The meaning of the 4 DIP switches on the IBM EGA vary depending on what
other video adapters are installed in the system, so there are 5
different tables, I'll summarize them below.  The amount of memory on
the IBM EGA card (64KB base, maximum of 256KB) is sensed, not
determined by switches or jumpers.

Table 1:  EGA is the only video adapter in the system

SW1   SW2   SW3   SW4   Type of Display Attached and Initial Mode
Selection
OFF   OFF   ON    OFF    Monochrome (5151)
ON    OFF   OFF   ON     Color 40x25 (5153)
OFF   OFF   OFF   ON     Color 80x25 (5153)
ON    ON    ON    OFF    Enhanced Color - Normal Mode (5154)
OFF   ON    ON    OFF    Enhanced Color - Enhanced Mode (5154)

Table 2: EGA and Monochrome (MDPA) installed, EGA primary

SW1   SW2   SW3   SW4   EGA Display        MDPA Display
ON    OFF   OFF   ON    Color 40x25        Monochrome
OFF   OFF   OFF   ON    Color 80x25        Monochrome
ON    ON    ON    OFF   5154 - Normal      Monochrome
OFF   ON    ON    OFF   5154 - Enhanced    Monochrome

Table 3:  EGA and MDPA, MDPA primary

SW1   SW2   SW3   SW4    EGA Display     MDPA Display
ON    ON    ON    ON     Color 40x25     Monochrome
OFF   ON    ON    ON     Color 80x25     Monochrome
ON    OFF   ON    ON     5154 - Normal   Monochrome
OFF   OFF   ON    ON     5154-Enhanced   Monochrome

Table 4:  EGA and CGA, EGA primary

SW1   SW2   SW3   SW4    EGA Display   CGA Display
ON    OFF   ON    OFF    Monochrome    Color 40x25
OFF   OFF   ON    OFF    Monochrome    Color 80x25

Table 5:  EGA and CGA, CGA primary

SW1   SW2   SW3   SW4    EGA Display   CGA Display
ON    ON    OFF   ON     Monochrome    Color 40x25
OFF   ON    OFF   ON     Monochrome    Color 80x25

You'll note that with two display adapters installed, one is always
acting as a monochrome adapter and the other as a color adapter, which
makes sense when you consider the memory address mapping.  It also
means that the EGA/CGA combination is really a waste, if you are going
to use 2 display adapters it might as well be the EGA/MDPA combination
(or a VGA/MDPA combination).

As to the jumpers, P1 should jumper positions 2-3 when attaching a
monochrome (5151) or CGA (5153) display to the EGA, P1 should jumper
positions 1-2 when attaching an ECD (5154).  P3 should jumper positions
1-2 to select I/O range 3xx, jumper positions 2-3 to select I/O range
2xx.  Only the 3xx range is supported by BIOS.  The P2 block is used
for a light pen input.

Rick Ekblaw