Add instruction breakpoint

[mikey]

Add a instruction breakpoint to microwatt. When the PC hits this value, take a trace exception at 0xD00.

The ISA defines the CIABR which is the "completed" instruction. This means take the exception after the instruction is run which is not ideal from a software perspective. An implementation that takes the breakpoint before the instruction is executed would be more useful but we'd have to call it an IABR not CIABR.

[mikey]

Extra credit for adding single stepping and/or branch tracing via MSR[TE].

[iamjpn]

I've got a bit of a WIP here: https://github.com/iamjpn/microwatt/tree/ciabr

[mikey]

Doesn't actually seem to complete first the instruction though.

What do you mean by that? @iamjpn

[iamjpn]

Doesn't actually seem to complete first the instruction though.

What do you mean by that? @iamjpn

What I mean is that currently the exception is taken before the instruction is completed.

breakpoint:
	addi	%r4, %r4, 1
	b	1f // should be skipped
	//cmpwi	%r4, 1
	//bne	1f

That's why I commented out that cmpwi in the test. I've got to fix that up.

[mikey]

As mentioned in the first comment, it would be useful to have it that way but it's not arch compliant.

[iagocaran]

I reimplemented @iamjpn work for a newer version of the code and wanted to make it arch compliant, but I'm having trouble getting the completed instruction address instead of the next instruction address.

https://github.com/iagocaran/microwatt