CS 110A Lecture Notes - Computer Basics - Chapter 1

Hardware and Software

• Hardware - physical machines
• Program - set of instructions to run on those machines
• Software - the collection of such programs

5 Main Components of Computer Hardware

1. Central Processing Unit (CPU) - Fetches and follows instructions, and does simple arithmetic calculations. (Following simple instructions given to it.)
2. Main Memory (RAM) - A silicon-wired card inside the computer box containing huge numbers of numbered memory locations (bits organized into bytes). High speed, low capacity. Volatile (memory is lost when power shuts down).
   • bit - a binary number; either 0 or 1. Stored electronically with 0 meaning off (no power flowing through a circuit) and 1 meaning on (power flowing).
   • Because computer memory is based upon this principle of 2 possible states, it's useful to work with powers of 2 for computer memory.
   • byte - 8 bits, giving 2⁸ (256) possible values - a more useful unit of measure
   • memory address - The location of the first byte of memory used for a variable. Stored numerically, often in hexidecimal (base 16) format.
3. Secondary Memory - Hard drive, flash memory, CD-ROM, etc. Low speed, high capacity. Non- volatile (nothing lost when power is cut.)
4. Input Device - Keyboard Mouse, etc. (input stream)
5. Output Device - Screen, Printer, etc. (output stream)

Making Software Work

• Operating System - Windows, Linux, Mac OS X, Android, iOS, etc. intermediary between hardware and software. Coordinates operation of tasks, allocates resources. The OS is a program itself.
• High-level language - Java, C, C++, PHP, JavaScript, Python, etc. Platform-independent (commands are the same for all kinds of computers). Designed to be understandable by people, and usually compiled to work on computers.
• Low-level language - Very simple computer instructions given one small step at a time. True Machine Language is made up of binary numeric codes for these instructions. We generally deal with its equivalent Assembly Language, which uses short abbreviations for commands, such as "add" for an addition calculation, or "sub" for subtraction. Platform-dependent (commands are different for different kinds of computers)
• Compiler - Converts a high level language to assembly language, saving it in a new file. Source code to Object code. A compiler is a program, generally written in a high level language itself. Compilers are generally platform-dependent. They produce assembly language that works only on the machine they are running on.
• Interpreter - a program which inputs a source code program (such as Perl or PHP) and executes it directly, converting each line of code to machine language as it goes. This is like compiling and running a program all in one step.

Question: Is Python Interpreted or Compiled?

Here is a discussion on StackOverflow

Here is an historical Glossary of Programming Languages

Python Interpreters

• Web-based Python interpreters:
  • Repl.it for Python programs with a text (console) interface
  • Repl.it for Python Turtle graphics programs with a graphical display
• You can install Python on your own computer by downloading it from: https://www.python.org/
• Python is already installed for you on hills.ccsf.edu, where all CS students have an account. It only works for programs with a text (console) interface.

Software Development

• Software Development Life Cycle - Analysis, Design, Implementation, Testing, Maintenance/Evolution, Obsolescence. Implementation (writing code in Java or other languages) occupies only 20% of the total development time for a project.
• Testing and Debugging - 3 kinds of errors: Syntax/Compile-time, Run-time, and Logic. Syntax errors often confuse the compiler, so attack the first error, then try compiling and/or running again.
• Algorithm - a sequence of precise instructions. May be conveyed using flowchart, logic diagram, pseudo-code, or such. Planning saves time! Think in terms of what you want, how you can do it, before you dive in.
• Here's an example of a pseudo-code algorithm to borrow required and optional textbooks for a class, using parameters for the course number and section number:

  To borrow books for CS 110A Section 001, you would write:

  BorrowBooks(CourseNumber="CS 110A", SectionNumber="001");

  To borrow books for English 1A Section 581, you would write:

  BorrowBooks(CourseNumber="ENGL 1A", SectionNumber="581");

  Here is the generic algorithm to borrow books:

  BorrowBooks(CourseNumber, SectionNumber):

  1. Go to the Library.
  2. Repeat for each required book for this course section:
     1. Determine the required book's title and author.
     2. Describe the path to the shelf.
     3. Go and pick up a copy of the required book and put it in your bag.
  3. Repeat for each optional book:
     1. Determine the optional book's title and author.
     2. Describe the path to the shelf.
     3. Go to the shelf.
     4. Look at optional book to decide if you want to borrow it:
     5. If you want to borrow the optional book:
        1. Pick up a copy of the required book and put it in your bag.
     6. If you do not want to borrow the optional book:
        1. Put the optional book back on shelf.
  4. Go to the checkout station.
  5. Scan identification and borrow book.
  6. Exit the library with your books.