INTRODUCTION

This chapter talks about how BASIC stores and manipulates data. The topics include:

1. A brief mention of the operating system components and functions as well as the character set used in the Commodore 64.
2. The formation of constants and variables. What types of variables there are. And how constants and variables are stored in memory.
3. The rules for arithmetic calculations, relationship tests, string handling, and logical operations. Also included are the rules for forming expressions, and the data conversions necessary when you're using BASIC with mixed data types.

SCREEN DISPLAY CODES (BASIC CHARACTER SET)

THE OPERATING SYSTEM (OS)

The Operating System is contained in the Read Only Memory (ROM) chips and is a combination of three separate, but interrelated, program modules.

1. The BASIC Interpreter
2. The KERNAL
3. The Screen Editor

1. The BASIC Interpreter is responsible for analyzing BASIC statement syntax and for performing the required calculations and/or data manipulation. The BASIC Interpreter has a vocabulary of 65 "keywords" which have special meanings. The upper and lower case alphabet and the digits 0-9 are used to make both keywords and variable names. Certain punctuation characters and special symbols also have meanings for the Interpreter. Table 1-1 lists the special characters and their uses.
2. The KERNAL handles most of the interrupt level processing in the system (for details on interrupt level processing, see Chapter 5). The KERNAL also does the actual input and output of data.
3. The Screen Editor controls the output to the video screen (television set) and the editing of BASIC program text. In addition, the Screen Editor intercepts keyboard input so that it can decide whether the characters put in should be acted upon immediately, or passed on to the BASIC Interpreter.