P R E L I M I N A R Y
AMD
109
Am79C965
Hardware Access
PCnet-32 Controller Master Accesses
The particular signals involved in a PCnet-32 controller
bus master transfer depends upon the bus mode that
has been selected. There are two bus modes to choose
from. They are:
s Am486 32-bit mode
s VESA VL-Bus mode
Complete descriptions of the signals involved in bus
master transactions for each mode may be found in the
pin description section of this document. Timing dia-
grams for master accesses may be found in the block
description section for the Bus Interface Unit. This sec-
tion simply lists the types of master accesses that will be
performed by the PCnet-32 controller with respect to
data size and address information.
The PCnet-32 controller will support master accesses
only to 32-bit peripherals in Am486 environments. The
PCnet-32 controller does not support master accesses
to 16-bit peripherals in the 486 local bus mode.
The PCnet-32 controller will support master accesses to
either 32-bit or 16-bit peripherals in VESA VL-Bus
mode. Support of master accesses to 16-bit peripherals
in VESA VL-Bus mode is provided through the
LBS16
input pin.
The PCnet-32 controller is not compatible with 8-bit sys-
tems, since there is no mode that supports PCnet-32
controller accesses to 8-bit peripherals.
Table 35 describes all possible bus master accesses
that the PCnet-32 controller will perform. The right-
most column lists all operations that may execute the
given access.
Table 35. Master Accesses
BE3–
Access
R/W
BE0
Possible Instance
4-Byte Read
RD
0000
Descriptor Read or
Initialization Block
Read or
Transmit Data
Buffer Read
4-Byte Write
WR
0000
Descriptor Write or
Receive Data
Buffer Write
3-Byte Write
WR
1000
Receive Data
Buffer Write
3-Byte Write
WR
0001
Receive Data
Buffer Write
2-Byte Write
WR
1100
Receive Data
Buffer Write
2-Byte Write
WR
1001*
Receive Data
Buffer Write
2-Byte Write
WR
0011
Receive Data
Buffer Write
1-Byte Write
WR
1110
Receive Data
Buffer Write
1-Byte Write
WR
1101*
Receive Data
Buffer Write
1-Byte Write
WR
1011*
Receive Data
Buffer Write
1-Byte Write
WR
0111
Descriptor Write or
Receive Data
Buffer Write
*Cases marked with an asterisk represent extreme bound-
ary conditions that are the result of programming one- and
two-byte buffer sizes, and therefore will not be seen under
normal circumstances.