Intel SE7520AF2 Technical Product Specification page 143

Intel® Server Board SE7520AF2 TPS
The BIOS copies the user binary into system memory before the first scan point. If the user
binary reports that it does not contain runtime code, it is located in conventional memory (0 -
640 KB).
Reporting that the user binary is POST has only the advantage that it does not use up limited
option ROM space, and more option ROMs can be fitted. If user binary code is required at run-
time, it is copied to option ROM space. At each scan-point during POST, the system BIOS
determines if this scan-point has a corresponding user binary entry point to which it transfers
control.
To determine this, the bitmap at byte 4 of the header is tested against the current mask bit that
has been determined / defined by the scan point. If the bitmap has the appropriate bit set, the
mask is placed in AL and execution is passed to the address computed by (ADR(Byte
5)+5*scan sequence #).
During execution, the user binary may access 11 bytes of Extended BIOS Data Area RAM
(EBDA). The segment of the EBDA can be found at address 40:0e. Offset 18 to offset 21h is
available for the user binary. The BIOS also reserves eight CMOS bits for the user binary.
These bits are in an unchecksummed region of CMOS with default values of zero, and will
always be located in the first bank of CMOS. These bits are contiguous, but are not in a fixed
location. Upon entry into the user binary, DX contains a 'token' that points to the reserved bits.
This token has the following format:
MSB
15 12
# of bits available –1
The most significant 4 bits are equal to the number of CMOS bits available, minus 1. This field is
equal to 7 since 8 CMOS bits are available. The 12 least significant bits define the position of
the CMOS bit in RTC. This is a bit address, not a byte address. The CMOS byte location is
1/8th of the 12-bit number, and the remainder is the starting bit position within that byte. For
example, if the 12-bit number is 0109h, user binary can use bit 1 of CMOS byte 0108h/8 or
021h.
The following code fragment shows the header and format for a user binary:
db 55h, 0AAh, 20h
MyCode
db
db
db
Revision 1.2
11
Bit offset from start of CMOS of first bit
PROC FAR
CBh
04h
CBh
Intel order number C77866-003
; 20h = 8KB USER Area. 40h=16KB.
; MUST be a FAR procedure
; Far return instruction
; Bit map to define call points, a 1 in any bit
; specifies that the BIOS is called at that scan
; point in POST
; First transfer address used to point
System BIOS
LSB
0
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