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42 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
Technical Description
2.3.5.2 Absolute Addressing Mode
With Absolute addressing, any GPR can be read or written by any of the eight Contexts in a
Microengine. Absolute addressing enables register data to be shared among all of the Contexts,
e.g., for global variables or for parameter passing. All 256 GPRs can be read by Absolute address.
2.3.5.3 Indexed Addressing Mode
With Indexed addressing, any Transfer or Next Neighbor register can be read or written by any one
of the eight Contexts in a Microengine. Indexed addressing enables register data to be shared
among all of the Contexts. For indexed addressing the register number comes from the T_INDEX
register for Transfer registers or NN_PUT and NN_GET registers (for Next Neighbor registers).
Example 9 shows the Index Mode usage. Assume that the numbered bytes have been moved into
the S_TRANSFER_IN registers as shown.
If the software wants to access a specific byte that is known at compile-time, it will normally use
context-relative addressing. For example to access the word in transfer register 3:
If the location of the data is found at run-time, indexed mode can be used, e.g., if the start of an
encapsulated header depends on an outer header value (the outer header byte is in a fixed location).
Example 9. Use of Indexed Addressing Mode
Transfer
Register
Number
Data
31:24 23:16 15:8 7:0
0 0x00 0x01 0x02 0x03
1 0x04 0x05 0x06 0x07
2 0x08 0x09 0x0a 0x0b
3 0x0c 0x0d 0x0e 0x0f
4 0x10 0x11 0x12 0x013
5 0x14 0x15 0x16 0x17
6 0x18 0x19 0x1a 0x1b
7 0x1c 0x1d 0x1e 0x1f
alu[dest, --, B, $xfer3] ; move the data from s_transfer 3 to gpr dest
; Check byte 2 of transfer 0
; If value==5 header starts on byte 0x9, else byte 0x14
br=byte[$0, 2, 0x5, L1#], defer_[1]
local_csr_wr[t_index_byte_index, 0x09]
local_csr_wr[t_index_byte_index, 0x14]
nop ; wait for index registers to be loaded
L1#:
; Move bytes right justified into destination registers
nop ; wait for index registers to be loaded
nop ;
byte_align_be[dest1, *$index++]
byte_align_be[dest2, *$index++] ;etc.
; The t_index and byte_index registers are loaded by the same instruction.