ARM ARM7TDMI User Manual Page 150

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Debugging Your System
5-46 Copyright © 2001 ARM Limited. All rights reserved. ARM DDI 0234B
5.18.5 System speed access
When a system speed access is performed during debug state, the value of the PC
increases by three addresses. System speed instructions access the memory system and
so it is possible for aborts to take place. If an abort occurs during a system speed
memory access, the ARM7TDMI-S processor enters abort mode before returning to
debug state.
This scenario is similar to an aborted watchpoint, but the problem is much harder to fix
because the abort was not caused by an instruction in the main program, and so the PC
does not point to the instruction that caused the abort. An abort handler usually looks at
the PC to determine the instruction that caused the abort and also the abort address. In
this case, the value of the PC is invalid, but because the debugger can determine which
location was being accessed, the debugger can be written to help the abort handler fix
the memory system.
5.18.6 Summary of return address calculations
The calculation of the branch return address is as follows:
for normal breakpoint and watchpoint, the branch is:
- (4 + N + 3S)
for entry through debug request (DBGRQ) or watchpoint with exception, the
branch is:
- (3 + N + 3S)
where N is the number of debug speed instructions executed (including the final branch)
and S is the number of system speed instructions executed.
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