#134 Expose the byte being read/written to breakpoints

[Philip Kendall]

While we have memory and I/O breakpoints, we don't expose the byte being read or written to the breakpoint; for example, it would often be useful to do something like break port write 0xfe if (byte being written) & 0x07 == 0x01 to spot the border being set to blue.

[Philip Kendall]

OK, a first pass at this which adds a bp:data debugger system variable is now in the feature-134-data-byte-to-breakpoints branch. Point for discussion: the read breakpoints were currently tested before the memory location/port was actually read. Obviously this needs to happen afterwards in order for us to get the value. This has a couple of consequences if people are using conditional breakpoints (or the Python hooks):

• The timing of the breakpoint will be different
• Any effects of the read will have happened

I don't see those as too much of an issue, but it's certainly worth flagging up. Should the write breakpoints be moved to after the action as well?