Porting Linux to the Digital Alpha

A presentation given at the Boston Computer Society Linux & Unix User Group meeting, March 15, 1995.

The slides from this talk are available here in the file slides.tar.gz

What is Linux/Alpha?

Small, cheap, high-performance operating system for entry-level Alpha systems (less $$$)
Freely-redistributable reference OS for Digital's OEM (chip & board) customers
Establish Digital's presence in "Super-PC" marketplace (Alpha, PowerPC, P6/P7, etc).
Stimulate "impulse" purchase of Alpha Chips [ Don't worry... we'll make more 8-) ]

Linux/Alpha should be:

Small: Minimal runtime on 1-2 floppies, full system on 340Mb disk; Text-only system in 8Mb memory, X-based system in 16Mb memory
Cheap: Linux/Alpha will be distributed under GPL (ftp://ftp.dec.com/pub/DEC/Linux-Alpha); DEC may develop added-value components for sale (e.g. high-performance X servers), but a fully functional system will be free.
Compatible: Source-compatible with Linux/Intel; Investigating binary compatibility options with Linux/Intel
Fast: Should not unduly hamper CPU performance; It's an Alpha; what can I say?

Complementary efforts
- Us: Fast time-to-market, 32-bit port, minimal changes to existing code structure.
- Linus: 64-bit port, extensive changes to existing code structure to accommodate multiple architectures
Different emphasis means each of us solves different problems and can save the other problems later on.
Once Linux/Alpha 1.0 is self-hosting (5/95 timeframe), it can be used for 1.2 development.
Eventual unification of code streams.
- We will contribute what we learned from 1.0 to Linus for the 1.2 codebase
- Will eventually have 64-bit 1.2 kernel with both 32 and 64-bit APIs and ABIs
  - Compatibility with Linux/Alpha 1.0 binaries
  - Useful for any possible Intel binary-compatibility strategy.

DEC 2000 AXP/DECpc 150 AXP (aka Jensen): Adaptec 1742 SCSI; Floppy, keyboard, text-mode VGA
DECchip Evaluation Boards: Floppy, keyboard, text-mode VGA
Digital AXPpci/33 Motherboard: Floppy, keyboard, text-mode VGA
Coming Soon:: Adaptec 1542 SCSI; NCR 53C810 SCSI
Care to contribute? 8-)

Similarities:: Little-endian; Support 32-bit integers & pointers; Paged memory management
Differences:: RISC vs. CISC; Alpha does not have byte/word memory access instructions; OSF PAL supports 7 interrupt levels, Intel has two (cli/sti); Intel has segmented as well as paged memory management; Alpha has lighter-weight procedure-call mechanism than Intel; Alpha has more register context to save than Intel. Intel has more non-register context to save than Alpha.

Make it work first, make it fast later
32-bit system: Default integer word size is 32 bits, and all addresses fit in 32 bits.
- Eliminates 64-bit pitfalls in existing 32-bit code
- Uses less memory & disk than 64-bit code
- PC-class systems do not need 64-bit addressing capability (yet!)
- 64-bit quantities still available for computation via long long and double datatypes.
Console and PALcode
- SRM console on all platforms that support it.
- Linux Mini-loader for most PC-class platforms (except DEC 2000)
- OSF PALcode
  - Writing new PAL is a pain
  - OSF PAL generally applicable for most UNIX ports
  - Freely-distributable version available
Memory Layout

0x00000000-0x3fffffff: User Code/Data/Stack
0x40000000-0x6fffffff: Unused
0x70000000-0x7bffffff: 32-bit KSEG
0x7c000000-0x7fffffff: Kernel code/data/stk
- Addresses cannot exceed 0x7fffffff due to sign-extension considerations
- Kernel and user in same memory space to allow easy kernel access to user memory
- KSEG: virtual = physical + constant
  - 64-bit: constant = 0xfffffc0000000000 (provided by PALcode)
  - 32-bit: constant = 0x70000000 (We map this ourselves)
- Current 32-bit KSEG boundaries limit us to 192Mb of physical memory. Shouldn't be a problem for the first release...
- Kernel pages protected: KRW or KRO, UNA
- User pages protected: KRW, URW or URO
- Kernel can access user memory, user cannot access kernel memory.

Kernel - context switching: Intel gets complete switch by calling task gate; OSF PAL switches minimal context, OS must switch rest. Most context lives in task_struct or on kernel stack.
Memory Management: Alpha 3-level page table "wastes" 2 pages per process; significant with big (8K) pages and small memory (8Mb system has only 1024 pages!) We have an answer 8-); Memory-management algorithms were portable, but implementations were not. We had to "rewrite" much of memory.c, keeping the algorithms but varying the details (Linus redid the memory management in 1.2 to make the code more general).
File System: Executable loading, mostly detail work; Other than that, filesystem worked flawlessly from the beginning!
Device Drivers: Bus access on Intel is straightforward (inb/outb, bus memory is physical memory); Bus access on Alpha requires glue logic in both hardware and software ("magic swizzles"); Many PC devices come with Intel initialization code in onboard BIOS. Intel PCs, of course, will run this code before Linux boots. Alpha PCs won't.; DMA engines operate differently on PC-class systems and on Alpha systems; Problems need to be solved only once; other drivers "should" fall into place