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 ╓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
Switch Settings
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