[tavish1]

Any comments on how breakpoints work on external targets, running bare-metal, and you can't replace instructions? Ex. debugging an 8-bit AVR, say atmega1280 over JTAG. I am guessing it has to do with the JTAG doing a simple compare of the PC with the breakpoint address, just want confirmation and more details.

[TickleSteve]

yeah, processors such as the ARM cortex-M series have built in debug features such as ~8 breakpoint registers that trigger a change in processor state when they match a code or data access address.

The breakpoint registers are accessed via JTAG/SWD using your j-link/FET/whatever.

Quite often, when you're debugging embedded systems, you run out of "hardware" breakpoints and have to resort to software-style breakpoints described in the article.

[sigill]

Yes. And these software breakpoints are written into flash. This is why they're very slow - and why they wear down your flash.

The situation is kinda OK when you don't often change breakpoints and your CPU has an instruction register writable by the debug probe via JTAG/SWD/etc. Upon stepping or continuing from a breakpoint, the debugger will write the actual instruction at the breakpoint into the instruction register and tell the CPU "run again, but don't load the instruction from memory as I have already loaded it into your instruction register.".

Another option is to emulate the effects of the instruction in the debugger and write the results back into registers/RAM/I/O. This is not always possible.

If you don't have the options above, your flash will wear down quickly, as stepping away or continuing from a breakpoint entails writing the actual instruction back, stepping, then writing the breakpoint again.

[cnvogel]

The program counter will not typically be compared via JTAG (this would be way too slow) but there will be dedicated registers inside the CPU that are compared against the program counter.

For AVR specifically, have a look at e.g. https://github.com/raimue/avarice/blob/master/src/jtagbp.cc (just random find on github, likely not the official repository).

This is, by the way, also possible on a typical PC (as in "personal computer" not program counter), also x86/64 has debug registers.

http://www.codeproject.com/Articles/28071/Toggle-hardware-da...

...and very likely also on ARM, m68k, MIPS, Sparc, PPC, ... I just didn't look it up.

[garaetjjte]

In case of small AVRs with debugWIRE interface, they don't have hardware breakpoints. Placing breakpoint automatically flashes memory overwriting instruction with BREAK instruction.

[sigill]

As other comments tell you, software breakpoints are used with flash parts such as the AVRs. You can write flash from they outside.