eCos Reference Manual
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# Architecture Characterization

These are definition that are related to the basic architecture of the CPU. These include the CPU context save format, context switching, bit twiddling, breakpoints, stack sizes and address translation.

Most of these definition are found in `cyg/hal/hal_arch.h`. This file is supplied by the architecture HAL. If there are variant or platform specific definitions then these will be found in `cyg/hal/var_arch.h` or `cyg/hal/plf_arch.h`. These files are include automatically by this header, so need not be included explicitly.

## Register Save Format

    typedef struct HAL_SavedRegisters
    {
        /* architecture-dependent list of registers to be saved */
    } HAL_SavedRegisters;

This structure describes the layout of a saved machine state on the stack. Such states are saved during thread context switches, interrupts and exceptions. Different quantities of state may be saved during each of these, but usually a thread context state is a subset of the interrupt state which is itself a subset of an exception state. For debugging purposes, the same structure is used for all three purposes, but where these states are significantly different, this structure may contain a union of the three states.

## Thread Context Initialization

    HAL_THREAD_INIT_CONTEXT( sp, arg, entry, id )

This macro initializes a thread's context so that it may be switched to by `HAL_THREAD_SWITCH_CONTEXT()`. The arguments are:

sp

  A location containing the current value of the thread's stack pointer. This should be a variable or a structure field. The SP value will be read out of here and an adjusted value written back.

arg

  A value that is passed as the first argument to the entry point function.

entry

  The address of an entry point function. This will be called according the C calling conventions, and the value of `arg` will be passed as the first argument. This function should have the following type signature `void entry(CYG_ADDRWORD arg)`.

id

  A thread id value. This is only used for debugging purposes, it is ORed into the initialization pattern for unused registers and may be used to help identify the thread from its register dump. The least significant 16 bits of this value should be zero to allow space for a register identifier.

## Thread Context Switching

    HAL_THREAD_LOAD_CONTEXT( to )
    HAL_THREAD_SWITCH_CONTEXT( from, to )

These macros implement the thread switch code. The arguments are:

from

  A pointer to a location where the stack pointer of the current thread will be stored.

to

  A pointer to a location from where the stack pointer of the next thread will be read.

For `HAL_THREAD_LOAD_CONTEXT()` the current CPU state is discarded and the state of the destination thread is loaded. This is only used once, to load the first thread when the scheduler is started.

For `HAL_THREAD_SWITCH_CONTEXT()` the state of the current thread is saved onto its stack, using the current value of the stack pointer, and the address of the saved state placed in `*from`. The value in `*to` is then read and the state of the new thread is loaded from it.

While these two operations may be implemented with inline assembler, they are normally implemented as calls to assembly code functions in the HAL. There are two advantages to doing it this way. First, the return link of the call provides a convenient PC value to be used in the saved context. Second, the calling conventions mean that the compiler will have already saved the caller-saved registers before the call, so the HAL need only save the callee-saved registers.

The implementation of `HAL_THREAD_SWITCH_CONTEXT()` saves the current CPU state on the stack, including the current interrupt state (or at least the register that contains it). For debugging purposes it is useful to save the entire register set, but for performance only the ABI-defined callee-saved registers need be saved. If it is implemented, the option `CYGDBG_HAL_COMMON_CONTEXT_SAVE_MINIMUM` controls how many registers are saved.

The implementation of `HAL_THREAD_LOAD_CONTEXT()` loads a thread context, destroying the current context. With a little care this can be implemented by sharing code with `HAL_THREAD_SWITCH_CONTEXT()`. To load a thread context simply requires the saved registers to be restored from the stack and a jump or return made back to the saved PC.

Note that interrupts are not disabled during this process, any interrupts that occur will be delivered onto the stack to which the current CPU stack pointer points. Hence the stack pointer should never be invalid, or loaded with a value that might cause the saved state to become corrupted by an interrupt. However, the current interrupt state is saved and restored as part of the thread context. If a thread disables interrupts and does something to cause a context switch, interrupts may be re-enabled on switching to another thread. Interrupts will be disabled again when the original thread regains control.

## Bit indexing

    HAL_LSBIT_INDEX( index, mask )
    HAL_MSBIT_INDEX( index, mask )

These macros place in `index` the bit index of the least significant bit in `mask`. Some architectures have instruction level support for one or other of these operations. If no architectural support is available, then these macros may call C functions to do the job.

## Idle thread activity

    HAL_IDLE_THREAD_ACTION( count )

It may be necessary under some circumstances for the HAL to execute code in the kernel idle thread's loop. An example might be to execute a processor halt instruction. This macro provides a portable way of doing this. The argument is a copy of the idle thread's loop counter, and may be used to trigger actions at longer intervals than every loop.

