How Breakpoints Work

Breakpoints are set using the 't', 'to', 'b', 'g', 'c', or 'when' commands. The 't', 'to', 'g', and 'c' commands set temporary breakpoints that are automatically deleted when execution halts. The 'b' and 'when' commands set persistent breakpoints that must be explicitly deleted (using the 'db' command).

When the user types a 'b' command to set a breakpoint. The Monitor decides which type of breakpoint to utilize using the following algorithm:

	if (is_writeable(addr)) bptype = RAM;
	else if (is_cacheable(addr) && cpu_has_ilock) bptype = ILOCK;
	else if (cpu_has_hwbp) bptype = HW;
	else error();

This information is saved in an array. When execution is started, the Monitor walks through the breakpoint array and implements the breakpoints using the following rules:

RAM
Replace the instruction at the specified address with a "break" instruction and save the previous contents (the instruction) so that it can be restored when execution halts.

ILOCK
Use the Icache locking feature to lock a "break" instruction into the Icache at the specified address.

HW
Program the hardware breakpoint register to generate an exception when execution reaches the specified address.

When an exception is encountered, the MIPS processor passes control to the General Exception Vector. The Monitor then restores the previous contents of all breakpoints, and transfers control to the prompt.

Thus, if the memory is inspected, the break instructions will never be seen, as they are only present in memory during execution of the application program.

The only exception to this is if the program hangs in such a way as to require the user to press reset on the target system. In this case the Monitor will "forget" that breakpoints had been set, and so you will see any RAM breakpoints as break instructions in memory. At this point the application program must be re-downloaded to obtain correct execution.

Diagram showing how breakpoints work in the SerialIce1 DLL


ab_cmd ---->setHwdbpt------+
                   '       |        +--------+
setbp -----------+  '''''''|''''''''|        |
                 |         |        |  Bpt[] |
when ------------+         |        |        |
	         |    '''''|''''''''|        |
	         v   '     |        +--------+
             setTrigger ---+
	                   |
trace (sstep) -------------+
                           |
c temp --------------------+
                           |
g . temp ------------------+              [imon95]
                           |
 .................................................................
                           |
ibpt32                     |             [serialice1.dll]
  |                        |
  v                        |
ibpt32m -------------------+
                           |
dbpt32                     |
  |                        |
  v                        |
dbpt32m -------------------+
                           |
                           |                   +-----------+
		           v                   |           |
                       setbp_target '''''''''''| brkList[] |
                        |       |              |           |
			v       |              +-----------+
		brkInstall      |
		                v
		             brkRemove

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