# Operating System Abstraction Layer (OSAL)

Some specific information on the provided OSALs are provided.

## Linux OSAL

This OSAL can be used to compile for Linux host systems like Ubuntu 20.04 or for 
embedded Linux targets like the Raspberry Pi. This OSAL generally requires threading support
and real-time functionalities. This is generally done by adding `-lrt` and `-lpthread` to the linked libraries in the compilation process. The CMake build support provided will do this automatically for the `fsfw` target. It should be noted that most UNIX systems need to be configured specifically to allow the real-time functionalities required by the FSFW.

More information on how to set up a Linux system accordingly can be found in the 
[Linux README of the FSFW example application](https://egit.irs.uni-stuttgart.de/fsfw/fsfw_example/src/branch/master/doc/README-linux.md#top)

## Hosted OSAL

This is the newest OSAL. Support for Semaphores has not been implemented yet and will propably be implemented as soon as C++20 with Semaphore support has matured. This OSAL can be used to run the FSFW on any host system, but currently has only been tested on Windows 10 and Ubuntu 20.04. Unlike the other OSALs, it uses dynamic memory allocation (e.g. for the message queue implementation). Cross-platform serial port (USB) support might be added soon.

## FreeRTOS OSAL

FreeRTOS is not included and the developer needs to take care of compiling the FreeRTOS sources and adding the `FreeRTOSConfig.h` file location to the include path. This OSAL has only been tested extensively with the pre-emptive scheduler configuration so far but it should in principle also be possible to use a cooperative scheduler. It is recommended to use the `heap_4` allocation scheme. When using newlib (nano), it is also recommended to add `#define configUSE_NEWLIB_REENTRANT` to the FreeRTOS configuration file to ensure thread-safety.

When using this OSAL, developers also need to provide an implementation for the `vRequestContextSwitchFromISR` function. This has been done because the call to request a context switch from an ISR is generally located in the `portmacro.h` header and is different depending on the target architecture or device.

## RTEMS OSAL

The RTEMS OSAL was the first implemented OSAL which is also used on the active satellite Flying Laptop.

## TCP/IP socket abstraction

The Linux and Host OSAL provide abstraction layers for the socket API. Currently, only UDP sockets have been imlemented. This is very useful to test TMTC handling either on the host computer directly (targeting localhost with a TMTC application) or on embedded Linux devices, sending TMTC packets via Ethernet.