1)         
 ON 12345678           Position 3 = 128Ks                        
  ^ ߳ܳܳ߳     256K      Position 2 =  64Ks                        
    Ľ           Position 1 =  32Ks                        
 ON 12345678                                                     
  ^ ߳߳߳ܳ     320K  Add them up, then add 64K (system board RAM)  
    Ľ       to get the total system memory.               
 ON 12345678      
  ^ ߳ܳ߳ܳ     384K
    Ľ
 ON 12345678
  ^ ߳߳ܳܳ     448K
    Ľ
 ON 12345678
  ^ ߳ܳܳܳ     512K
    Ľ
 ON 12345678
  ^ ܳܳܳܳ     544K
    Ľ

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 12345678      Note: on PC-2, SW2 positions 6-8 are always OFF
  ^ ߳߳߳߳߳    64K
    Ľ
 ON 12345678      Ŀ
  ^ ߳ܳ߳߳߳    128K  The settings reflect a binary number of 32K   
    Ľ       blocks where an OFF is a 1-bit.  To set the   
 ON 12345678       switches without using a chart, just remember:
  ^ ߳߳ܳ߳߳    192K                                                
    Ľ           Position 5 = 512Ks  (ON=0, OFF=1)         
 ON 12345678           Position 4 = 256Ks                        
  ^ ߳ܳܳ߳߳    256K      Position 3 = 128Ks                        
    Ľ           Position 2 =  64Ks                        
 ON 12345678           Position 1 =  32Ks                        
  ^ ߳߳߳ܳ߳    320K                                                
    Ľ       Add them up, then add 64K to get the total RAM
 ON 12345678      
  ^ ߳ܳ߳ܳ߳    384K
    Ľ
 ON 12345678
  ^ ߳߳ܳܳ߳    448K
    Ľ
 ON 12345678
  ^ ߳ܳܳܳ߳    512K
    Ľ
 ON 12345678
  ^ ߳߳߳߳ܳ    576K
    Ľ
 ON 12345678
  ^ ߳ܳ߳߳ܳ    640K
    Ľ

See Also: Equipment List
          Switch Settings
                                    -*-