Intel IXP2800 2.3.6 Local CSRs, 2.3.7 Execution Datapath

Hardware Reference Manual 43
Intel® IXP2800 Network Processor
Technical Description
2.3.6 Local CSRs
Local Control and Status registers (CSRs) are external to the Execution Datapath, and hold specific
data. They can be read and written by special instructions (local_csr_rd and local_csr_wr) and are
accessed less frequently than datapath registers.
Because Local CSRs are not built in the datapath, there is a write-to-use delay of three instructions,
and a read-to-consume penalty of two instructions.
2.3.7 Execution Datapath
The Execution Datapath can take one or two operands, perform an operation, and optionally write
back a result. The sources and destinations can be GPRs, Transfer registers, Next Neighbor
registers, and Local Memory. The operations are shifts, add/subtract, logicals, multiply, byte align,
and find first one bit.
2.3.7.1 Byte Align
The datapath provides a mechanism to move data from source register(s) to any destination
register(s) with byte aligning. Byte aligning takes four consecutive bytes from two concatenated
values (8 bytes), starting at any of four byte boundaries (0, 1, 2, 3), and based on the endian-type
(which is defined in the instruction opcode), as shown in Example 5. The four bytes are taken from
two concatenated values. Four bytes are always supplied from a temporary register that always
holds the A or B operand from the previous cycle, and the other four bytes from the B or A operand
of the Byte Align instruction.
The operation is described below, using the block diagram in Figure6. The alignment is controlled
by the two LSBs of the BYTE_INDEX Local CSR.
Table 6. Align Value and Shift Amount
Align Value
(in Byte_Index[1:0])
Right Shift Amount (Number of Bits)
(Decimal)
Little-Endian Big-Endian
0032
1824
21616
3248