Chapter 5. Exception Handling

Most of the HAL consists of simple macros or functions that are called via the interfaces described in the previous section. These just perform whatever operation is required by accessing the hardware and then return. The exception to this is the handling of exceptions: either synchronous hardware traps or asynchronous device interrupts. Here control is passed first to the HAL, which then passed it on to eCos or the application. After eCos has finished with it, control is then passed back to the HAL for it to tidy up the CPU state and resume processing from the point at which the exception occurred.

The HAL exceptions handling code is usually found in the file vectors.S in the architecture HAL. Since the reset entry point is usually implemented as one of these it also deals with system startup.

The exact implementation of this code is under the control of the HAL implementer. So long as it interacts correctly with the interfaces defined previously it may take any form. However, all current implementation follow the same pattern, and there should be a very good reason to break with this. The rest of this section describes these operate.

Exception handling normally deals with the following broad areas of functionality:

  • Startup and initialization.
  • Hardware exception delivery.
  • Default handling of synchronous exceptions.
  • Default handling of asynchronous interrupts.

5.1. HAL Startup

Execution normally begins at the reset vector with the machine in a minimal startup state. From here the HAL needs to get the machine running, set up the execution environment for the application, and finally invoke its entry point.

The following is a list of the jobs that need to be done in approximately the order in which they should be accomplished. Many of these will not be needed in some configurations.

  • Initialize the hardware. This may involve initializing several subsystems in both the architecture, variant and platform HALs. These include:

    • Initialize various CPU status registers. Most importantly, the CPU interrupt mask should be set to disable interrupts.
    • Initialize the MMU, if it is used. On many platforms it is only possible to control the cacheability of address ranges via the MMU. Also, it may be necessary to remap RAM and device registers to locations other than their defaults. However, for simplicity, the mapping should be kept as close to one-to-one physical-to-virtual as possible.
    • Set up the memory controller to access RAM, ROM and I/O devices correctly. Until this is done it may not be possible to access RAM. If this is a ROMRAM startup then the program code can now be copied to its RAM address and control transferred to it.
    • Set up any bus bridges and support chips. Often access to device registers needs to go through various bus bridges and other intermediary devices. In many systems these are combined with the memory controller, so it makes sense to set these up together. This is particularly important if early diagnostic output needs to go through one of these devices.
    • Set up diagnostic mechanisms. If the platform includes an LED or LCD output device, it often makes sense to output progress indications on this during startup. This helps with diagnosing hardware and software errors.
    • Initialize floating point and other extensions such as SIMD and multimedia engines. It is usually necessary to enable these and maybe initialize control and exception registers for these extensions.
    • Initialize interrupt controller. At the very least, it should be configured to mask all interrupts. It may also be necessary to set up the mapping from the interrupt controller's vector number space to the CPU's exception number space. Similar mappings may need to be set up between primary and secondary interrupt controllers.
    • Disable and initialize the caches. The caches should not normally be enabled at this point, but it may be necessary to clear or initialize them so that they can be enabled later. Some architectures require that the caches be explicitly reinitialized after a power-on reset.
    • Initialize the timer, clock etc. While the timer used for RTC interrupts will be initialized later, it may be necessary to set up the clocks that drive it here.

    The exact order in which these initializations is done is architecture or variant specific. It is also often not necessary to do anything at all for some of these options. These fragments of code should concentrate on getting the target up and running so that C function calls can be made and code can be run. More complex initializations that cannot be done in assembly code may be postponed until calls to hal_variant_init() or hal_platform_init() are made.

    Not all of these initializations need to be done for all startup types. In particular, RAM startups can reasonably assume that the ROM monitor or loader has already done most of this work.

  • Set up the stack pointer, this allows subsequent initialization code to make proper procedure calls. Usually the interrupt stack is used for this purpose since it is available, large enough, and will be reused for other purposes later.
  • Initialize any global pointer register needed for access to globally defined variables. This allows subsequent initialization code to access global variables.
  • If the system is starting from ROM, copy the ROM template of the .data section out to its correct position in RAM. (Section 4.8, “Linker Scripts”).
  • Zero the .bss section.
  • Create a suitable C call stack frame. This may involve making stack space for call frames, and arguments, and initializing the back pointers to halt a GDB backtrace operation.
  • Call hal_variant_init() and hal_platform_init(). These will perform any additional initialization needed by the variant and platform. This typically includes further initialization of the interrupt controller, PCI bus bridges, basic IO devices and enabling the caches.
  • Call cyg_hal_invoke_constructors() to run any static constructors.
  • Call cyg_start(). If cyg_start() returns, drop into an infinite loop.

