The 8051 microcontroller supports six interrupt sources – reset, two hardware interrupts, two timer interrupts, and a serial communication interrupt:
| Interrupt number | Description | Offset in bytes |
| Reset | 0 | |
| 0 | External interrupt INT0 | 3 |
| 1 | Timer 0 (TF0) | 11 |
| 2 | External interrupt INT1 | 19 |
| 3 | Timer 1 (TF1) | 27 |
| 4 | Serial | 36 |
The interrupt vector array is located at address 0; each entry except reset is 8 bytes in size. Though a minimal ISR can fit into 8 bytes, normally, the entries contain code that redirects execution to the actual ISR located elsewhere.
The reset entry is special. It is activated by the reset signal and immediately jumps to the address where the main program is located.
8051 defines a special register called Interrupt Enable (EA), which is used to enable and disable interrupts. Its 8-bits are allocated in the following way:
| Bit | Name | Meaning |
| 0 | EX0 | External Interrupt 0 |
| 1 | ET0 | Timer 0 Interrupt |
| 2 | EX1 | External Interrupt 1 |
| 3 | ET1 | Timer 1 Interrupt |
| 4 | ES | Serial Port Interrupt |
| 5 | - | Not used |
| 6 | - | Not used |
| 7 | EA | Global Interrupt Control |
Setting these bits to 1 enables corresponding interrupts, to 0 disables them. The EA bit enables or disables all interrupts.