174
Am79C973/Am79C975
P R E L I M I N A R Y
GNT at the beginning of the burst
transaction. If EXTREQ is set to
1, REQ stays asserted until the
last but one data phase of the
burst transaction is done. This
mode is useful for systems that
implement an arbitration scheme
without preemption and require
that REQ stays asserted through-
out the transaction.
EXTREQ should not be set to 1
when the Am79C973/Am79C975
controller is used in a PCI bus ap-
plication.
Read accessible always, write
accessible only when either the
STOP or the SPND bit is set. EX-
TREQ is cleared by H_RESET
and is not affected by S_RESET
or STOP.
7
DWIO
Double Word I/O. When set, this
bit indicates that the Am79C973/
Am79C975 controller is pro-
grammed for DWord I/O (DWIO)
mode. When cleared, this bit indi-
cates
that
the
Am79C975 controller is pro-
grammed for Word I/O (WIO)
mode. This bit affects the I/O Re-
source Offset map and it affects
the
defined
Am79C973/Am79C975 control-
lers I/O resources. See the DWIO
and WIO sections for more de-
tails.
Am79C973/
width
of
the
The initial value of the DWIO bit is
determined by the programming
of the EEPROM.
The value of DWIO can be al-
tered
automatically
Am79C973/Am79C975 control-
ler. Specifically, the Am79C973/
Am79C975 controller will set
DWIO if it detects a DWord write
access to offset 10h from the
Am79C973/Am79C975 controller
I/O base address (corresponding
to the RDP resource).
by
the
Once the DWIO bit has been set
to a 1, only a H_RESET or an EE-
PROM read can reset it to a 0.
(Note that the EEPROM read op-
eration will only set DWIO to a 0 if
the appropriate bit inside of the
EEPROM is set to 0.)
Read accessible always. DWIO
is read only, write operations
have no effect. DWIO is cleared
by H_RESET and is not affected
S_RESET or by setting the STOP
bit.
6
BREADE
Burst Read Enable. When set,
this bit enables burst mode during
memory read accesses. When
cleared, this bit prevents the de-
vice from performing bursting
during
read
accesses.
Am79C973/Am79C975 controller
can perform burst transfers when
reading the initialization block,
the descriptor ring entries (when
SWSTYLE = 3) and the buffer
memory.
The
BREADE should be set to 1 when
the Am79C973/Am79C975 con-
troller is used in a PCI bus appli-
cation to guarantee maximum
performance.
Read accessible always; write
accessible only when either the
STOP or the SPND bit is set.
BREADE is cleared by H_RESET
and is not affected by S_RESET
or STOP.
5
BWRITE
Burst Write Enable. When set,
this bit enables burst mode during
memory write accesses. When
cleared, this bit prevents the de-
vice from performing bursting
during
write
accesses.
Am79C973/Am79C975 controller
can perform burst transfers when
writing the descriptor ring entries
(when SWSTYLE = 3) and the
buffer memory.
The
BWRITE should be set to 1 when
the Am79C973/Am79C975 con-
troller is used in a PCI bus appli-
cation to guarantee maximum
performance.
Read accessible always, write
accessible only when either the
STOP or the SPND bit is set.
BWRITE is cleared by H_RESET