5.2. Vectors and VSRs

The CPU delivers all exceptions, whether synchronous faults or asynchronous interrupts, to a set of hardware defined vectors. Depending on the architecture, these may be implemented in a number of different ways. Examples of existing mechanisms are:

Exceptions are vectored to locations 256 bytes apart starting at either zero or 0xFFF00000. There are 16 such vectors defined by the basic architecture and extra vectors may be defined by specific variants. One of the base vectors is for all external interrupts, and another is for the architecture defined timer.
Most exceptions and all interrupts are vectored to a single address at either 0x80000000 or 0xBFC00180. Software is responsible for reading the exception code from the CPU cause register to discover its true source. Some TLB and debug exceptions are delivered to different vector addresses, but these are not used currently by eCos. One of the exception codes in the cause register indicates an external interrupt. Additional bits in the cause register provide a first-level decode for the interrupt source, one of which represents an architecture defined timer.
Exceptions are delivered via an Interrupt Descriptor Table (IDT) which is essentially an indirection table indexed by exception number. The IDT may be placed anywhere in memory. In PC hardware the standard interrupt controller can be programmed to deliver the external interrupts to a block of 16 vectors at any offset in the IDT. There is no hardware supplied mechanism for determining the vector taken, other than from the address jumped to.
All exceptions, including the FIQ and IRQ interrupts, are vectored to locations four bytes apart starting at zero. There is only room for one instruction here, which must immediately jump out to handling code higher in memory. Interrupt sources have to be decoded entirely from the interrupt controller.

With such a wide variety of hardware approaches, it is not possible to provide a generic mechanism for the substitution of exception vectors directly. Therefore, eCos translates all of these mechanisms in to a common approach that can be used by portable code on all platforms.

The mechanism implemented is to attach to each hardware vector a short piece of trampoline code that makes an indirect jump via a table to the actual handler for the exception. This handler is called the Vector Service Routine (VSR) and the table is called the VSR table.

The trampoline code performs the absolute minimum processing necessary to identify the exception source, and jump to the VSR. The VSR is then responsible for saving the CPU state and taking the necessary actions to handle the exception or interrupt. The entry conditions for the VSR are as close to the raw hardware exception entry state as possible - although on some platforms the trampoline will have had to move or reorganize some registers to do its job.

To make this more concrete, consider how the trampoline code operates in each of the architectures described above:

A separate trampoline is contained in each of the vector locations. This code saves a few work registers away to the special purposes registers available, loads the exception number into a register and then uses that to index the VSR table and jump to the VSR. The VSR is entered with some registers move to the SPRs, and one of the data register containing the number of the vector taken.
A single trampoline routine attached to the common vector reads the exception code out of the cause register and uses that value to index the VSR table and jump to the VSR. The trampoline uses the two registers defined in the ABI for kernel use to do this, one of these will contain the exception vector number for the VSR.
There is a separate 3 or 4 instruction trampoline pointed to by each active IDT table entry. The trampoline for exceptions that also have an error code pop it from the stack and put it into a memory location. Trampolines for non-error-code exceptions just zero the memory location. Then all trampolines push an interrupt/exception number onto the stack, and take an indirect jump through a precalculated offset in the VSR table. This is all done without saving any registers, using memory-only operations. The VSR is entered with the vector number pushed onto the stack on top of the standard hardware saved state.
The trampoline consists solely of the single instruction at the exception entry point. This is an indirect jump via a location 32 bytes higher in memory. These locations, from 0x20 up, form the VSR table. Since each VSR is entered in a different CPU mode (SVC,UNDEF,ABORT,IRQ or FIQ) there has to be a different VSR for each exception that knows how to save the CPU state correctly.

