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Bitfields for BIOS equipment list

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Bit Description

0

floppy disk(s) installed (number specified by bits 7-6)

1

80x87 coprocessor installed

3 - 2

number of 16K banks of RAM on motherboard PC only
number of 64K banks of RAM on motherboard XT only

2

pointing device installed PS

3

unused PS

5 - 4

initial video mode
00 EGA, VGA, or PGA
01 40x25 color
10 80x25 color
11 80x25 monochrome

7 - 6

number of floppies installed less 1 (if bit 0 set)

8

DMA support installed PCjr, Tandy 1400LT
DMA support *not* installed Tandy 1000's

11 - 9

number of serial ports installed

12

game port installed

13

serial printer attached PCjr
internal modem installed PC/Convertible

15 - 14

number of parallel ports installed

23

page tables set so that Weitek coprocessor addressable in real mode Compaq, Dell, and many other 386/486 machines

24

Weitek math coprocessor present Compaq, Dell, and many other 386/486 machines
25 internal DMA parallel port available Compaq Systempro
26 IRQ for internal DMA parallel port (if bit 25 set)
0 = IRQ5
1 = IRQ7
Compaq Systempro

28 - 27

parallel port DMA channel
00 DMA channel 0
01 DMA channel 0 ???
10 reserved
11 DMA channel 3
Compaq Systempro

Note:

Some implementations of Remote (Initial) Program Loader (RPL/RIPL) don't set bit 0 to indicate a "virtual" floppy drive, although the RPL requires access to its memory image through a faked drive A:. This may have caused problems with releases of DOS 3.3x and earlier, which assumed A: and B: to be invalid drives then and would discard any attempts to access these drives. Implementations of RPL should set bit 0 to indicate a "virtual" floppy.

The IBM PC DOS 3.3x-2000 IBMBIO.COM contains two occurences of code sequences like:
INT 11h
JMP SHORT skip
DB 52h, 50h, 53h; "RPS"

skip:
OR AX, 1
TEST AX,1

While at the first glance this seems to be a bug since it just wastes memory and the condition is always true, this could well be a signature for an applyable patch to stop it from forcing AX bit 0 to be always on. MS-DOS IO.SYS does not contain these signatures, however.

Bugs: Some old BIOSes didn't properly report the count of floppy drives installed to the system. In newer systems INT 13h/AH=15h can be used to retrieve the number of floppy drives installed.
Award BIOS v4.50G and v4.51PG erroneously set bit 0 even if there are no floppy drives installed; use two calls to INT 13/AH=15h to determine whether any floppies are actually installed

See Also:

INT 12"BIOS",#03215 at INT 4B"Tandy 2000"