Original post: https://www.seds.nl/posts/writting-a-kernel-part-1/
In this series of posts, I intend to document my re-go on writing a small, portable kernel for the RISC-V architecture. I developed a micro-kernel for the RISC-V ISA in my bachelor thesis, however, due to the time it takes on developing a kernel, and the time I had on writing a bachelor's thesis, I choose on writing the kernel for the Sifive's HiFive1 Rev B development board due to their well written bare metal compatibility Library for the board, low cost and most importantly, RISC-V.
By using Sifive's API, it took out the overhead of having to deal with the configurations of the clock, interrupt, CPU, and generally required hardware for the kernel to run. Long story short, I submitted my thesis, got approved, and now I am willing to rewrite the code all over. During the development of my thesis, there were components I needed to write and have a better understanding, so I was not able to pay as much attention as I wanted to the project design, structural decisions, and general kernel/architectural decisions. One of the decisions I made during the development phase was not to document my ideas, questions, and answers I had. This post attempts to fix this problem, by documenting on how to write a Kernel for the RISC-V architecture.
Before we start, let's remember some basic computer concepts and try to assemble them together so we can better understand how each component fits within a kernel.
A modern computer
A modern computer consists of at least one central processing unit, main memory, some data storage, and another type of input and output devices. Computer architecture is the specification on which describes how software and hardware may interact with each other. Computer processors provide an abstract model interface known as the instruction set, which serves as an interface between the hardware and software.
The need for a kernel
Application developers need to communicate with these types of hardware, and by having to learn each computer specification is difficult and time-consuming. The kernel is the middle section of the abstraction between hardware and user software (Figure 1). It manages computer resources to allow application programmers to communicate with them.
By having a simpler model of the computer, application programmers can write less error-prone software by leaving the hardware complexity to the kernel.
Kernels may provide little to no abstraction at all. Kernels may be necessary for specific purposes instead of offering any resource to an upper layer. We can take the traffic light system as an example. The system may need to change states every 60 seconds, and that is all. For that, instead of having a Kernel, we could do some simple bare-metal programming, enable a timer interrupt and have a timer interrupt handler handle state changes1.
The computer organization and architecture
Although there are different distinctions made between computer architecture and organization, the first refers to what systems and application programmers see, which are the attributes that have a direct impact on the execution of a program, for example, whether a computer will have a multiply instruction, where the latter refers to the operational unit and its interconnections that make the architectural specifications, such as whether the multiply instruction will be implemented by a multiply unit or by a mechanism of repeated add unit. The ISA, the numbers of bits used to represent data types, the IO structure, and approaches for memory addressing are all organizational issues that need to be structured (Stallings 2011).
The computer organization creates a hierarchy of hardware attributes details that are transparent to the programmer, such the interface between the computer and peripherals, the memory technology used, the type of processor and control signals (Stallings 2011). Computer architecture should offer a clean abstract set to simplify design, modeling, and allow running software to communicate with the hardware available (Patterson and Hennessy 2017).
The processor needs extra hardware in order to do its job, RAM to store program and data, support for logic and at least one I/O device to transfer data between the computer and the outside world (Catsoulis 2005).
Processors should be designed to process, store and retrieve data, but for that to happen, the processor has to go through several stages, where (i) fetch the instruction from memory, which could be the register, cache or main memory, (ii) decode the instruction to figure out what action is required to run, (iii) fetch data from memory or a IO module if required, (iv) perform arithmetic or logical operation on the fetched data if required and (v) the results of an execution may require to be written back to memory or the IO module (Stallings 2011).
Final conclusions
We understood a modern computer is a complex set of hardware with different factors and use-cases. We saw how a kernel should interact with the hardware and how it should abstract the underlying hardware resources and provide some sort of API for programmers and users to interact. Not all problems are solvable with a kernel, some times pure bare-metal programming is what it takes to solve the problem, however, when we are willing to multitask, exchange communication between different resources, a Kernel might come in handy dealing with those problems.
Computer organization is hierarchy of hardware attribute details that are transparent to the programmer, like IO functions, inter-process communication, memory management and etc.
I hope with this post you were able to understand or remember key concepts of fundamental "modern" computing. Part 2 we will start implementing the initial boot of our kernel, starting by initializing the CPU and booting into QEMU's RISC-V emulator and as we go along, I will introduce more concepts like those introduced in this post.
References
Catsoulis, John. 2005. Designing Embedded Hardware. O'Reilly Media, Inc.
Patterson, D. A., and J. L. Hennessy. 2017. Computer Organization and Design Risc-V Edition: The Hardware Software Interface. The Morgan Kaufmann Series in Computer Architecture and Design. Elsevier Science https://books.google.com.br/books?id=H7wxDQAAQBAJ.
Stallings, William. 2011. Operating Systems: Internals and Design Principles. 7th ed. Upper Saddle River, NJ, USA: Prentice Hall Press.
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I don't know how an actual traffic light system works, but I am assuming it's some state-machine that handles interrupts of some kind. ↩
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