
4-10
SYM53C876/876E Data Manual
Registers
PCI Configuration Registers
Register 2Eh
Subsystem ID
Read Only
This register uniquely identifies the add-in board
or subsystem where this PCI device resides. It
provides a mechanism for an add-in card vendor
to distinguish its cards from one another even if
the cards have the same PCI controller installed
on them (and therefore the same Vendor ID and
Device ID). This register loads automatically at
powerup from an external serial EEPROM if in
operating mode A and the load from EEPROM is
successful. The 16-bit value that should be stored
in the external serial EEPROM for this register is
vendor-specific. If in operating mode D, this reg-
ister is loaded with a default value of 1000h. If an
error occurs during a load from EEPROM or if
the operating mode is B, this register defaults to a
value of 0000h. See the
Serial EEPROM Interface
section in Chapter 2 for information about the
values to load in this register.
Register 30h
Expansion ROM Base Address
Read/Write
This four-byte register handles the base address
and size information for the expansion ROM. It
functions exactly like the Base Address 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 controls
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 Command register.
The host system detects the size of the external
memory by first writing the Expansion ROM
Base Address register with all ones and then read-
ing back the register. The SCSI functions of the
SYM53C876 respond with zeroes in all don’t care
locations. The ones in the remaining bits repre-
sent the binary version of the external memory
size. For example, to indicate an external memory
size of 32 KB, this register, when written with
ones and read back, returns ones in the upper 17
bits.
The ROM is accessed through the MAD bus
which is common to both SCSI functions in this
device.
SID
15-12
SID
11-8
SID
7-4
SID
3-0
If EEPROM not enabled <<<Default>>> Mode A
0
If EEPROM not enabled <<<Default>>> Mode D
1
EEPROM value if EEPROM enabled <<<Default>>>
X
0
0
0
0
0
0
X
X
X
ERBA
31-28
Default>>>
0
ERBA
27-24
ERBA
23-20
ERBA
19-16
ERBA
15-12
ERBA
11-8
ERBA
7-4
ERBA
3-0
0
0
0
0
0
0
0