If a high priority fpu interrupt routine does not make any function calls, does not use any save-on-call registers, and uses the RPTB instruction, then the interrupt routine will not include RB register save/restores .

For high priority fpu interrupt routines, the RB register should be save/restored if the interrupt routine uses the RPTB instruction.

However, high priority fpu interrupt routines will only save/restore RB for below cases:  

• the interrupt routine makes any function calls since all save-on-call registers are saved/restored
• the interrupt routine generates any save-on-call register save/restores