Pwnos design Document Version 0a
Design Organisational Overview
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DesignOrganisational OverviewThis section outlines the overall organisation of PwnOS, the relations between its major modules, and design aspects common to all or most major modules of PwnOS. To optimise the use of modern general-purpose CPUs, PwnOS is a 64-bit operating system for processors supporting x86-64 architecture and SSE3. Portability and backward compatibility often conflict with the goals of low time and space overhead, and so are not considered in the design of PwnOS. PwnOS is designed to best work with many CPU cores. Because of the similarity from software, the terms “CPU” and “CPU core” are used interchangeably in the rest of this document. The following figure presents the overall code architecture of PwnOS in terms of its modules and the relevant interfaces. 
Figure : Overall Architecture The reasoning behind having the heap memory management module and part of the synchronisation module accessible directly from Privilege Level 3 (PL3) is that some of the time overhead of system calls can be avoided by using regular function calls where possible. Managing heap memory does not require actively changing page tables or updating core data structures, so can be done from PL3. Likewise, synchronisation actions such as getting a lock that is currently free and releasing a lock on which no threads are blocking are both operations that can be done in PL3, and upon failure (e.g. the lock is not free when attempting to get it) can call PL0 to properly handle all cases. That these modules are in special, read-only, pages of memory common to all processes is also important. There will be a heap for modules in PL0 and a heap for each application, and duplication of the heap management code would be wasteful, so it is kept in common for all tasks using global, read-only pages for efficiency and accident avoidance. This is discussed further in the Page Memory Management section. Custom libraries in this read-only memory may include libraries developed by/for a user of PwnOS, the Code Cortex libraries, or any other libraries to be loaded here. The following diagram presents the dependencies between the modules of PwnOS. 
Figure : Module Dependencies Dependencies on (Fast) Sync have been omitted since all modules but Thread Scheduler and (Full) Sync have such a dependency. There are cyclic dependencies involving I/O, Page and Heap Memory, and Sync, and as such, they must be initialised without using any of the code therein. See the Booting section for more detail on initialisation. API SpecificationThe following table specifies the core Application Programming Interface of PwnOS. It has been designed to be simple, with meaningful names, and still have sufficient functionality. Parameters are passed by registers, and system calls are made using the special SYSCALL instruction for optimal performance. Library calls can be made as normal calls (after relocation).
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