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# How the memory subsystem handles FIR bits
In order to make handling of FIR bits a bit more straight forward, we have
created a small data structure mss::fir::reg. Given the FIR register which
reports FIR errors, a fir::reg will encapsulate the 2 action registers and
the mask register as well as the reporting register.
FIR actions, and when FIR are unmasked are defined by the team as a whole,
and are documented outside of the ekb.
## FIR actions
FIR actions are configured in 3 parallel registers: action0, action1 and
mask. Bit patterns assigned across these three registers define a response
the system is to take when a FIR bit is asserted. Because they span three
parallel registers, the actions are defined as tripples:
```
Action 0 Action 1 Mask
+---+ +---+ +---+
| 0 | | 0 | | 0 |
+---+ +---+ +---+
```
So, for any bit postion representing a FIR, that corresponding bit in the
three registers defines the system response. Consider "bit 12," to define
a response, we must set the three bits in position 12 in the three
egisters:
```
bit 0, ..., bit 12
Action 0 Action 1 Mask Action 0 Action 1 Mask
+---+ +---+ +---+ +---+ +---+ +---+
| 0 | | 0 | | 1 | | 0 | | 0 | | 0 |
+---+ +---+ +---+ ..., +---+ +---+ +---+ ...,
```
This represents that the FIR "bit 0" is masked (1 in the mask register bit
position 0) and the FIR "bit 12" should respond with a check-stop (0 in
the action 0 and action 1 registers in bit position 12, and a 0 to represent
the FIR is unmasked.
## Action Tripples
The actions are defined as:
```
Action 0 Action 1 Mask
+---+ +---+ +---+
| 0 | | 0 | | 0 | System Checkstop
+---+ +---+ +---+
+---+ +---+ +---+
| 0 | | 1 | | 0 | Recoverable Error
+---+ +---+ +---+
+---+ +---+ +---+
| 1 | | 0 | | 0 | Attention
+---+ +---+ +---+
+---+ +---+ +---+
| 1 | | 1 | | 0 | Local Checkstop
+---+ +---+ +---+
+---+ +---+ +---+
| X | | X | | 1 | Masked
+---+ +---+ +---+
```
## The fir::reg class
Given this information, the fir::reg class provides a read-modify-write
interface for specific actions, keyed off of the FIR register. For example
fir::reg::checkstop() configures the appropriate action0, action1 and mask bits.
fir::reg::write() ensures all of the configuration is written to the proper
register. Refer to the fir.H documentation for further explanation on using
the fir::reg class.
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