The interrupt flag register (IFR) contains the status of the maskable interrupts (INT15-INT4) and the NMI interrupt. Each corresponding bit in the IFR is set to 1 when that interrupt occurs; otherwise, the bits are cleared to 0. The IFR is shown in Figure 8-33 and described in Table 8-317.
The update to IFR via a write to the interrupt set register (ISR)/interrupt clear register (ICR) has one effective delay slot; IFR cannot be set/cleared using one MVC instruction (a write to ISR/ICR) and read the corresponding changed value of IFR via a very next MVC instruction. Software should use a NOP after a write to ISR/ICR, if the changed value has to be read immediately.
MVC 0x10, ISR ; Set IFR[4]
NOP ; a NOP to fill the delay slot
MVC IFR, R0 ; R0 would contain 0x10
Figure 8-33 Interrupt Flag Register (IFR) | IF15 | IF14 | IF13 | IF12 | IF11 | IF10 | IF9 | IF8 | IF7 | IF6 | IF5 | IF4 | Reserved | NMIF | Rsvd |
| R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 | R-0 |
| LEGEND: R = Readable by the MVC instruction; -n = value after reset |
Table 8-317 Interrupt Flag Register (IFR) Field Descriptions| Bit | Field | Value | Description |
|---|
| 31-16 | Reserved | 0 | Reserved. The reserved bit location is always read as 0. A value written to this field has no effect. |
| 15-4 | IFn | | Interrupt flag. Indicates the status of the corresponding maskable interrupt. An interrupt flag may be manually set by setting the corresponding bit (ISn) in the interrupt set register (ISR) or manually cleared by setting the corresponding bit (ICn) in the interrupt clear register (ICR). |
| 0 | Interrupt has not occurred. |
| 1 | Interrupt has occurred. |
| 3-2 | Reserved | 0 | Reserved. The reserved bit location is always read as 0. A value written to this field has no effect. |
| 1 | NMIF | | Nonmaskable interrupt flag. |
| 0 | Interrupt has not occurred. |
| 1 | Interrupt has occurred. |
| 0 | Reserved | 0 | Reserved. The reserved bit location is always read as 0. A value written to this field has no effect. |