
8/01/00
Am79C976
197
P R E L I M I N A R Y
Bus Configuration Registers
The Bus Configuration Registers (BCRs) are included
for compatibility with older PCnet Family software All
BCR functions can be accessed more efficiently
through the memory-mapped registers.
BCRs are used to program the configuration of the bus
interface and other special features of the Am79C976
controller that are not related to the IEEE 8802-3 MAC
functions. The BCRs are accessed by first setting the
appropriate RAP value and then by performing a slave
access to the BDP. See Table 89.
All BCR registers are 16 bits in width in Word I/O mode
(DWIO = 0, BCR18, bit 7) and 32 bits in width in DWord
I/O mode (DWIO = 1). The upper 16 bits of all BCR reg-
isters is undefined when in DWord I/O mode. These
bits should be written as zeros and should be treated
as undefined when read. The default value given for
any BCR is the value in the register after H_RESET.
Some of these values may be changed shortly after
H_RESET when the contents of the external EEPROM
is automatically read in. None of the BCR register val-
ues are affected by the assertion of the STOP bit or
S_RESET.
Note that several registers have no default value.
BCR0, BCR1, BCR3, BCR8, BCR10-17, and BCR21
are reserved and have undefined values. BCR2 and
BCR34 are not observable without first being pro-
grammed through the EEPROM read operation or a
user register write operation.
BCR0, BCR1, BCR16, BCR17, and BCR21 are regis-
ters that are used by other devices in the PCnet family.
Writing to these registers have no effect on the opera-
tion of the Am79C976 controller.
Writes to those registers marked as
“
Reserved
”
will
have no effect. Reads from these locations will produce
undefined values.
Table 89. BCR Registers
RAP
Mnemonic
Default
Name
Programmability
User
EEPROM
0
MSRDA
0005h
Reserved
No
No
1
MSWRA
0005h
Reserved
No
No
2
MC
0000h
Miscellaneous Configuration
Yes
Yes
3
Reserved
N/A
Reserved
No
4
LED0
00C0h
LED0 Status
Yes
Yes
5
LED1
0094h
LED1 Status
Yes
Yes
6
LED2
1080h
LED2 Status
Yes
Yes
7
LED3
0081h
LED3 Status
Yes
Yes
8
Reserved
N/A
Reserved
No
No
9
FDC
0004h
Full-Duplex Control
Yes
Yes
10-15
Reserved
N/A
Reserved
No
No
16
IOBASEL
N/A
Reserved
No
No
17
IOBASEU
N/A
Reserved
No
No
18
BSBC
9000h
Burst and Bus Control
Yes
Yes
19
EECAS
0000h
EEPROM Control and Status
Yes
No
20
SWS
0000h
Software Style
Yes
No
21
Reserved
N/A
Reserved
No
No
22
PCILAT
1818h
PCI Latency
Yes
Yes
23
PCISID
0000h
PCI Subsystem ID
No
Yes
24
PCISVID
0000h
PCI Subsystem Vendor ID
No
Yes