Interrupt Mask Register
| RDRF | Receive Data Register Full Interrupt Mask |
| TDRE | Transmit Data Register Empty Interrupt Mask |
| TXEMPTY | Transmission Registers Empty Mask |
| OVRES | Overrun Error Interrupt Mask |
| CSR | Chip Select Rise Interrupt Mask |
| CSS | Chip Select Status Interrupt Mask |
| INSTRE | Instruction End Interrupt Mask |