5.3. Default Synchronous Exception Handling

Most synchronous exception VSR table entries will point to a default exception VSR which is responsible for handling all exceptions in a generic manner. The default VSR simply saves the CPU state, makes any adjustments to the CPU state that is necessary, and calls cyg_hal_exception_handler().

cyg_hal_exception_handler() needs to pass the exception on to some handling code. There are two basic destinations: enter GDB or pass the exception up to eCos. Exactly which destination is taken depends on the configuration. When the GDB stubs are included then the exception is passed to them, otherwise it is passed to eCos.

If an eCos application has been loaded by RedBoot then the VSR table entries will all point into RedBoot's exception VSR, and will therefore enter GDB if an exception occurs. If the eCos application wants to handle an exception itself, it needs to replace the the VSR table entry with one pointing to its own VSR. It can do this with the HAL_VSR_SET_TO_ECOS_HANDLER() macro.

5.4. Default Interrupt Handling

Most asynchronous external interrupt vectors will point to a default interrupt VSR which decodes the actual interrupt being delivered from the interrupt controller and invokes the appropriate ISR.

The default interrupt VSR has a number of responsibilities if it is going to interact with the Kernel cleanly and allow interrupts to cause thread preemption.

To support this VSR an ISR vector table is needed. For each valid vector three pointers need to be stored: the ISR, its data pointer and an opaque (to the HAL) interrupt object pointer needed by the kernel. It is implementation defined whether these are stored in a single table of triples, or in three separate tables.

The VSR follows the following approximate plan:

  1. Save the CPU state. In non-debug configurations, it may be possible to get away with saving less than the entire machine state. The option CYGDBG_HAL_COMMON_INTERRUPTS_SAVE_MINIMUM_CONTEXT is supported in some targets to do this.
  2. Increment the kernel scheduler lock. This is a static member of the Cyg_Scheduler class, however it has also been aliased to cyg_scheduler_sched_lock so that it can be accessed from assembly code.
  3. (Optional) Switch to an interrupt stack if not already running on it. This allows nested interrupts to be delivered without needing every thread to have a stack large enough to take the maximum possible nesting. It is implementation defined how to detect whether this is a nested interrupt but there are two basic techniques. The first is to inspect the stack pointer and switch only if it is not currently within the interrupt stack range; the second is to maintain a counter of the interrupt nesting level and switch only if it is zero. The option CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK controls whether this happens.
  4. Decode the actual external interrupt being delivered from the interrupt controller. This will yield the ISR vector number. The code to do this usually needs to come from the variant or platform HAL, so is usually present in the form of a macro or procedure callout.
  5. (Optional) Re-enable interrupts to permit nesting. At this point we can potentially allow higher priority interrupts to occur. It depends on the interrupt architecture of the CPU and platform whether more interrupts will occur at this point, or whether they will only be delivered after the current interrupt has been acknowledged (by a call to HAL_INTERRUPT_ACKNOWLEDGE() in the ISR).
  6. Using the ISR vector number as an index, retrieve the ISR pointer and its data pointer from the ISR vector table.
  7. Construct a C call stack frame. This may involve making stack space for call frames, and arguments, and initializing the back pointers to halt a GDB backtrace operation.
  8. Call the ISR, passing the vector number and data pointer. The vector number and a pointer to the saved state should be preserved across this call, preferably by storing them in registers that are defined to be callee-saved by the calling conventions.
  9. If this is an un-nested interrupt and a separate interrupt stack is being used, switch back to the interrupted thread's own stack.
  10. Use the saved ISR vector number to get the interrupt object pointer from the ISR vector table.
  11. Call interrupt_end() passing it the return value from the ISR, the interrupt object pointer and a pointer to the saved CPU state. This function is implemented by the Kernel and is responsible for finishing off the interrupt handling. Specifically, it may post a DSR depending on the ISR return value, and will decrement the scheduler lock. If the lock is zeroed by this operation then any posted DSRs may be called and may in turn result in a thread context switch.
  12. The return from interrupt_end() may occur some time after the call. Many other threads may have executed in the meantime. So here all we may do is restore the machine state and resume execution of the interrupted thread. Depending on the architecture, it may be necessary to disable interrupts again for part of this.

The detailed order of these steps may vary slightly depending on the architecture, in particular where interrupts are enabled and disabled.