am79c972b Advanced Micro Devices, am79c972b Datasheet - Page 52

• Manufacturer Part Number: am79c972b
• Description: Pcnet?-fast+ Enhanced 10/100 Mbps Pci Ethernet Controller With Onnow Support
• Manufacturer: Advanced Micro Devices

(SSIZE32 (BCR20, bit 8) is set to 0). Note that even
though the Am79C972 controller treats the descriptor
entries as 16-bit structures, it will always perform 32-bit
bus transfers to access the descriptor entries. The
value of CSR2, bits 15-8, is used as the upper 8-bits for
all memory addresses during bus master transfers.
When SWSTYLE is set to 2 or 3, the descriptor ring
base addresses must be aligned to a 16-byte bound-
ary, and a maximum of 512 ring entries is allowed when
the ring length is set through the TLEN and RLEN fields
of the initialization block. Each ring entry is organized
as three 32-bit message descriptors (SSIZE32
(BCR20, bit 8) is set to 1). The fourth DWord is re-
served. When SWSTYLE is set to 3, the order of the
message descriptors is optimized to allow read and
write access in burst mode.
For any software style, the ring lengths can be set be-
yond this range (up to 65535) by writing the transmit
and receive ring length registers (CSR76, CSR78) di-
rectly.
Each ring entry contains the following information:
n The address of the actual message data buffer in
n The length of the message buffer
n Status information indicating the condition of the
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user or host memory
buffer
Am79C972
To permit the queuing and de-queuing of message
buffers, ownership of each buffer is allocated to either
the Am79C972 controller or the host. The OWN bit
within the descriptor status information, either TMD or
RMD, is used for this purpose.
When OWN is set to 1, it signifies that the Am79C972
controller currently has ownership of this ring descrip-
tor and its associated buffer. Only the owner is permit-
ted to relinquish ownership or to write to any field in the
descriptor entry. A device that is not the current owner
of a descriptor entry cannot assume ownership or
change any field in the entry. A device may, however,
read from a descriptor that it does not currently own.
Software should always read descriptor entries in se-
quential order. When software finds that the current de-
scriptor is owned by the Am79C972 controller, then the
software must not read ahead to the next descriptor.
The software should wait at a descriptor it does not own
until the Am79C972 controller sets OWN to 0 to release
ownership to the software. (When LAPPEN (CSR3, bit
5) is set to 1, this rule is modified. See the LAPPEN de-
scription. At initialization, the Am79C972 controller
reads the base address of both the transmit and re-
ceive descriptor rings into CSRs for use by the
Am79C972 controller during subsequent operations.
Figure 31 illustrates the relationship between the initial-
ization base address, the initialization block, the re-
ceive and transmit descriptor ring base addresses, the
receive and transmit descriptors, and the receive and
transmit data buffers, when SSIZE32 is cleared to 0.