Chapter 8. Future developments

The HAL is not complete, and will evolve and increase over time. Among the intended developments are:

  • Common macros for interpreting the contents of a saved machine context. These would allow portable code, such as debug stubs, to extract such values as the program counter and stack pointer from a state without having to interpret a HAL_SavedRegisters structure directly.

  • Debugging support. Macros to set and clear hardware and software breakpoints. Access to other areas of machine state may also be supported.

  • Static initialization support. The current HAL provides a dynamic interface to things like thread context initialization and ISR attachment. We also need to be able to define the system entirely statically so that it is ready to go on restart, without needing to run code. This will require extra macros to define these initializations. Such support may have a consequential effect on the current HAL specification.

  • CPU state control. Many CPUs have both kernel and user states. Although it is not intended to run any code in user state for the foreseeable future, it is possible that this may happen eventually. If this is the case, then some minor changes may be needed to the current HAL API to accommodate this. These should mostly be extensions, but minor changes in semantics may also be required.

  • Physical memory management. Many embedded systems have multiple memory areas with varying properties such as base address, size, speed, bus width, cacheability and persistence. An API is needed to support the discovery of this information about the machine's physical memory map.

  • Memory management control. Some embedded processors have a memory management unit. In some cases this must be enabled to allow the cache to be controlled, particularly if different regions of memory must have different caching properties. For some purposes, in some systems, it will be useful to manipulate the MMU settings dynamically.

  • Power management. Macros to access and control any power management mechanisms available on the CPU implementation. These would provide a substrate for a more general power management system that also involved device drivers and other hardware components.

  • Generic serial line macros. Most serial line devices operate in the same way, the only real differences being exactly which bits in which registers perform the standard functions. It should be possible to develop a set of HAL macros that provide basic serial line services such as baud rate setting, enabling interrupts, polling for transmit or receive ready, transmitting and receiving data etc. Given these it should be possible to create a generic serial line device driver that will allow rapid bootstrapping on any new platform. It may be possible to extend this mechanism to other device types.

2017-02-09
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