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Computing Terminology

What is a computer?

• any device, entity, or object that can perform computations
• need not be electronic in nature
  • e.g. mechanical, biological computers, optical computing, molecular computing, quantum computing, etc

Processor/microprocessor/central processing unit (CPU)

• the core unit that does the actual computations.
• only does a few things, such as:
  • jump to a particular location in RAM
  • read a number from RAM
  • save a number to RAM
  • add two numbers together
  • compare two numbers to see which is larger
  • send a number to a particular device (e.g. a monitor)
  • receive a number from a particular device (e.g. a keyboard)
• the processors is often called “the brains” of the computer.
• processors are grouped in families that all share similar characteristics
• usually each family has a specific variety of machine code that it “understands”
• all instructions to the processor as to what it should do must be specified as a sequence of these machine codes
• watch video: See How the CPU Works In One Lesson

Clockspeed

• speed of processor’s electric pulses (oscillating at a certain frequency to keep time).
• measured in Hertz, (i.e. cycles per second)
• the processor can only perform a fixed number of computations with each pulse
• the pulses that the clock sends to the processor are typically 5 Volts or less.
• see more on this topic

Source code

• the language written by the creator of the program (often a human).
• typically written as plain text
• there are many popular “high level” source code programming languages, such as Python, Javascript, Java, C, C++, Objective C, C#, etc.
• “high level” means that the language is relatively easy for humans to understand when reading it.
• a “low level” lanuage is one written in a code that is more closely related to what the processor “understands” and can execute. Low level languages may be difficult for a human to easily read and write, given that it likely looks nothing like human language.
• high level source code must usually be translated into the ultimate low level language – specific sequences of “machine code” instructions that the processor can natively understand – before it can be executed by the processor.

Machine code

• language “understood” by processor
• represents the lowest level of code - code that the processor natively knows how to execute (i.e. how to “run”)
• instructions in machine code can be directly executed by the processor it is designed for
• each family of processors has its own variety of machine code, e.g. Intel x86, ARM, etc.
• consists of binary code: each instruction is written as a specific sequence of 0’s and 1’s
• see more on number systems

1101101010011010

Assembling

• assembly language is a simple set of mnemonics, such as ‘add’ or ‘sub’ for subtract.
• each mnemonic is a shorthand code that directly maps to one of the binary machine code instructions available on the processor
• for every machine code instruction, there is a mnemonic using alphabetic characters that can be written instead
• assembling is the process of translating assembly language mnemonic codes into machine code for a specific processor family
• an assembler is the software that assembles
• once assembled, the machine code is executable on the processors for which it was designed

Example

The following is an example of a simple program in both assembly language and its equivalent in native machine code for an x86-64 processor:

Assembly Code (x86-64, AT&T syntax):

mov    $60, %rax    # syscall: exit
mov    $42, %rdi    # exit code 42
syscall             # invoke syscall

Each of the instructions corresponds to the specific numeric sequence indicated in the machine code equivalent below.

Equivalent Machine Code (hexadecimal):

b8 3c 00 00 00    # mov $60, %rax
bf 2a 00 00 00    # mov $42, %rdi
0f 05             # syscall

The machine code instructions are represented in hexadecimal number form, not binary, for better readability.

This program, when assembled and run, will exit with code 42.

Compiling

• compiling is translating an entire program from one programming languages to another
• this term is most often used to refer to the process of translating high-level source code directly to machine code
• but also used to refer to the process of translating high-level programming code to an intermediate-level language, such as byte code
• if compiled directly to machine code, that machine code is then “executable”, meaning it can be run on the processor, if desired
• another program, called an executor performs the execution of the machine code, if desired… this is not part of the compilation process
• if compiled to an intermediate-level language instead of machine code, another compiler or interpreter, such as as virtual machine (VM), must then be used to translate that intermediate-level language to machine code before it can be executed.

Interpreting

• at a simplified level, an interpreter translates a single statement of source code at a time from one higher-level programming language into machine code and immediately executing that machine code on the processor
• the code is being translated “on the fly” into machine code and immediately executed
• contrast this with compiling, which involves bulk analysis and translation of an entire source code base to machine code, but does not execute the machine code

The reality of compiling vs interpreting

• read this short explanation by a user on stackoverflow.com for a bit more of the reality of modern-day interpreters, which are more complex than my simplified explanation

Implementations

Many real world high-level programming language implementations use one or both of compiling and interpreting. Here are a few examples:

C:

• high-level C code is usually compiled directly to machine code for a specific processor family
• that compiled machine code can then be executed on the processors for which it was compiled

Shell scripts:

• bash and similar shell scripting languages are interpreted directly into machine code by the shell interpreter.
• each shell statement is immediately translated to machine code and executed on the processor

Java:

• high-level Java code is typically compiled to intermediate-level bytecode
• bytecode can then be interpreted into machine code by any Java Virtual Machine

Python:

• high-level Python code is typically compiled to intermediate-level bytecode
• bytecode can then be interpreted into machine code by any Python Virtual Machine

Bytecode

• Python and Java both are usually compiled to their respective versions of bytecode
• Bytecode is called such because each instruction in these languages is one byte (8 bits) of data
• Bytecode is an intermediate-level code that is interpreted “natively” by the corresponding virtual machine - a software simulation of a computer that has bytecode is its machine code.
• The advantage of bytecode is that all processors for which a Python Virtual Machine or Java Virtual Machine have been created can execute their respective bytecodes, whereas typically machine code differs from processor to processor. So any given machine code can only run on the processor family for which it was created, whereas bytecode can run on any computer that has a virtual machine installed.
• this allows Python and Java to be “write once, run anywhere” languages.
• see this explanation by a Python core developer for a more detailed description of what bytecode is (starting at 2’10” in the video)

Java bytecode

• Java was one of the early languages that relied on the bytecode compiler/interpreter paradigm
• in Java, any family of processors that has had a “Java Virtual Machine” (JVM) designed for it can run the Java bytecode
• the community who work on Java have made JVMs for most popular processor families
• a programmer writes high-level Java source code, this is typically compiled to bytecode, this bytecode is saved onto whatever computer the user wants it to run the program on, and the JVM interprets the bytecode to the appropriate machine code for that processor family
• thus Java is a “write once, run anywhere” sort of language

This paradigm is now common in more modern programming languages like Python, Ruby, and even PHP

Documentation

Documenting code is important for readability and maintenance of that code. Most languages have common conventions for how developers leave notes and document their code.

Python

• See Python docstring conventions
• All programs must be documented following these conventions

Java

• See documenting source code in Java
• All programs must be documented following these conventions