ARM ARM7TDMI - Priorities and Exceptions

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Debug in Depth

B.10 Priorities and exceptions

When a breakpoint or a debug request occurs, the normal flow of the program is

interrupted. Debug can be treated as another type of exception. The interaction of the

debugger with other exceptions is described in Behavior of the program counter in

debug state on page B-30. This section covers the following priorities:

• Breakpoint with Prefetch Abort

• Interrupts

• Data Aborts on page B-34.

B.10.1 Breakpoint with Prefetch Abort

When a breakpointed instruction fetch causes a Prefetch Abort, the abort is taken and

the breakpoint is disregarded. Usually, Prefetch Aborts occur when, for example, an

access is made to a virtual address that does not physically exist and the returned data

is therefore invalid. In such a case, the normal action of the operating system is to swap

in the page of memory and to return to the previously-invalid address. This time, when

the instruction is fetched and providing the breakpoint is activated, it can be

data-dependent, the ARM7TDMI core enters debug state.

The Prefetch Abort, therefore, takes higher priority than the breakpoint.

B.10.2 Interrupts

When the ARM7TDMI core enters halt debug state, interrupts are automatically

disabled.

If an interrupt is pending during the instruction prior to entering debug state, the

ARM7TDMI core enters debug state in the mode of the interrupt. On entry to debug

state, the debugger cannot assume that the ARM7TDMI core is in the mode expected

by the user program. The debugger must check the PC, the CPSR, and the SPSR to

accurately determine the reason for the exception.

Debug, therefore, takes higher priority than the interrupt, but the ARM7TDMI core

does remember that an interrupt has occurred.

If bit [4], monitor mode enable, of the Debug control register is set, FIQs remain

enabled. An entry to the abort exception routine disables IRQs, so in monitor mode the

abort exception routine must re-enable IRQs before they can be recognized and

serviced.

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