Figure 4 -- Calculating the address of an interrupt routine

   
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+-----------------------------------------------+
|                                               |
| +---+                                         |
| |INT|                                         |
| +---+                                         |
|   |                                           |
|   |  +---------+ BS:0000h                     |
|   |  | SEGMENT |                              |
|   |  + ------- |                              |
|   |  | OFFSET  |                              |
|   +->+---------+ BS:0004h                     |
|      | SEGMENT +-->----->----->----+          |
|      + ------- |                   |          |
|      | OFFSET  +-->----->----->----+          |
|      +---------+ BS:0008h          |          |
|      | SEGMENT |             +-----------+    |
|      + ------- |             | INTERRUPT |    |
|      | OFFSET  |             |  SERVICE  |    |
|      +---------+ BS:000Ch    |  ROUTINE  |    |
|        .......               +-----------+    |
|        .......                                |
|        .......                                |
|      +---------+ BS:03FCh                     |
|      | SEGMENT |                              |
|      + ------- |                              |
|      | OFFSET  |                              |
|      +---------+ BS:0400h                     |
|                                               |
| BS = interrupt table Base Segment:            |
|      80286 = Always 0                         |
|      80386 = Relocatable via IDTR             |
|                                               |
| (a) Interrupt service addressing in real-mode |
|                                               |
+-----------------------------------------------+
|                                               |
|                                +---+          |
|                                |INT|          |
|                                +---+          |
|                                  |            |
|                                  |            |
|  4                   3 3 2 2     |        0   |
|  7                   2 1 4 3     |        0   |
| +---------------------+---+----------------+  |
| |(8x# of interrupts)-1| 0 |24-bit base addr|  |
| +---------------------+---+----------------+  |
|  Interrupt base register (descriptor cache)   |
|                                  |            |
|                                  |            |
|         +-<-----<-----<-----<----+            |
|         |                                     |
|         |                Interrupt Descriptor |
|         |                      Table          |
|         |                +----------------+   |
|         |                |Segment selector|   |
|         |                |--------------- |   |
|         |                |     Offset     |   |
|         +->----->----->--+----------------+   |
| +---<-----<-----<-----<--|Segment selector|   |
| |                        |--------------- |   |
| +---<-----<-----<-----<--|     Offset     |   |
| |                        +----------------+   |
| |                        |Segment selector|   |
| |                        |--------------- |   |
| |                        |     Offset     |   |
| |                        +----------------+   |
| |                            . . . . . .      |
| |  Global (or Local)         . . . . . .      |
| |  Descriptor table          . . . . . .      |
| |  +----------------+    +----------------+   |
| |  |  Base address  |    |Segment selector|   |
| |  |--------------- |    |--------------- |   |
| |  |     Offset     |    |     Offset     |   |
| |  +----------------+    +----------------+   |
| |  |  Base address  |      # of interrupts    |
| |  |--------------- |                         |
| |  |     Offset     |                         |
| |  +----------------+                         |
| |  |  Base address  |-->----->----+           |
| +->|--------------- |             |           |
|    |     Offset     |-->----->----+           |
|    +----------------+             |           |
|        . . . . . .                |           |
|        . . . . . .        +-----------------+ |
|        . . . . . .        |Interrupt Service| |
|    +----------------+     |     Routine     | |
|    |  Base address  |     +-----------------+ |
|    |--------------- |                         |
|    |     Offset     |                         |
|    +----------------+                         |
|                                               |
| b) Interrupt service addressing in Prot. Mode |
|                                               |
+-----------------------------------------------+

   
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