LAN91C100 Datasheet, PDF (64/101 Pages) SMSC Corporation – FEAST™ Fast Ethernet Controller

MEMORY PARTITIONING
Unlike other controllers, the LAN91C100 does
not require a fixed memory partitioning between
transmit and receive resources. The MMU
allocates and de-allocates memory upon
different events. An additional mechanism
allows the CPU to prevent the receive process
from starving the transmit memory allocation.
Memory is always requested by the side that
needs to write into it, that is: the CPU for
transmit or the MAC for receive. The CPU can
control the number of bytes it requests for
transmit but it cannot determine the number of
bytes the receive process is going to demand.
Furthermore, the receive process requests will
be dependent on network traffic, in particular on
the arrival of broadcast and multicast packets
that might not be for the node, and that are not
subject to upper layer software flow control.
In order to prevent unwanted traffic from using
too much memory, the CPU can program a
"memory reserved for transmit" parameter. If
the free memory falls below the "memory
reserved for transmit" value, MMU requests
from the MAC block will fail and the packets will
overrun and be ignored. Whenever enough
memory is released, packets can be received
again. If the reserved value is too large, the
node might lose data which is an abnormal
condition. If the value is kept at zero, memory
allocation is handled on first-come first-served
basis for the entire memory capacity.
Note that with the memory management built
into the LAN91C100, the CPU can dynamically
program this parameter. For instance, when the
driver does not need to enqueue transmissions,
it can allow more memory to be allocated for
receive (by reducing the value of the reserved
memory). Whenever the driver needs to burst
transmissions it can reduce the receive memory
allocation. The driver program the parameter as
a function of the following variables:
1) Free memory (read only register)
2) Memory size (read only register)
The reserved memory value can be changed on
the fly. If the MEMORY RESERVED FOR TX
value is increased above the FREE MEMORY,
receive packets in progress are still received,
but no new packets are accepted until the FREE
MEMORY increases above the MEMORY
RESERVED value.
INTERRUPT GENERATION
The interrupt strategy for the transmit and
receive processes is such that it does not
represent the bottleneck in the transmit and
receive queue management between the
software driver and the controller. For that
purpose there is no register reading necessary
before the next element in the queue (namely
transmit or receive packet) can be handled by
the controller. The transmit and receive results
are placed in memory.
The receive interrupt will be generated when the
receive queue (FIFO of packets) is not empty
and receive interrupts are enabled. This allows
the interrupt service routine to process many
receive packets without exiting, or one at a time
if the ISR just returns after processing and
removing one.
There are two types of transmit interrupt
strategies:
1) One interrupt per packet.
2) One interrupt per sequence of packets.
The strategy is determined by how the transmit
interrupt bits and the AUTO RELEASE bit are
used.
TX INT bit - Set whenever the TX completion
FIFO is not empty.
TX EMPTY INT bit - Set whenever the TX FIFO
is empty.
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