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Chapter 4 – Programming
4-8
Access to the PCI BIOS special functions for 16-bit callers is provided through interrupt
1Ah. Thirty-two bit (i.e., protect mode) access is provided by calling through a 32-bit
protect mode entry point.
Calling Conventions
The PCI BIOS functions preserve all registers and flags except those used for return
parameters. The Carry Flag [CF] will be altered as shown to indicate completion status.
The calling routine will be returned to with the interrupt flag unmodified and interrupts
will not be enabled during function execution. These routines, which are re-entrant,
require 1024 bytes of stack space and the stack segment must be the same size (i.e., 16 or
32 bit) as the code segment.
The PCI BIOS provides a 16-bit real and protect mode interface and a 32-bit protect
mode interface.
16-Bit Interface
The 16-bit interface is provided through the Int 1Ah software interrupt. The PCI BIOS
Int 1Ah interface operates in either real mode, virtual-86 mode, or 16:16 protect mode.
The Int 1Ah entry point supports 16-bit code only.
32-Bit Interface
The protected mode interface supports 32-bit protect mode callers. The protected mode
PCI BIOS interface is accessed by calling through a protected mode entry point in the
PCI BIOS. The entry point and information needed for building the segment descriptors
are provided by the BIOS32 Service Directory. Thirty-two-bit callers invoke the PCI
BIOS routines using CALL FAR.
The BIOS32 Service Directory is implemented in the BIOS in a contiguous 16-byte data
structure, beginning on a 16-byte boundary somewhere in the physical address range
0E00000h-0FFFFFh. The address range should be scanned for the following valid,
checksummed data structure containing the following fields:
Offset
Size
Description
0
4 bytes
Signature string in ASCII. The string is “_32_”. This puts an “underscore”
at offset 0, a “3” at offset 1, a “2” at offset 2, and another “underscore” at
offset 3.
4
4 bytes
Entry point for the BIOS32 Service Directory. This is a 32-bit physical
address.
8
1 byte
Revision level.
9
1 byte
Length of the data structure in 16-byte increments. (This data structure is
16 bytes long, so this field contains 01h.)
0Ah
1 byte
Checksum. This field is the checksum of the complete data structure. The
sum of all bytes must add up to 0.
0Bh
5 bytes
Reserved. Must be zero.
The BIOS32 Service Directory is accessed by doing a FAR CALL to the entry point
obtained from the Service data structure. There are several requirements about the
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