## Reorder barrier

    HAL_REORDER_BARRIER()

When optimizing the compiler can reorder code. In some parts of multi-threaded systems, where the order of actions is vital, this can sometimes cause problems. This macro may be inserted into places where reordering should not happen and prevents code being migrated across it by the compiler optimizer. It should be placed between statements that must be executed in the order written in the code.

## Breakpoint support

    HAL_BREAKPOINT( label )
    HAL_BREAKINST
    HAL_BREAKINST_SIZE

These macros provide support for breakpoints.

`HAL_BREAKPOINT()` executes a breakpoint instruction. The label is defined at the breakpoint instruction so that exception code can detect which breakpoint was executed.

`HAL_BREAKINST` contains the breakpoint instruction code as an integer value. `HAL_BREAKINST_SIZE` is the size of that breakpoint instruction in bytes. Together these may be used to place a breakpoint in any code.

## GDB support

    HAL_THREAD_GET_SAVED_REGISTERS( sp, regs )
    HAL_GET_GDB_REGISTERS( regval, regs )
    HAL_SET_GDB_REGISTERS( regs, regval )

These macros provide support for interfacing GDB to the HAL.

`HAL_THREAD_GET_SAVED_REGISTERS()` extracts a pointer to a `HAL_SavedRegisters` structure from a stack pointer value. The stack pointer passed in should be the value saved by the thread context macros. The macro will assign a pointer to the `HAL_SavedRegisters` structure to the variable passed as the second argument.

`HAL_GET_GDB_REGISTERS()` translates a register state as saved by the HAL and into a register dump in the format expected by GDB. It takes a pointer to a `HAL_SavedRegisters` structure in the `regs` argument and a pointer to the memory to contain the GDB register dump in the `regval` argument.

`HAL_SET_GDB_REGISTERS()` translates a GDB format register dump into a the format expected by the HAL. It takes a pointer to the memory containing the GDB register dump in the `regval` argument and a pointer to a `HAL_SavedRegisters` structure in the `regs` argument.

## Setjmp and longjmp support

    CYGARC_JMP_BUF_SIZE
    hal_jmp_buf[CYGARC_JMP_BUF_SIZE]
    hal_setjmp( hal_jmp_buf env )
    hal_longjmp( hal_jmp_buf env, int val )

These functions provide support for the C `setjmp()` and `longjmp()` functions. Refer to the C library for further information.

## Stack Sizes

    CYGNUM_HAL_STACK_SIZE_MINIMUM
    CYGNUM_HAL_STACK_SIZE_TYPICAL

The values of these macros define the minimum and typical sizes of thread stacks.

`CYGNUM_HAL_STACK_SIZE_MINIMUM` defines the minimum size of a thread stack. This is enough for the thread to function correctly within eCos and allows it to take interrupts and context switches. There should also be enough space for a simple thread entry function to execute and call basic kernel operations on objects like mutexes and semaphores. However there will not be enough room for much more than this. When creating stacks for their own threads, applications should determine the stack usage needed for application purposes and then add `CYGNUM_HAL_STACK_SIZE_MINIMUM`.

`CYGNUM_HAL_STACK_SIZE_TYPICAL` is a reasonable increment over `CYGNUM_HAL_STACK_SIZE_MINIMUM`, usually about 1kB. This should be adequate for most modest thread needs. Only threads that need to define significant amounts of local data, or have very deep call trees should need to use a larger stack size.

## Address Translation

    CYGARC_CACHED_ADDRESS(addr)
    CYGARC_UNCACHED_ADDRESS(addr)
    CYGARC_PHYSICAL_ADDRESS(addr)

These macros provide address translation between different views of memory. In many architectures a given memory location may be visible at different addresses in both cached and uncached forms. It is also possible that the MMU or some other address translation unit in the CPU presents memory to the program at a different virtual address to its physical address on the bus.

`CYGARC_CACHED_ADDRESS()` translates the given address to its location in cached memory. This is typically where the application will access the memory.

`CYGARC_UNCACHED_ADDRESS()` translates the given address to its location in uncached memory. This is typically where device drivers will access the memory to avoid cache problems. It may additionally be necessary for the cache to be flushed before the contents of this location is fully valid.

`CYGARC_PHYSICAL_ADDRESS()` translates the given address to its location in the physical address space. This is typically the address that needs to be passed to device hardware such as a DMA engine, ethernet device or PCI bus bridge. The physical address may not be directly accessible to the program, it may be re-mapped by address translation.

## Global Pointer

    CYGARC_HAL_SAVE_GP()
    CYGARC_HAL_RESTORE_GP()

These macros insert code to save and restore any global data pointer that the ABI uses. These are necessary when switching context between two eCos instances - for example between an eCos application and RedBoot.

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