| Port | Function / Purpose |
|---|---|
| 0020 | Programmable Interrupt Controller 8259 Initialization Command Word 1 (ICW1) |
| 00A0 | NMI mask register (same as port 0020) |
| 00F0 | Math coprocessor clear busy latch |
| Bit | Set | Function when set | Unset | Function when clear |
|---|---|---|---|---|
| 0 | 1 | ICW4 is needed | 0 | No ICW4 needed |
| 1 | 1 | Single 8259 | 0 | Cascading 8259's |
| 2 | 1 | 4 byte interrupt vectors | 0 | 8 byte interrupt vectors |
| 3 | 1 | Level triggered mode | 0 | Edge triggered mode |
| 4 | 1 | Must be set for ICW1 | 0 | |
| 5 | 1 | 0 | Must be clear for PC Systems |
See also: Identify 8087/80287/80387 FPU Go back