DS271PP3
73
CS8900A
Crystal LAN™ ISA Ethernet Controller
CIRRUS LOGIC PRODUCT DATA SHEET
4.7 Receive and Transmit Frame Locations
The Receive and Transmit Frame PacketPage loca-
tions are used to transfer Ethernet frames to and
from the host. The host sequentially writes to and
reads from these locations, and internal buffer
memory is dynamically allocated between transmit
and receive as needed. One receive frame and one
transmit frame are accessible at a time.
4.7.1 Receive PacketPage Locations
In IO mode, the receive status/length/frame loca-
tions are read through repetitive reads from one IO
port at the IO base address. See Section 4.10 on
page 76.
In memory mode, the receive status/length/frame
locations are read using memory reads of a block of
memory starting at memory base a 0400h.
Typically the memory locations are read sequen-
tially using repetitive Move instructions (REP
MOVS). See Section 4.9 on page 74.
Random access is not needed. However, the first
118 bytes of the receive frame can be accessed ran-
domly if word reads, on even word boundaries, are
used. Beyond 118 bytes, the memory reads must be
sequential. Byte reads, or reads on odd-word
boundaries, can be performed only in sequential
read mode. See Section 4.8 on page 73.
The RxStatus word reports the status of the current
received frame. RxEvent register 4 (PacketPage
base + 0124h) has the same contents as the RxSta-
tus register, except RxEvent is cleared when Rx-
Event is read.
The RxLength (receive length) word is the length,
in bytes, of the data to be transferred to the host
across the ISA bus. The register describes the
length from the start of Destination Address to the
end of CRC, assuming that CRC has been selected
(via Register 3 RxCFG, bit BufferCRC). If CRC
has not been selected, then the length does not in-
clude the CRC, and the CRC is not present in the
receive buffer.
After the RxLength has been read, the receive
frame can be read. When some portion of the frame
is read, the entire frame should be read before read-
ing the RxEvent register either directly or through
the ISQ register. Reading the RxEvent register sig-
nals to the CS8900A that the host is finished with
the current frame, and wants to start processing the
next frame. In this case, the current frame will no
longer be accessible to the host. The current frame
will also become inaccessible if a Skip command is
issued, or if the entire frame has been read. See
Section 5.2 on page 79.
4.7.2 Transmit Locations
The host can write frames into the CS8900A buffer
using Memory writes using REP MOVS to the Tx-
Frame location. See Section 5.7 on page 99.
4.8 Eight and Sixteen Bit Transfers
A data transfer to or from the CS8900A can be
done in either I/O or Memory space, and can be ei-
ther 16 bits wide (word transfers) or 8 bits wide
(byte transfers). Because the CS8900A’s internal
architecture is based on a 16-bit data bus, word
transfers are the most efficient.
To transfer transmit frames to the CS8900A and re-
ceive frames from the CS8900A, the host may mix
word and byte transfers, provided it follows three
rules:
1) The primary method used to access CS8900A
memory is word access.
2) Word accesses to the CS8900A’s internal
memory are kept on even-byte boundaries.
3) When switching from byte accesses to word ac-
cesses, a byte access to an even byte address
must be followed by a byte access to an odd-
byte address before the host may execute a
word access (this will realign the word transfers
to even-byte boundaries). On the other hand, a
byte access to an odd-byte address may be fol-
lowed by a word access.
Содержание Crystal LAN CS8900A
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