From Code to CPU: The Journey of Execution

In the world of computing, the journey from source code to execution on a Central Processing Unit (CPU) involves several steps that can be broadly categorized into three types: machine code, intermediate code, and interpreted code.

Source Code Types

There are three primary types of source code used in modern computing: machine code, intermediate code, and interpreted code. Each type has its unique characteristics and implications for the execution process.

Machine Code

Machine code is the native language of the CPU. It is generated directly from the source code using an assembler or compiler. For example, machine code for a PC BIOS is compiled from high-level source code, optimized, and then converted into machine-specific instructions. This machine code is the only language a CPU can execute.

Intermediate Code

Intermediate code, also known as static code, is generated from the source code by a compiler but is not directly executable on the CPU. It is designed to be compatible with multiple CPU architectures. A good example of this is Java, which uses intermediate code known as Java bytecode. This bytecode is pre-compiled and optimized but requires a Java Virtual Machine (JVM) to run. The JVM acts as an interpreter that adjusts the intermediate code to run on the specific CPU.

Interpreted Code

Interpreted code is executed line by line as it is read from the source files. Languages like Python fall into this category, as do web pages. Interpreted code does not need to be compiled before execution, which makes it slower but highly portable. However, it poses a security risk since the source code can be modified by anyone.

Execution Environments

Executing code on a CPU requires two primary environments: bare-metal and operating system (OS) environments.

Bare-Metal Environment

In a bare-metal environment, code is written specifically for a computer and is stored in non-volatile memory such as ROM. Examples include BIOS and firmware in various devices like microwaves. This code runs directly on the CPU without the need for an OS.

Operating System Environment

An OS acts as a bridge between application programs and the hardware of a computer. It manages the execution of other programs stored in external storage and loaded into RAM. The OS is itself a bare-metal program, and it is responsible for coordinating the execution of various user applications.

A Practical Example: Web Browsers and the Compilation Process

Let's consider a web browser as an example to illustrate the complexity of the source code execution process. When a user boots a PC and opens a web browser, the following steps occur:

The operating system loads the browser executable from disk into RAM. The operating system calls the entry point, which starts the execution of the web browser program. The web browser then parses and interprets web pages, which can include intermediate code (like Java bytecodes) and source code (like JavaScript). Interpreters and virtual machines (like the JVM for Java) are loaded and used to execute the code within the web page.

Web pages often contain a combination of these elements. For instance, a web page might use JavaScript to dynamically change the appearance of the content. When a user clicks a button, JavaScript code is executed, modifying the page content accordingly.

Examples of Source Code Execution

The following is an HTML example that demonstrates the use of JavaScript:

!DOCTYPE htmlhtmlbody  h2What Can JavaScript Do?/h2  p idtext/p  button typebutton onclick(#39;text#39;).innerHTML  Text has changed!Click Me!/button/body/html

When this HTML is rendered in a browser and the user clicks the button, the JavaScript code is executed, changing the text content as shown:

Before button click: What Can JavaScript Do?

After button click: What Can JavaScript Do? Text has changed!

Note that this simple example showcases how JavaScript can dynamically alter the content of a web page based on user interaction.

Java, on the other hand, requires more complex compilation steps. Java source code is compiled into bytecode, which is then delivered to the PC and executed by the JVM. This process involves several stages but ultimately results in the same kind of execution as JavaScript on a web page.

Programs written in a generic way, such as those using C or Java, can run on multiple CPUs and OSes, increasing portability. However, even when a program is compiled for a specific CPU and OS, it still relies on specific OS APIs, which can limit its portability.

In summary, the journey from source code to CPU execution is a multi-step process that involves significant complexity, with different types of code (machine code, intermediate code, and interpreted code) and various environments (bare-metal and OS environments) playing crucial roles in the execution process.