Specifications

62 AMD Geode™ GeodeROM Functional Specification
Non-Removable Media
32087C
6.4 INT 13h ATA Device (Fixed Disk) Data Structures
The lower 1 MB of RAM in a PC/AT compatible system contains a hodgepodge of “compatible” locations and data struc-
tures. Several of these locations are related directly to fixed disk support.
6.4.1 BIOS Data Area (BDA)
Initially, the BIOS existed in ROM. It had no RAM to store information about the machine it was executing. For this reason,
early BIOS designers chose to use a 256-byte region in lower RAM for themselves. They named this region the BIOS Data
Area (BDA).
The BDA resides between 40:0h and 40:FFh. It contains a variety of dissimilar items, ranging in function from the INT 09h
scan code buffer, to serial/parallel port timeout values, to installed RAM and equipment descriptors.
The BDA is a 256 byte region starting at Offset 0:400h. The BIOS creates and uses this area because the BIOS exists in
ROM and has no dynamic RAM in its own address space for storing dynamic information such as device status and config-
uration. Table 6-3 describes the layout of the BDA.
Table 6-3. Format of the BIOS Data Area
Offset Size Description
00h WORD Base I/O address of first serial I/O port; zero if none.
02h WORD Base I/O address of second serial I/O port; zero if none.
04h WORD Base I/O address of third serial I/O port; zero if none.
06h WORD Base I/O address of fourth serial I/O port; zero if none.
Above fields filled in by POST as it finds serial ports. POST never leaves gaps. DOS and
BIOS serial device numbers may be redefined by reassigning these fields.
08h WORD Base I/O address of first parallel I/O port; zero if none.
0Ah WORD Base I/O address of second parallel I/O port; zero if none.
0Ch WORD Base I/O address of third parallel I/O port; zero if none.
0Eh WORD Segment of Extended BIOS Data Segment (see Section 6.4.2 "Extended BIOS Data Area
(EBDA)" on page 66).
10h WORD Installed hardware:
Bits [15:14] = Number of parallel devices
Bits [13:12] = Unused
Bits [11:9] = Number of serial devices
Bits [8:7] = Unused
Bit 6 = Secondary floppy installed
Bits [5:4] = Initial video mode:
00 = EGA,VGA
01 = 40x25 CGA
10 = 80x25 CGA
11 = 80x25 Monochrome
Bit 3 = Unused
Bit 2 = PS/2 pointing device installed
Bit 1 = Coprocessor install
Bit 0 = Primary (boot) floppy installed
12h BYTE POST scratchpad byte.
13h WORD Base RAM (0-640 KB) size.
15h BYTE Reserved.
16h BYTE Reserved.