Xycom XVME-653, Manual

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Chapter 4 – Programming
4-7
Calling Conventions
The PCI BIOS functions preserve all registers and flags except those used for return pa-
rameters. 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, re-
quire 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 inter-
face 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
0E0000h-0FFFFFh. The address range should be scanned for the following valid, check-
summed data structure containing the following fields:
Table 4-4 BIOS32 Service Table
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 ob-
tained from the Service data structure. There are several requirements about the calling
environment that must be met. The CS code segment selector and the DS data segment
selector must be set up to encompass the physical page holding the entry point as well as
the immediately following physical page. They must also have the same base. The SS
stack segment selector must be 32-bit and provide at least 1 KB of stack space. The
calling environment must also allow access to I/O space.
The BIOS32 Service Directory provides a single function call to locate the PCI BIOS
service. All parameters to the function are passed in registers. Parameter descriptions are
provided below. Three values are returned by the call. The first is the base physical ad-
dress of the PCI BIOS service, the second is the length of the service, and the third is the
entry point to the service encoded as an offset from the base. The first and second values
can be used to build the code segment selector and data segment selector for accessing
the service.