Expansion rom base address – LSI 53C875A User Manual
Page 104
4-12
Registers
value that should be stored in the external serial
EEPROM is vendor specific. Please see the
in
for additional
information on downloading a value for this register.
Registers: 0x30–0x33
Expansion ROM Base Address
Read/Write
ERBA
Expansion ROM Base Address
[31:0]
This four-byte register handles the base address and size
information for the expansion ROM. It functions exactly
like the
Base Address Register Zero (I/O)
and One
(Memory) registers, except that the encoding of the bits
is different. The upper 21 bits correspond to the upper
21 bits of the expansion ROM base address.
The expansion ROM Enable bit, bit 0, is the only bit
defined in this register. This bit is used to control whether
or not the device accepts accesses to its expansion
ROM. When the bit is set, address decoding is enabled,
and a device is used with or without an expansion ROM
depending on the system configuration. To access the
external memory interface, also set the Memory Space
bit in the
register.
The host system detects the size of the external memory
by first writing the
register
with all ones and then reading back the register. The
LSI53C875A responds with zeros in all don’t care
locations. The ones in the remaining bits represent the
binary version of the external memory size. For example,
to indicate an external memory size of 32 Kbytes, this
register, when written with ones and read back, returns
ones in the upper 17 bits.
The size of the external memory is set through MAD[3:1].
Please see the section on
for the
possible size encodings available.
31
0
ERBA[31:0]
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0