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Hardware Reference Manual 45
Intel
®
IXP2800 Network Processor
Technical Description
Example 11 shows a little-endian sequence of instructions and the value of the various operands.
Table 8 shows the data in the registers for this example. The value in BYTE_INDEX[1:0] CSR
(which controls the shift amount) for this example is 2.
As the examples show, byte aligning “n” words takes “n+1” cycles due to the first instruction
needed to start the operation.
Another mode of operation is to use the T_INDEX register with post-increment, to select the
source registers. T_INDEX operation is described later in this chapter.
2.3.7.2 CAM
The block diagram in Figure 7 is used to explain the CAM operation.
The CAM has 16 entries. Each entry stores a 32-bit value, which can be compared against a source
operand by instruction:
CAM_Lookup[dest_reg, source_reg]
All entries are compared in parallel, and the result of the lookup is a 9-bit value that is written into
the specified destination register in bits 11:3, with all other bits of the register 0 (the choice of bits
11:3 is explained below). The result can also optionally be written into either of the LM_Addr
registers (see below in this section for details).
The 9-bit result consists of four State bits (dest_reg[11:8]), concatenated with a 1-bit Hit/Miss
indication (dest_reg[7]), concatenated with 4-bit entry number (dest_reg[6:3]). All other bits of
dest_reg are written with 0. Possible results of the lookup are:
• miss (0) — lookup value is not in CAM, entry number is Least Recently Used entry (which
can be used as a suggested entry to replace), and State bits are 0000.
• hit (1) — lookup value is in CAM, entry number is entry that has matched; State bits are the
value from the entry that has matched.
Table 8. Register Contents for Example 11
Register
Byte 3
[31:24]
Byte 2
[23:16]
Byte 1
[15:8]
Byte 0
[7:0]
03210
17654
2BA98
3FEDC
Example 11. Little-Endian Align
Instruction A Operand B Operand Prev A Result
Byte_align_le[--, r0] 3210 -- -- --
Byte_align_le[dest1, r1] 7654 3210 3210 5432
Byte_align_le[dest2, r2] BA98 7654 7654 9876
Byte_align_le[dest3, r3] FEDC BA98 BA98 DCBA
NOTE: B Operand comes from Prev_A register during byte_align_le instructions.