Cypress DCT-1D Guide de l'utilisateur télécharger pdf (Page 61)

Chapter 6

Lab task 4 - Custom Instructions

6.1 Introduction

In this lab task you will learn how to design and integrate a new instruction into the

processor. This part will target the bit alignment problem arising when trying to write

bit streams to the memory. A bit stream is here deﬁned as a stream of bits with no

particular memory alignment. In accordance with jpegﬁles the bit pattern to write

to the stream will be called the code and the number of bits to write the size of the

code. The extension you will make to the instruction set is called a Variable Length

eXtension, or VLX for short.

6.1.1 Huffman Coding

First a short review of the principles of Huffman encoding. Huffman encoding works

by encoding the most frequently occurring item with the fewest number of bits and

then using more bits for items occurring less frequently. For instance take the sequence

ABAACDAAAB, the letter’s frequencies are calculated and listed in 6.1

Four different values imply that we at least need two bits per value thus we would

need 10 · 2 = 20 bits to encode the string. If we were to use the Huffman codes in 6.1

we would need only 6 · 1 + 2 · 2 + 3 + 3 = 16 bits, thus saving four bits. How the

Huffman codes are calculated is not in the scope of this course, for further information

about Huffman coding see for example [14].

When the Huffman tables are calculated the encoding process is simple, just do a

lookup and replace, e.g. simply replace the data with its corresponding code.

In our case jpegﬁles uses already pre calculated Huffman tables, the JPEG standard

Letter Frequency Huffman code

A 6 0

B 2 10

C 1 110

D 1 111

Table 6.1: Letter frequencies and Huffman codes

1 2 ... 56 57 58 59 60 61 62 63 64 65 66 ... 81 82

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Cypress DCT-1D Guide de l'utilisateur Page 61