AMD. The PCI Device ID register is located at offset
02h in the PCI Configuration Space. It is read only.
data parity error occurred during
a master cycle. PERREN also
enables reporting address parity
errors through the SERR pin and
the SERR bit in the PCI Status
register.
PCI Command Register
Offset 04h
The PCI Command register is a 16-bit register used to
control the gross functionality of the Am79C978 con-
troller. It controls the Am79C978 controller’s ability to
generate and respond to PCI bus cycles. To logically
disconnect the Am79C978 device from all PCI bus cy-
cles except configuration cycles, a value of 0 should be
written to this register.
PERREN
is
cleared
by
H_RESET and is not affected by
S_RESET or by setting the STOP
bit.
5
4
VGASNOOP VGA Palette Snoop. Read as ze-
ro; write operations have no ef-
fect.
The PCI Command register is located at offset 04h in
the PCI Configuration Space. It is read and written by
the host.
MWIEN
SCYCEN
BMEN
Memory Write and Invalidate Cy-
cle Enable. Read as zero; write
operations have no effect. The
Am79C978 controller only gener-
ates Memory Write cycles.
Bit
Name
Description
15-10 RES
Reserved locations. Read as ze-
ros; write operations have no ef-
fect.
3
2
Special Cycle Enable. Read as
zero; write operations have no ef-
fect. The Am79C978 controller
ignores all Special Cycle opera-
tions.
9
8
FBTBEN
Fast Back-to-Back Enable. Read
as zero; write operations have no
effect. The Am79C978 controller
will not generate Fast Back-to-
Back cycles.
Bus Master Enable. Setting
BMEN enables the Am79C978
controller to become a bus mas-
ter on the PCI bus. The host must
set BMEN before setting the INIT
or STRT bit in CSR0 of the
Am79C978 controller.
SERREN
SERR Enable. Controls the as-
sertion of the SERR pin. SERR is
disabled when SERREN is
cleared. SERR will be asserted
on detection of an address parity
error and if both SERREN and
PERREN (bit 6 of this register)
are set.
BMEN is cleared by H_RESET
and is not effected by S_RESET
or by setting the STOP bit.
SERREN
is
cleared
by
H_RESET and is not effected by
S_RESET or by setting the STOP
bit.
1
MEMEN
Memory Space Access Enable.
The Am79C978 controller will ig-
nore all memory accesses when
MEMEN is cleared. The host
must set MEMEN before the first
memory access to the device.
7
6
RES
Reserved location. Read as ze-
ros; write operations have no ef-
fect.
PERREN
Parity Error Response Enable.
Enables the parity error response
functions. When PERREN is 0
and the Am79C978 controller de-
tects a parity error, it only sets the
Detected Parity Error bit in the
PCI Status register. When PER-
REN is 1, the Am79C978 control-
ler asserts PERR on the
detection of a data parity error. It
also sets the DATAPERR bit (PCI
Status register, bit 8), when the
For memory mapped I/O, the
host must program the PCI Mem-
ory Mapped I/O Base Address
register with a valid memory ad-
dress before setting MEMEN.
For accesses to the Expansion
ROM, the host must program the
PCI Expansion ROM Base Ad-
dress register at offset 30h with a
valid memory address before set-
ting MEMEN. The Am79C978
Am79C978
103
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