readme.txt

网络处理器例子程序5

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Byte align example code readme.txt

Introduction
============

This example illustrates the use of the byte_align_le instruction. 

It takes as input an ethernet packet in little endian format, strips off
the ethernet header and byte aligns the contained IP packet.

Input : 64 byte ethernet packet from SRAM
Output: Byte aligned 50 byte IP packet into SRAM


Example 
=======

64 byte ethernet packet: 00010203 04050607 08091011 12131415 .... 60616263

Bytes : 0-3       4-7       8-11      12 13 14 15  16-19     ....  48 49 50 51   ....   60-63
------------------------------------------------------------------------------------------------------------
Input : 03020100  07060504  11100908  15 14 13 12  19181716  ....  .. .. .. ..   ....   60616263

Processing - Strip off first 14 bytes and byte align the rest
             of the bytes. Pad last 2 bytes with zeroes.

Output: 17161514  21201918  25242322  .. .. .. ..  ........  ....  00 00 63 62

        <--- Bytes 14-63 (50 bytes) of the ethernet packet ----->        <--->

So using a 2 byte shift, bytes 14-63 are aligned on a 4 byte boundary
when stored in SRAM.


Initialization script
=====================

There is a script file "byte_align_le_init.ind" which is called
to create an ethernet packet at the SRAM location _SRC_SRAM_OFFSET.
If you already have an ethernet packet stored at the this location,
remove this script . Its only purpose is to store predictable
data if it doesn't already exist.


How to run the code and verify the result
=========================================

Open the project in the workbench.

To run the code, assemble and start debugging.

Select Go.

Run for at least 1000 cycles.

Select Stop.

Open the memory watch window.

sram[0:63] should be

	0x03020100
	0x07060504
	0x11100908
	0x15141312
	0x19181716
	0x23222120
	0x27262524
	0x31302928
	0x35343332
	0x39383736
	0x43424140
	0x47464544
	0x51504948
	0x55545352
	0x59585756
	0x63626160

sram[80:131] should be

	0x17161514
	0x21201918
	0x25242322
	0x29282726
	0x33323130
	0x37363534
	0x41403938
	0x45444342
	0x49484746
	0x53525150
	0x57565554
	0x61605958
	0x00006362

End of Readme.txt