Lecture Overview

Operating Systems, Continued

In addition to the kernel (see day7), there are two more components usually associated to the operating system.

• The shell. The shell is a process or group of processes that start when the operating system boots and provides the ability to launch other programs. The shell is what draws the taskbar, start menu, dock, android home screen, terminal, etc. It launches and kills other processes based on user interaction. The shell is implemented as a normal process running on top of the kernel: it competes with other processes for the CPU, makes syscalls into the kernel when it wants things done (like draw the taskbar), and in almost all respects acts just like any other process, albeit one tasked with controlling the screen and launching other programs.

• Utilities. An operating system usually comes with a large number of utilities (for example, the task manager). These utilities are normal programs and some of them might even be written in python. They are scheduled by the kernel and make syscalls to get things done. Some examples are the file manager and settings/control panel.

Programming Languages

Programming languages are made up of three components:

• Syntax. The syntax of a programming language is the rules on which sequences of ASCII or Unicode characters constitute a valid program.
• Semantics. For each syntactically valid statement, the semantics provide the meaning.
• Implementation. The implementation is a program that translates between syntactically valid ASCII or Unicode files to machine code and syscalls.

Python defines the syntax and semantics here. As an example, consider the if statement.

• The syntax is that you have the ASCII letter "i" followed by the letter "f" followed by a space followed by any expression followed by a colon. This is the rule on which sequences of letters constitute a valid statement, and if the input does not match this rule you will get a syntax error. For example, forgetting the colon causes a syntax error.
• The semantics are that a valid if expression means the following. First, the expression is evaluated. If the expression does not produce a boolean an error is raised. If the expression is a boolean and is True, the indented block is executed and if not the indented block is skipped.
• The python implementation translates the sequence of ASCII letters making a syntactically valid statement into machine instructions so that the machine instructions implement the semantics.
  • Programming languages use machine instructions to implement semantics, and there are several ways this can occur. Either the implementation can do direct translation (called compiled languages), or JIT, or a mix of the two. For us, we won't concentrate so closely on what these methods are.

The syntax of a programming language is described using Backus-Naur Form. This gives slightly more detail than we discussed in class so I won't rewrite it all.

For example, the python reference uses (a variant) of BNF to describe the syntax of statements, for example the if statement. The BNF syntax is then followed by a description of the semantics. Section 1.2 describes the python variant of BNF.

Exercises

On day6, you wrote code and used Wallis' and Leibniz's approximations of pi. Convert both of these implementations to functions and put them into the same file. Each function should take one paramter for the number of terms to compute, and return the approximation of pi. That is, create a python file like

def wallis(terms):
    # Code for wallis here, where terms is an integer for the number of terms to compute
    # return the pi approximation

def leibnitz(terms):
    # Code for leibnitz here, where terms is an integer for the number of terms to compute
    # return the pi approximation

Add some test code to make sure your code works, for example

print(wallis(1000))