3.4.4. TapActionIRScan

3.4.4. `TapActionIRScan`

This class implements capture, shift and update of a value through the instruction register. It has a number of private instance variables.

```
uint32_t  iRegIn
	    
```
This is the value being shifted in through TDI. It holds the remaining bits to be shifted.
```
int  iRegSize
	    
```
This is the number of bits to be shifted in through TDI and out through TDO
```
uint32_t  iRegOut
	    
```
This is the value being shifted out through TDO. It holds the bits shifted out so far.
```
int       bitsShifted
	    
```
The number of bits shifted in so far.
```
enum {
    SHIFT_IR_PREPARING,
    SHIFT_IR_SHIFTING,
    SHIFT_IR_UPDATING
  } iRScanState
	    
```
This enumeration records where the action is in its process. The process method follows a state machine (see below) whose state is recorded here.

Constructor

The primary responsibility of the constructor is to initialize all the instance variables. The input register and size are taken from the arguments, the output register and count of bits shifted are zeroed and the IR-Scan process state machine is set to SHIFT_IR_PREPARING.

`process`

This method generates the appropriate values of TDI and TMS to process the action and capture TDI. It is controlled by its own state machine, whose state is recorded in iRScanState as shown in Figure 3.2

Figure 3.2. State machine for the JtagActionIRScan process.

A subtlety of the implementation is that, because TDO changes on the falling edge, its value is always one step later than that of TDI. Thus TDO is not captured on the first cycle in Shift-IR state and its last bit is captured immediately after leaving Shift-IR state.

`getIRegOut`

This function is part of the public API, allowing the user to access the value shifted through TDO on completion.