The Implementation of the ULIX Literate Operating System








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A diary documenting the implementation of ULIX-i386

Welcome to the ULIX blog.

Ulix (Literate Unix) is a Unix-like operating system developed at University of Erlangen-Nürnberg. I use D. E. Knuth's concept of Literate Programming for the implementation and documentation. The goal was a fully working system which can be used in operating system courses to show students how OS concepts (such as paging and scheduling) can be implemented. Literate programs are very accessible because they can be read like a book; the order of presentation is not enforced by program logic or compiler restrictions, but instead is guided by the implementer's creative process.

Ulix is written in C and assembler for the Intel architecture. The literate programming part is handled by noweb.

On this page I document my progress with the implementation.

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v0.03: Paging with Higher Half Kernel (12.08.2011)

It works: now the kernel uses virtual addresses starting at 0xC0000000 (the last of four gigabytes). This is a preparation for allowing user mode code (processes) to run in the lower 3 GBytes. It took me all a whole day to get this to work because my original code did not setup enough page -> frame mappings; it mapped some virtual memory to [0..1MB], but the system was loaded to [1MB..2MB], so after enabling paging nothing useful was found...
Now the next step will be user mode. For a starter I will just try to switch to user mode and back; there's some meddling with the stack involved because Intel CPUs have only one instruction that can be used to enter user mode: IRET (return from interrupt) which finds the next instruction pointer on the stack. This will later turn into the init process from which all other processes will be forked.
Another thing to think of now is memory allocation. When I start handling processes I will need a list of free page frames, so the physical memory should be identified in the beginning. There's a lot of untested frame allocation code in Felix Freiling's non-Intel Ulix, so I'll look how that works on an Intel chip.

Resources used:
Higher Half With GDT, http://wiki.osdev.org/Higher_Half_With_GDT

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