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IXP42X product line and IXC1100 control plane processors—Intel XScale
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Intel
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IXP42X Product Line of Network Processors and IXC1100 Control Plane Processor
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For the indirect branch cases, a temporary scratch register may be necessary to hold
intermediate values while computing the final target address. DBG_r13 can be used for
this purpose (see “Debug Handler Restrictions” on page 128 for restrictions on
DBG_r13 usage).
3.6.15.2 Implementing a Debug Handler
The debugger uses the debug handler to examine or modify processor state by sending
commands and reading data through JTAG. The API between the debugger and debug
handler is specific to a debugger implementation. Intel provides a standard debug
handler and API which can be used by third-party vendors. Issues and details for
writing a debug handler are discussed in this section and in the Intel Debug Handler.
3.6.15.2.1 Debug Handler Entry
When the debugger requests an external debug break or is waiting for an internal
break, it should poll the TR bit through JTAG to determine when the processor has
entered Debug Mode. The debug handler entry code must do a write to TX to signal the
debugger that the processor has entered Debug Mode. The write to TX sets the TR bit,
signalling the host that a debug exception has occurred and the processor has entered
Debug Mode. The value of the data written to TX is implementation defined (debug
break message, contents of register to save on host, etc.).
3.6.15.2.2 Debug Handler Restrictions
The Debug Handler executes in Debug Mode which is similar to other privileged
processor modes, however, there are some differences. Following are restrictions on
Debug Handler code and differences between Debug Mode and other privileged modes.
• The processor is in Special Debug State following a debug exception, and thus has
special functionality as described in “Halt Mode” on page 93.
• Although address translation and PID remapping are disabled for instruction
accesses (as defined in Special Debug State), data accesses use the normal
address translation and PID remapping mechanisms.
• Debug Mode does not have a dedicated stack pointer, DBG_r13. Although DBG_r13
exists, it is not a general purpose register. Its contents are unpredictable and
should not be relied upon across any instructions or exceptions. However, DBG_r13
can be used, by data processing (non RRX) and MCR/MRC instructions, as a
temporary scratch register.
• The following instructions should not be executed in Debug Mode, they may result
in unpredictable behavior:
—LDM
—LDR w/ Rd=PC
— LDR w/ RRX addressing mode
—SWP
—LDC
—STC
• The handler executes in Debug Mode and can be switched to other modes to access
banked registers. The handler must not enter User Mode; any User Mode registers
that need to be accessed can be accessed in System Mode. Entering User Mode
may cause unpredictable behavior.