University of helsinki
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| Linux: a Portable Operating System Linus Torvalds Helsinki January 31, 1997 Master of Science Thesis UNIVERSITY OF HELSINKI Department of Computer Science Tiedekunta/Osasto — Fakultet/Sektion — Faculty Laitos — Institution — Department Tekijä — Författare — Author Työn nimi — Arbetets titel — Title Oppiaine — Läroämne — Subject Työn laji — Arbetets art — Level Aika — Datum — Month and year Sivumäärä — Sidoantal — Number of pages Tiivistelmä — Referat — Abstract Avainsanat — Nyckelord — Keywords Säilytyspaikka — Förvaringsställe — Where deposited Muita tietoja — Övriga uppgifter — Additional information HELSINGIN YLIOPISTO — HELSINGFORS UNIVERSITET — UNIVERSITY OF HELSINKI Faculty of Science Department of Computer Science Linus Torvalds Linux: a Portable Operating System Computer Science Master of Science thesis January 1997 52 p. Linux, Operating System, Portability, Alpha, Sparc Department of Computer Science Library, serial number C–1997–12 We explore the hardware portability issues in Linux that were uncovered when porting the operating system to multiple CPU and bus architectures. We also discuss software interface portability issues, especially with regard to binary compatibility with other operating systems that can share the same hardware platform. The approach taken in Linux is described, with a few example architectures covered in some more detail. Computing Reviews Classification: D.4 (Operating Systems), C.1 (Processor Architectures), D.2.7 (Software Engineering — Portabil- ity), D.4.7 (Operating Systems — Organization and Design) Acknowledgements While the Linux project has been closely associated with me personally, partly due to the name, I would like to make it very clear that the Linux operating system is a huge project done co-operatively by lots of people all over the world. Even if you discount all the user-level programs that are an integral part of any running Linux system, just the kernel contains code from hundreds of people from all around the world. Thanks to all of you. Contents 1 Introduction 1 2 Linux — the Design and Implementation 7 2.1
The design — compatibility . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3 Kernel organization . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4 The virtual machine implementation . . . . . . . . . . . . . . . . . 11 3 Software Interface Portability Issues 14 3.1
The implementation of the software interface . . . . . . . . . . . . . 15 3.2 Personalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4 Hardware Portability Issues 19 4.1
Data representation portability issues . . . . . . . . . . . . . . . . . 19 4.1.1 Different data sizes . . . . . . . . . . . . . . . . . . . . . . . 20 4.1.2 The impact of alignment restrictions and byte order . . . . . 22 4.2 Kernel memory management . . . . . . . . . . . . . . . . . . . . . . 24 4.2.1 Memory management through virtual page tables . . . . . . 26 4.2.2 Page table mapping coherence . . . . . . . . . . . . . . . . . 29 4.3 Ensuring cache coherency and atomic operations . . . . . . . . . . . 30 4.3.1 Instruction cache coherency . . . . . . . . . . . . . . . . . . 30 4.3.2 Data cache coherency . . . . . . . . . . . . . . . . . . . . . . 33 4.3.3 The case against virtual data caches . . . . . . . . . . . . . 34 4.4 Multiprocessor issues . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.4.1 Symmetric multi-processing . . . . . . . . . . . . . . . . . . 39 4.4.2 Massively parallel systems . . . . . . . . . . . . . . . . . . . 42 4.5 Device drivers: accessing the I/O bus . . . . . . . . . . . . . . . . . 43 4.5.1 Proprietary buses . . . . . . . . . . . . . . . . . . . . . . . . 44 4.5.2 PCI — the emerging standard . . . . . . . . . . . . . . . . . 45 5 Example Architectures 46 5.1
The Alpha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5.2 The Sparc line of computers . . . . . . . . . . . . . . . . . . . . . . 48 6 Future Work 51 References 53
1 Introduction This thesis is an introduction to the portability issues of the Linux kernel. It is not only meant to be my Masters thesis, I hope it will actually be useful for people who want to port Linux to other architectures, or for people who just want to know the design issues and goals of Linux on different platforms (as opposed to the more general goals of Linux in general). In an attempt to make this practical, I try to explain the general issues, usually followed by a comment on one or more specific ports with some pointers on how something is implemented in practice 1 .
is built up from the basic kernel and all the system and user applications that are usually running on top of the kernel, in the context of this thesis only the kernel itself is considered. The portability of system and user applications is here considered a totally different issue, even though there are obviously some common concerns. Also, you should not expect this to be a line-by-line (or even very low-level) explanation of the kernel — rather the opposite. Instead of trying to explain what all the specific functions do, this paper tries to explain the basic ideas, and the specific examples should be seen as just clarifying how Linux works on some specific hardware in some specific case. The thesis generally follows a common format: each section first explains the generic problems and potential solutions, to then be followed by an “Implementa- tion” section that gives Linux-specific implementation details. The reader should be able to follow the text even with little or no knowledge of Linux itself, although a general knowledge of the issues is obviously expected. History of Linux portability People who have followed Linux from the very beginning may find the title of this paper, “Linux: a Portable Operating System”, a rather ironic statement. Being portable was not what Linux was about initially; the early versions of Linux were extremely unportable. 1 For the same reason this paper will also be available in an electronic version which I will try to keep up-to-date as people send me comments or as changes to the system occur. Linux is by no means a static system, and some of the issues, especially when it comes to multiple CPU’s, are so young that major changes can still happen. 1
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