
PRS28.4G
IBM Packet Routing Switch
prs28.03.fm
August 31, 2000
Internal Registers
Page 75 of 131
5.2.19 Look-Up Tables and Memory Row
The Table Pointer and Table Data Registers (the two registers described next) work in combination to access
the Look-Up Table or the Memory Row. The lower byte of the Table Pointer Register is set to ‘0’ (for Memory
Row) or ‘1’ (for Look-Up Table).
A Look-Up Table is a 16-entry table that allows the first 16 bytes of each data row of a byte stream to be
arranged before transmission. There are two Look-Up Tables; one for the master stream and one for the
slave stream. For each table, entry at location
A
of the lookup table points to the data byte to be sent as the
A
th
byte in the same data stream. A Look-Up Table is accessed by first setting the Table Select Bit (bit 7) in
the Table Pointer register to ‘1’, then performing a Table Data register access. Note that the Table Pointer
register is auto incremented by two after every read or write operation to the Table Data register. The Look-
Up Table reset values are the normal byte order, from 0 to 15 (no rearranging).
This register is a direct mapping of one shared memory row/location, specified by the Memory Row Address
Register. It is a 20 byte wide register. Because it is the address of the ROW (16 bits) and the address of a
byte in a row which is 4 which makes it a total of 20. Read/write to this register is performed by first setting the
Table Select Bit (bit 7) in the Table Pointer Register to ‘0’, then performing a Table Data Register access.
Note that the Table Pointer register is auto incremented by two after every read or write operation to the
Table Data register.