ARM-specific Functions

10.6except.s

This file contains the low-level exception handlers. For this simple configuration, the only exception handler used is an interrupt handler, irqHandler(). The code in irq.c initializes the interrupt vector such that irqHandler() is called whenever an interrupt is pending. In turn, irqHandler() uses the interrupt mode stack (also established by code in irq.c) to save all the APCS registers necessary, and then calls the C interrupt handler function, irqDispatch(), in irq.c. The irqHandler() function is coded as an interrupt handler function and must not be called from C code.

Note

The handler irqHandler() is always assembled as ARM code (never Thumb). If you assemble except.s with the -arch 4T assembler option, it assumes that the C interrupt dispatch function irqHandler() is written in Thumb code, so it switches to Thumb mode before calling it. In all other cases, the code assumes that irqDispatch() is written in ARM code (not Thumb) and calls it directly.

ARM-specific Functions

10.7irq.c

This file contains functions that are used to manage interrupts. These functions are as follows:

•ENTER_CRIT_SECTION() and EXIT_CRIT_SECTION()

•irqDispatch()

•irq_Enable() and irq_Disable()

•irqInit().

10.7.1ENTER_CRIT_SECTION() and EXIT_CRIT_SECTION()

These functions are used to protect sections of code from pre-emption. They work by disabling all interrupts from the PID hardware for the duration of the critical section, so they should be used sparingly.

10.7.2irqDispatch()

The C interrupt dispatch function, irqDispatch(), reads the interrupt controller status register on the PID board to determine the source of the interrupt. It then calls the device-specific interrupt handler associated with that interrupt. Timer ticks are handled as a special case. The only work required on a timer tick interrupt is to increment the cticks variable, so this is performed directly by irqDispatch().

Note

The UART and Olicom Ethernet device driver interrupt handlers are declared as extern ___weak to allow the code to reference them without forcing the linker to include them in the final object code. If no other module references any of the symbols in the device driver module, the device driver module will not be linked into the final image. When this happens, the ___weak exported symbols are initialized with the value 0. Code in the dispatch function checks for this before attempting to call the handler function.

10.7.3irq_Enable() and irq_Disable()

Device drivers that need to enable and disable specific interrupts, such as the timer tick interrupt or the UART interrupt, use these two functions. They keep track of which interrupts are currently enabled. They also interact with the critical section-locking functions, ENTER_CRIT_SECTION() and EXIT_CRIT_SECTION(), to determine which hardware interrupts to enable.

ARM-specific Functions

10.7.4irqInit()

The irqInit() function is used to enable the system to handle interrupts. In the demonstration sources, there are three potential interrupt sources:

•the regular clock tick interrupt

•the UARTs

•the Olicom Ethernet PC-Card.

The irqInit() function uses code in stack.s to establish an interrupt mode stack and reprograms the interrupt vector to point at the low-level interrupt handler, irqHandler(), found in except.s.