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_be instruction. 

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

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


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  :  00010203   04050607   08091011   12 13 14 15   16171819   ....   .. .. .. ..   ....   60616263

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

Output :  14151617   18192021   22232425   .. .. .. ..   ........   ....   62 63 00 00

          <---------- 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.


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 3000 cycles.

Select Stop.

Open the data watch window.

rbuf[0:63] should be

	0x00112233
	0x44556677
	0x8899aabb
	0x05dd4500
	0x002e0000
	0x00000406
	0x18a61023
	0x4567891b
	0xcdef0001
	0x02030405
	0x06070809
	0x0a0b0c0d
	0x0e0f1011
	0x12131415
	0x16171819
	0xe7ef019b

Open the memory watch window.

dram[80:135] should be

	0x00000000 0x4500002e
	0x01234567 0x040618a6
	0x00010203 0x89abcdef
	0x08090a0b 0x04050607
	0x10111213 0x0c0d0e0f
	0x1819e7ef 0x14151617
	0x00000000 0x019b0000

End of Readme.txt