第4章アルゴリズムの言語による実現：低水準プログラミング言語

By the end of this section, you will be able to:

• Discuss the future of low-level programming

• Understand how the C language is used to develop firmware for embedded systems

• Develop kernel code using the C programming language

High-level languages are popular, and have their place, but there are certain applications where only middle-level languages such as C will do. This section showcases two such applications: firmware and kernel development.

The economic trends that diminish interest in low-level programming are expected to continue and even accelerate. We are using wider varieties of computer hardware—not just personal computers, mobile devices, and servers, but also narrower segments such as tablets, set-top boxes, video streamers, and system-on-chip (SoC) computers such as the Raspberry Pi (Figure 4.33).

The IoT is the growing network of products that are not used as a computer, but nevertheless contain an Internet-connected computer. IoT devices include but are not limited to smart voice assistant speakers, thermostats, home appliances, speakers, and tap payment systems. The computer embedded inside an IoT device is limited in terms of size, energy use, and cost, so it typically has a slow CPU and small memory. This makes middle-level languages well suited to writing IoT software; writing Internet-connected applications in low-level languages is impractical, and high-level languages may not be efficient enough.

Rust is a relatively new middle-level language created by the Mozilla Foundation in the 2010s. Many of C’s positive features are also found in Rust: efficient execution, portability, modularity, procedural and structured programming, and recursion. Rust has the capability to manipulate pointers but adds native safety features so that the compiler can help the programmer prevent bugs related to pointers. The language also includes some features that are more common to high-level languages and are unavailable in C, including higher-level data types (lists, maps, and sets), macros, templates, and parallel programming. These features do make Rust more complicated than C, though. Since Rust has the same positive attributes as C, with some additional desirable features, we expect to see increasing use of Rust as a middle-level language.

It is a best practice to give procedures descriptive names that help a reader understand what the procedure does. English has been globally accepted as the language of programming, but this can cause some problems in non–English-speaking countries. For example, “open_file” is more descriptive than “fopen”; it is also difficult to tell what “fopen” might mean out of context and especially if you are not a native English speaker. Unfortunately, procedure names in C are almost always written in English. How can this practice of using English names affect aspiring programmers whose first language is not English? How will it make it more difficult for programmers around the world to collaborate on source code?

Hardware is purely physical machinery, and software is purely digital code. To bridge this gap, we have firmware, which is very low-level code that communicates directly with hardware, and provides a convenient interface for other software. (“Firm” is the halfway point between “hard” and “soft.”)

If you want to learn to be an embedded systems engineer, it would be best to start from a simple hardware kit, rather than starting with the latest Intel or ARM chipset. Arduino is a hardware platform intended for creating simple, low-cost hardware for educational or hobbyist purposes. There are many series of Arduinos, but their "Arduino PLC Starter Kit" has a simple processor and comes with a guide book. Atmega328P has an 8-bit core, which is a good place to start digital circuit design and firmware development. You do not need to know how to draw schematics and layouts and assemble the chips. But you do need to know how to read schematics and understand how the chips are connected. Firmware developers should be able to read the schematics and figure out how to send data to the target device.