"csharp" entries

Where apps end and the system begins

Exploring the system calls and control flow that underpin high-level languages.

Editor’s note: Sometimes we can forget how important it is to truly understand how a system works, and how the nuts and bolts affect our everyday activities. Marty Kalin asks us to dive a little deeper and strengthen our computational thinking abilities.

It’s clear that applications need system resources to execute: a processor, memory, and usually I/O devices such as the keyboard and screen. It’s less clear how applications gain access to these shared resources, which are under operating system (OS) control. The OS, like any good manager, is efficient and unobtrusive as it handles resource requests from applications. Let’s take a look at how applications interact with the OS, in both routine and dramatic fashion.

Consider what happens when a print statement executes. Here’s a Ruby example:

The Ruby puts statement wraps a call to a high-level I/O function in the standard C library (in this case, printf), which acts as the interface between resource-requesting applications and resource-granting OS routines. In this example, the screen is the requested resource. The standard library interacts seamlessly with the OS, which also is written in C with some assembly language. The library function printf is high-level because, as the f in the name indicates, the function can format the bytes to be written as integers, floating-point values, and character strings such as Hello, world!. In systems-speak, the Ruby application and the C library function execute in user space, which does not bestow the rights and privileges needed to control system resources such as the screen.

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