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參數(shù)資料
| 型號： | DM9008F |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | ISA/Plug & Play Super Ethernet Contoller |
| 中文描述： | 的ISA /插頭 |
| 文件頁數(shù)： | 40/68頁 |
| 文件大?。?/td> | 403K |
| 代理商： | DM9008F |
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DM9008
ISA/Plug & Play Super Ethernet Contoller
used to initialize the Remote DMA for removing a packet, and
is incremented when a packet is removed. A simple analogy
for remembering the function of these registers is that the
Current Page Register acts as a Write Pointer, whereas the
Boundary Pointer acts as a Read Pointer.
40
Final
Version :DM9008-DS-F02
June 14, 2000
Packet Reception
The local DMA receive channel uses a Buffer Ring Structure
comprised of a series of contiguous fixed length 256-byte
(128-word) buffers for storage of received packets. The
location of the Receive Buffer Ring is programmed in two
registers: Page Start and Page Stop. An ethernet packet
consists of a distribution of shorter link control packets and
longer data packets. The 256 byte buffer length provides a
good compromise between short packets and longer packets
for using memory most efficiently. In addition, these buffers
provide memory resources for storage of back-to-back packets
in loaded networks. The assignment of buffers for storing
packets is controlled by DM9008's Buffer Management Logic,
which provides three basic functions: linking of receive buffers
for long packets, recovery of buffers when a packet is rejected,
and recirculation of buffer pages that have been read by the
host. At initialization, a portion of the 64K byte (or 32K word)
address space is reserved for the receive buffer ring. For
applications, DM9008 should be programmed to 16K byte (or
8K word) address space (4000H-7FFFH) in NE2000 16-bit
mode, and 8K byte address space (4000-H-5FFFH) in NE2000
8-bit mode. Two eight-bit registers, the Page Start Address
Register (PSTART) and the Page Stop Address Register
(PSTOP), define the physical boundaries where the buffers
reside. DM9008 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
Register, the Page Stop Register (both described previously),
the Current Page Register (CURR) and the Boundary Pointer
Register (BNRY). The Current Page Register points to the first
buffer used to store a packet, and is used to restore the DMA to
Buffer Ring writing status. It also restores 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 reaches the
boundary, reception is aborted. The Boundary Pointer is also
Beginning of Reception
When the first packet arrives, DM9008 begins storing the
packet at the location pointed to by CURR. An offset of 4 bytes
is saved in this first buffer to store the packet's corresponding
receive status.
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 maximum 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
comparisons. The first comparison tests for equalit between
the DMA address of the next buffer and the contents of
PSTOP. If the buffer address equals PSTOP, the buffer
management logic will restore the DMA to the first buffer in the
Receive Buffer Ring value programmed in PSTART. The
second comparison tests for equality between the DMA
address of the next buffer address and the contents of BNRY.
If the two values are equal, the reception is aborted. BNRY 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 BNRY or PSTOP, 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 BNRY. If
neither is reached, the DMA is allowed to use the next buffer.
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