140
Am79C971
P R E L I M I N A R Y
dress pins are always driven, re-
gardless of the state of the
SSIZE32 bit.
Note that the setting of the
SSIZE32 bit has no effect on the
defined width for I/O resources.
I/O resource width is determined
by the state of the DWIO bit
(BCR18, bit 7).
7-0
SWSTYLE
Software Style register. The val-
ue in this register determines the
style of register and memory re-
sources that shall be used by the
Am79C971 controller. The Soft-
ware Style selection will affect the
interpretation of a few bits within
the CSR space, the order of the
descriptor entries and the width of
the descriptors and initialization
block entries.
All Am79C971 controller CSR
bits and BCR bits and all descrip-
tor, buffer, and initialization block
entries not cited in Table 26 are
unaffected by the Software Style
selection and are, therefore, al-
ways fully functional as specified
in the CSR and BCR sections.
Read/Write accessible only when
either the STOP or the SPND bit
is set. The SWSTYLE register will
contain the value 00h following
H_RESET and will be unaffected
by S_RESET or STOP.
CSR60: Previous Transmit Descriptor Address
Lower
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-0
PXDAL
Contains the lower 16 bits of the
previous transmit descriptor ad-
dress pointer. Am79C971 con-
troller has the capability to stack
multiple transmit frames.
Read/Write accessible only when
either the STOP or the SPND bit
is set. These bits are unaffected
by H_RESET, S_RESET, or
STOP.
CSR61: Previous Transmit Descriptor Address
Upper
Bit
Name
Description
31-16 RES
Reserved locations. Written as
zeros and read as undefined.
15-0
PXDAU
Contains the upper 16 bits of the
previous
transmit
descriptor
address pointer. The Am79C971
controller has the capability to
stack multiple transmit frames.
Read/Write accessible only when
either the STOP or the SPND bit
is set. These bits are unaffected
by H_RESET, S_RESET, or
STOP.
Table 26.
Software Styles
SWSTYLE
[7:0]
Style
Name
SSIZE32
Initialization Block
Entries
Descriptor Ring Entries
00h
LANCE/
PCnet-ISA
controller
0
16-bit software structures,
non-burst or burst access
16-bit software structures,
non-burst access only
01h
RES
1
RES
RES
02h
PCnet-PCI
controller
1
32-bit software structures,
non-burst or burst access
32-bit software structures,
non-burst access only
03h
PCnet-PCI
controller
1
32-bit software structures,
non-burst or burst access
32-bit software structures,
non-burst access only
All Other
Reserved
Undefined
Undefined
Undefined