(*
- Copyright 2014, NICTA
- This software may be distributed and modified according to the terms of
- the BSD 2-Clause license. Note that NO WARRANTY is provided.
- See "LICENSE_BSD2.txt" for details.
- @TAG(NICTA_BSD) *)
This repo contains a skeleton OS for AOS students to begin with
** Design Features **
- Memory Management An abstraction layer exists to hide the dirty details of untyped memory. The key interface functions provided to students include
steal_mem(sizebits) -> steals 1<<sizebits bytes of memory get_mem(&low, &high) -> returns the low and high addresses of available memory
It also has a wrapper for UntypedRetype for convenience
-
DMA memory The main initialisation function calls steal_mem early to reserve memory for dma_malloc. It is important to call steal_mem as the size of DMA memory is much larger than the 16K max allocation size that will be supported by the memory manager. It is important to do this early to ensure the frame table has not been initialised yet and also to reduce fragmentation cause be alignment.
-
CSpace the cspace of sos is managed by libcsm. 2nd Level CNodes are limited in size to be the euivelent size of a page directory (16K) to simplify object allocation single level cspaces for user apps are exposed to the students who must manage this themselves (need only include EP caps for sos, timer_server, pager? etc). These caps can be at a fixed offset and therefore fee/allocated slots need not be managed.
- Circular dependancy: to allocate an object, we need to reserve memory and allocate a cap. The process of allocating a cap may require more memory to be reserved for a new CNode. By seperating the memory allocation from the object retype/creating, we remove this circular dependancy.
The interface is a simple "sos_cspace_init" which does magic behind the scenes to transform the cspace given the restrictions mentioned above. From here, the students will use the libcsm interface to allocate/copy/free caps as needed
- Process Management No process management the function "create_initial_thread" is too long! but it serves as a step by step guide of how to setup a thread. It does not do any book keeping at all and maps in a single 4K page of stack as no pager is implemented.
There is no interface here
To initialise the system and intial threads, a skeleton frame table is implemented with a alloc function but no free. Alloc does nothing but reserve memory and return base address. It is up to the caller to retype the memory aquired.