SCO Ed6 Page1-2 - Phase1 0002/2384

[sorcerer_see]

P.S. I synthesize and re-write in my own words what books teach, write any example programs they contain, solve all exercises they offer, and comment cursively. I list pages done vs total pages, including any roman and appendix numbering.

SCO Ed6 Page1-2 - Phase1 0002/2384


1 INTRODUCTION

A digital computer = a machine that does (computing) tasks for people. To make the machine do specific tasks, instructions are given to it. A set of such instructions is called a program. These instructions are directed to the core of the computer, which consists of circuits that are affected by these instructions. At the (near to) base level those instructions are very simple:

• check the state of what is already set
• add 2 digits
• check whether a digit stored is zero
• copy a state of one memory location to another memory location
• ...

A collection of possible instructions forms a language that users use to communicate with the computer. This language, to which the computer can respond, we call machine language.

People who design a new computer also record the body of instructions with which to operate it. Because the machine can only understand very simplistic instructions, this language is tedious to handle by humans, especially once instruction sets become elaborate.

For this reason people created a language that consist of prepared instruction sets that are triggered by human-understandable commands. These commands are easier to understand, oversee and organize by humans. This language formed a first layer on top of the machine language. Later more layers were built, each on top of the former, in the same way. These languages became ever more clear and intuitive to use by humans.

The way that computer instructions are thus organized, is called structured computer organization. This is also the name of this book, and in the next section we will explain what this term exactly means. After this explanation we will look into historical developments, and the current state of the art of programming computers.


1.1 STRUCTURED COMPUTER ORGANIZATION

Computers can not understand human language and humans can not easily understand computer language.  This book explains how this gap can be bridged.

1.1.1 Languages, Levels, and Virtual Machines
There are 2 main methods in vogue to solve the problem. Both ways involved designing a new,set of instructions that is more convenient for humans to use. These sets form each a new language which we will, for the sake of example, call Level 1 languages. The collection of machine instructions we already had, we will call the Level 0 language. Our 2 L1 languages differ from one another in their approach and we will call them A and B.

• In the case of the L1 A  language, a compiler replaces all instructions in a program with the equivalent sequence of instructions in L0 machine language before executing the newly begotten program in its entirety. This method is called translation, and the program that replaces the original text a compiler.

• In the case of the L1 B language, a runtime environment replaces and immediately runs the program's instructions, one after the next. This method is called interpretation and the program that interprets the interpreted instructions the interpreter.

In both cases the program is created in L1 language and then converted and ran as L0 instructions. But in the first case the program is first converted entirely and only then ran, while in the second case the program is converted and ran instruction per instruction. Both types of languages are commonly used today and even mixed with one another.