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Machine Language For Beginners

by Richard Mansfield
Introduction

Why Machine Language?
Sooner or later, many programmers find that they want to learn machine language. BASIC is a fine general-purpose tool, but it has its limitations. Machine language (often called assembly language) performs much faster. BASIC is fairly easy to learn, but most beginners do not realize that machine language can also be easy. And, just as learning Italian goes faster if you already know Spanish, if a programmer already knows BASIC, much of this knowledge will make learning machine language easier. There are many similarities.
This book is designed to teach machine language to those who have a working knowledge of BASIC. For example, Chapter 9 is a list of BASIC statements. Following each is a machine language routine which accomplishes the same task. In this way, if you know what you want to do in BASIC, you can find out how to do it in machine language.
To make it easier to write programs in machine language (called "ML" from here on), most programmers use a special program called an assembler. This is where the term "assembly language" comes from. ML and assembly language programs are both essentially the same thing. Using an assembler to create ML programs is far easier than being forced to look up and then POKE each byte into RAM memory. That's the way it used to be done, when there was too little memory in computers to hold languages (like BASIC or Assemblers) at the same time as programs created by those languages. That old style hand-programming was very laborious.
There is an assembler (in BASIC) at the end of this book which will work on most computers which use Microsoft BASIC, including the Apple, PET/CBM, VIC, and the Commodore 64. There is also a separate version for the Atari. It will let you type in ML instructions (like INC 2) and will translate them into the right numbers and POKE them for you wherever in memory you decide you want your ML program. Instructions are like BASIC commands; you build an ML program using the ML "instruction set." A complete table of all the 6502 ML instructions can be found in Appendix A.
It's a little premature, but if you're curious, INC 2 will increase the number in your computer's second memory cell by one. If the number in cell 2 is 15, it will become a 16 after INC 2. Think of it as "increment address two."
Throughout the book we'll be learning how to handle a variety of ML instructions, and the "Simple Assembler" program will be of great help. You might want to familiarize yourself with it. Knowing what it does (and using it to try the examples in this book), you will gradually build your understanding of ML, hexadecimal numbers, and the new possibilities open to the computerist who knows ML.
Seeing It Work
Chapters 2 through 8 each examine a major aspect of ML where it differs from the way BASIC works. In each chapter, examples and exercises lead the programmer to a greater understanding of the methods of ML programming. By the end of the book, you should be able to write, in ML, most of the programs and subroutines you will want or need.
Let's examine some advantages of ML, starting with the main one - ML runs extremely fast.
Here are two programs which accomplish the same thing. The first is in ML, and the second is in BASIC. They get results at very different speeds indeed, as you'll see:
Machine Language
169 1 160 0 153 0 128 153 0 129 153 130 153 0 131 200 208 241 96
BASIC
5 FOR I=1 TO 1000: PRINT "A";: NEXT I
These two programs both print the letter "A" 1000 times on the screen. The ML version takes up 28 bytes of Random Access Memory (RAM). The BASIC version takes up 45 bytes and takes about 30 times as long to finish the job. If you want to see how quickly the ML works, you can POKE those numbers somewhere into RAM and run the ML program with a SYS (Commodore computers) or USR (Atari) or CALL (Apple). In both BASIC and ML, many instructions are followed by an argument. The instructions SYS and CALL have numbers as their arguments. In these cases, the instruction is going to turn control of the computer over to the address given as the argument. There would be an ML program waiting there. To make it easy to see this ML program's speed, we'll load it into memory without yet knowing much about it.
A disassembly is like a BASIC program's LISTing. You can give the starting address of an ML program to a disassembler and it will translate the numbers in the computer's memory into a readable series of ML instructions. See Appendix D for a disassembler that you can use to examine and study ML programs.
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