FPB (Flash Patch and Breakpoint)
The Cortex-M unit that provides hardware breakpoints in flash — and explains why your debugger suddenly refuses to set a seventh one.
On a PC, GDB sets a breakpoint by writing a trap instruction into the code. In flash you can't do that without erasing and rewriting a whole sector. The FPB gets around it with a handful of comparators that watch the instruction-fetch address and halt the core on a match. Cortex-M3 and M4 have six of them for code, the M0 at most four. Data accesses are watched by a different unit, the DWT.
Once the comparators run out you either get an error or the probe quietly reprograms flash behind your back; SEGGER markets that as unlimited breakpoints. The other half of the name, "flash patch", let the unit substitute words in flash with values from RAM, a way to fix bugs in ROM code after manufacturing. It also turned out to be a handy way to hide code, and later cores dropped it, but the name stuck.