RX/286

From Ruud Koot

RX/286 is a microkernel designed around the Intel 80286's 16-bit segmented protected virtual address mode.

Contents 1 Design 1.1 Memory management 1.2 Protection 1.3 Task management 1.4 File system 2 Other operating systems

[edit] Design

[edit] Memory management

Keep track of physical memory in circular doubly linked lists for block sizes of 2^k aligned on 2^k boundaries (for 8 ≤ k ≤ 16; e.g. blocks from 256 bytes through 65536 bytes.) Pointers are stored in the unallocated physical memory.

The full 16 MB address space is divided into 2¹⁶ 256 B pages. As segments are never larger than 64 kB it is not useful to track larger contagious regions of physical memory.

allocate takes O(1), although with a higher constant if:

• no pages of the correct size are available and a larger page needs to be split,
• the allocations is not of size 2^k and the remaining space needs to be split into several new pages.

free takes O(1). Contagious smaller pages are not consolidated. This could be done by using a red-black tree instead of a linked list, but would increase the runtime of free to O(log n).

Memory can still get fragmented, so a defragmentation process and additional allocation heuristics would be useful.

It might be necessary to keep track of different regions of memory (memory below 1 MB, memory above 1 MB) and different kinds of memory (regular memory, device memory which should not be reallocatable if freed) and support for bank switched memory (like expanded memory and possible a swap file).

[edit] Protection

Ring 0	Hypervisor
Ring 1	Supervisor
Ring 2	Drivers
Ring 3	Applications

[edit] Task management

[edit] File system

[edit] Other operating systems

• Coherent 3.x
• Novell NetWare 286
• Microsoft OS/2 1.x
• Microsoft Windows 3.1 (in "standard mode")
• SCO Xenix 2.3.2
• Various Unices (INTERACTIVE UNIX, Microport System V/286, Venix)