LAN91C100 Datasheet, PDF (29/101 Pages) SMSC Corporation – FEAST™ Fast Ethernet Controller

I/O SPACE - BANK 0
OFFSET
NAME
TYPE
A
MEMORY CONFIGURATION REGISTER Lower Byte -
READ/WRITE
Upper Byte -
READ ONLY
SYMBOL
MCR
HIGH
BYTE
MEMORY SIZE MULTIPLIER
(M)
0
0
1
1
0
1
0
1
LOW
BYTE
MEMORY RESERVED FOR TRANSMIT (IN BYTES * 256 * M)
0
0
0
0
0
0
0
0
MEMORY RESERVED FOR TRANSMIT
Programming this value allows the host CPU to
reserve memory to be used later for transmit,
limiting the amount of memory that receive
packets can use up. When programmed for
zero, the memory allocation between transmit
and receive is completely dynamic. When
programmed for a non-zero value, the allocation
is dynamic if the free memory exceeds the
programmed value, while receive allocation
requests are denied if the free memory is less or
equal to the programmed value. This register
defaults to zero upon reset. It is not affected by
the RESET MMU command.
The value written to the MCR is a reserved
memory space IN ADDITION TO ANY
MEMORY CURRENTLY IN USE. If the memory
allocated for transmit plus the reserved space
for transmit is required to be constant (rather
than grow with transmit allocations), the CPU
should update the value of this register after
allocating or releasing memory.
The contents of MIR as well as the low byte of
MCR are specified in 256 * M bytes. The
multiplier M is determined by bits 11, 10, and 9
as follows (Bits 11, 10 and 9 are read only bits
used by the software driver to transparently run
on different controllers of the LAN9000 family):
DEVICE bit 11 Bit 10 bit 9 M
MAX MEMORY SIZE
LAN91C100 0
1
0
2
256*256*2=128k
LAN91C90
0
0
1
1
256*256*1=64k
FUTURE
0
1
1
4
256k
FUTURE
1
0
0
8
512k
FUTURE
1
0
1
16
1M
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