DP8390 NSC [National Semiconductor], DP8390 Datasheet - Page 8

• Manufacturer Part Number: DP8390
• Description: NIC Network Interface Controller
• Manufacturer: NSC [National Semiconductor]

7 0 Packet Reception
for storing packets is controlled by Buffer Management Log-
ic in the NIC The Buffer Management Logic provides three
basic functions linking receive buffers for long packets re-
covery of buffers when a packet is rejected and recircula-
tion of buffer pages that have been read by the host
At initialization a portion of the 64k byte (or 32k word) ad-
dress space is reserved for the receive buffer ring Two
eight bit registers
(PSTART) and the Page Stop Address Register (PSTOP)
define the physical boundaries of where the buffers reside
The NIC treats the list of buffers as a logical ring whenever
the DMA address reaches the Page Stop Address the DMA
is reset to the Page Start Address
INITIALIZATION OF THE BUFFER RING
Two static registers and two working registers control the
operation of the Buffer Ring These are the Page Start Reg-
ister Page Stop Register (both described previously) the
Current Page Register and the Boundary Pointer Register
The Current Page Register points to the first buffer used to
store a packet and is used to restore the DMA for writing
status to the Buffer Ring or for restoring the DMA address in
the event of a Runt packet a CRC or Frame Alignment
error The Boundary Register points to the first packet in the
Ring not yet read by the host If the local DMA address ever
reaches the Boundary reception is aborted The Boundary
Pointer is also used to initialize the Remote DMA for remov-
ing a packet and is advanced when a packet is removed A
simple analogy to remember the function of these registers
is that the Current Page Register acts as a Write Pointer and
the Boundary Pointer acts as a Read Pointer
Note 1 At initialization the Page Start Register value should be loaded into
Note 2 The Page Start Register must not be initialized to 00H
BEGINNING OF RECEPTION
When the first packet begins arriving the NIC begins storing
the packet at the location pointed to by the Current Page
both the Current Page Register and the Boundary Pointer Register
Receive Buffer Ring At Initialization
the Page Start Address Register
(Continued)
Linking Receive Buffer Pages
TL F 8582– 30
8
Register An offset of 4 bytes is saved in this first buffer to
allow room for storing receive status corresponding to this
packet
LINKING RECEIVE BUFFER PAGES
If the length of the packet exhausts the first 256 byte buffer
the DMA performs a forward link to the next buffer to store
the remainder of the packet For a maximal length packet
the buffer logic will link six buffers to store the entire packet
Buffers cannot be skipped when linking a packet will always
be stored in contiguous buffers Before the next buffer can
be linked the Buffer Management Logic performs two com-
parisons The first comparison tests for equality between
the DMA address of the next buffer and the contents of the
Page Stop Register If the buffer address equals the Page
Stop Register the buffer management logic will restore the
DMA to the first buffer in the Receive Buffer Ring value
programmed in the Page Start Address Register The sec-
ond comparison tests for equality between the DMA ad-
dress of the next buffer address and the contents of the
Boundary Pointer Register If the two values are equal the
reception is aborted The Boundary Pointer Register can be
used to protect against overwriting any area in the receive
buffer ring that has not yet been read When linking buffers
buffer management will never cross this pointer effectively
avoiding any overwrites If the buffer address does not
match either the Boundary Pointer or Page Stop Address
the link to the next buffer is performed
Linking Buffers
Before the DMA can enter the next contiguous 256 byte
buffer the address is checked for equality to PSTOP and to
the Boundary Pointer If neither are reached the DMA is
allowed to use the next buffer
Received Packet Enters Buffer Pages
TL F 8582 – 32
1) Check for
2) Check for
e
e
to PSTOP
to Boundary
TL F 8582– 31