83C795 Datasheet, PDF (102/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

BUFFER STRUCTURING AND DATA MOVEMENT PROCESSES
T he LAN controller keeps a pointer (R egis ters
R T AB H and R T ABL) to the R eceive Descriptor
table. T his pointer is initialized by the driver when
the table is firs t built and s hould not need
re-initialization thereafter.
When the LAN controller’s DMA becomes active to
receive aframe, it will use RT AB as a pointer to the
firs t free receive buffer. R S TAT field of descriptor is
checked to s ee if the buffer is free (R S T AT =00). If
free, the s ize and s tarting location of the buffer are
loaded from the des criptor and rounded down to
even values. S torage of the frame into the buffer
begins . T he value of RT AB is s aved at the s tart of
reception for each frame in cas e the frame gets
abortedandRT AB has already been updatedtothe
next table entry.
While a frame is being buffered, the LAN controller
will followtheentries in thedes criptor tabletoobtain
additional buffers as needed to receive the entire
frame. Upon completion, the des criptor for the firs t
buffer of thatframeis written with thereceiver status
regis ter and the total byte count for that frame. T he
byte count includes storage of the received F CS .
S houldapartially receivedframeberejectedby the
LAN controller, it will reclaimthebuffers by res etting
the RT AB pointer to its value previous to s tart of
reception of this frame.
Toaddfreebuffers totheendof thedes criptor table,
fill in thefirst newtableentry with R S T AT =non-zero,
build all other table entries with R S T AT =0, then
change the firs t R S T AT to 00 to indicate availability.
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