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STP2002QFP
Fast Ethernet, Parallel Port, SCSI (FEPS) - STP2002QFP
Sun Microsystems,
5.4.5 Local Memory Data StructuresThe local memory data structures are organized as wrap-around FIFOs that
can store an unlimited number of packets. The transmit and receive data
structures are very similar, except for the format of the control/status word
that is appended to the end of a packet and the alignment of the first byte of a
packet when it is loaded into the FIFO. Also, the RxFIFO does not have a
shadow read pointer. The logical organization of the FIFOs changes depend-
ing on the SBus configuration. For a 32-bit SBus, the FIFO organization is
512 words× 33 bits. For a 64-bit SBus, the FIFOs are organized as 256 words
× 65 bits. The 512 words× 33 bits configuration makes use of both the Tag_0
and the Tag_1 bits in the FIFO, while the 256 words× 65 bits configuration
uses only the Tag_0 bit.
On the diagrams shown below, frames #1 and #2 represent a 512 words×
33 bits configuration, and frame #n represents a 256 words × 65 bits config-
uration. In reality, of course, only one configuration is used at a given time.
The configuration is selected by programming the extended transfer mode bit
in global configuration register. The amount of “junk” at the beginning of a
frame in the RxFIFO is determined by the first_byte_offset field in the ERX
configuration register.