Ports
Ports used by the System Library.
| 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 |
Initialization Command Word 1 (Port 20/A0)
| 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