
3.0 Device Architecture and Configuration
(Continued)
Revision 1.2
77
www.national.com
P
Table 29. BIOS-FWH Memory Space Definition
Upon reset in BIOS mode, both the BIOLPCEN and the BIOFWHEN bits in the XMEMCNF1 register are set. The PC8741x
device automatically detects the type of host boot protocol in use via the first completed BIOS read transaction after reset.
If the first read is an LPC memory read, the BIOFWHEN bit is cleared. If the first read is an LPC-FWH read, the BIOLPCEN
bit is cleared. The succeeding LPC or LPC-FWH transactions do not influence the BIOLPCEN and BIOFWHEN bits. The
software can later enable the response to both address spaces by setting the cleared bit. Figure 8 illustrates this behavior.
Figure 8. BIOS Mapping Enable Scheme
The two User-Defined Memory Zones (MEM0 and MEM1) are specified via a 32-bit base address. This address is formed
by eight bits of the XMEMBAL register, eight bits of the XMEMBAH register and 16 least significant bits of 0. The size of
each zone is specified through the XMEMSIZE register. The base address must be aligned to the block size. Figure 9 and
Figure 10 illustrate the mapping of the LPC and FWH spaces to the different memory zones.
The Memory Address Map Lock bit in X-Bus Memory Configuration Register 2 enables protection of the contents of the
memory mapping, thus preventing modifications to them that may cause access rights violation through aliasing.
The address used for the X-Bus transaction is the 28 least significant bits of the address bus. In read transactions, the data
read from the X-Bus is passed to the LPC bus. In write transactions, the data from the LPC is passed to the X-Bus.
Memory Address Range
FFFC 0000h - FFFF FFFFh
FFF8 0000h - FFFF FFFFh
FFF0 0000h - FFFF FFFFh
FFE0 0000h - FFFF FFFFh
FFC0 0000h - FFFF FFFFh
FF80 0000h - FFFF FFFFh
FF00 0000h - FFFF FFFFh
FE00 0000h - FFFF FFFFh
Description
386 mode BIOS Range.
This is the upper 256 Kbytes to 32 Mbytes of the memory
space, depending on the setting of BIOSIZE and SEL2BIOS
in XMEMCNF2 (see Section 3.15.12 on page 84). The
PC8741x devices use the ID field and address bits A18-A27
to A25-A27, respectively, to identify FWH access to the BIOS
memory.
Only hardware controlled transitions are shown.
Other transitions are possible via software
writing to the bits.
Note:
BIOFWHEN = 1
BIOLPCEN = 0
BIOFWHEN = 0
BIOLPCEN = 1
BIOFWHEN = 0
BIOLPCEN = 0
V
SB
Power-Up Reset
XCNF2 = Disable BIOS
XCNF2 = Enable BIOS
First Memory Read
is LPC
First Memory Read
is FWH
BIOFWHEN = 1
BIOLPCEN = 1