
3-38
S5935
PCI CONFIGURATION REGISTERS
EXPANSION ROM BASE ADDRESS
REGISTER (XROM)
Register Name:
Address Offset:
Power-up value:
Boot-load:
Expansion ROM Base Address
30h
00000000h
External nvRAM offset
70h
bits 31:11, bit 0 Read/Write; bits
10:1 Read Only
32 bits
Attribute:
Size:
31
0
0
0
1
10
Bit
Value
Address Decode
Enable (RW)
0=Disabled
1=Enabled
Reserved (RO)
Programmable (R/W)
11
Figure 12. Expansion ROM Base Address Register
The expansion base address ROM register provides
a mechanism for assigning a space within physical
memory for an expansion ROM. Access from the PCI
bus to the memory space defined by this register will
cause one or more accesses to the S5935 control-
lers’ external BIOS ROM (or nvRAM) interface. Since
PCI bus accesses to the ROM may be 32 bits wide,
repeated operations to the ROM are generated by
the S5935 and the wider data is assembled internal
to the S5935 controller and then transferred to the
PCI bus by the S5935.
Table 25. Expansion ROM Base Address Register
Bit
Description
31:11
Expansion ROM Base Address Location. These bits are used to position the decoded region in
memory space. Only bits which return a 1 after being written as 1 are usable for this purpose. These
bits are individually enabled by the contents sourced from the external boot memory (EPROM or
nvRAM). The desired size for the ROM memory is determined by writing all ones to this register and
then reading back the contents. The number of bits returned as zeros, in order from least significant
to most significant bit, indicates the size of the expansion ROM. This controller limits the expansion
ROM area to 64K bytes. The allowable returned values after all ones are written to this register are
shown in Table 26.
10:1
Reserved. All zeros.
0
Address Decode Enable. The Expansion ROM address decoder is enabled or disabled with this bit.
When this bit is set, the decoder is enabled; when this bit is zero, the decoder is disabled. It is required
that the PCI command register also have the memory decode enabled for this bit to have an effect.