Answer:

Introduction:
The 8051 microcontroller is an 8-bit microcontroller widely used in various embedded systems applications. It has a set of external interrupts that can be used to respond to external events or signals. These external interrupts can be enabled or disabled based on the specific requirements of the application.

External interrupts in 8051:
The 8051 microcontroller has two external interrupts, INT0 and INT1. These interrupts can be used to respond to external events such as a button press, a sensor output, or any other external event that needs to be captured by the microcontroller.

Enabling external interrupts:
The external interrupts of the 8051 can be enabled by multiple means. Let's discuss each of them in detail:

1. 4 LSBs of TCON register:
The TCON (Timer/Counter Control) register is an 8-bit register in the 8051 microcontroller. The 4 least significant bits of the TCON register, i.e., TCON.0, TCON.1, TCON.2, and TCON.3, are used to enable or disable the external interrupts. When any of these bits are set to 1, the corresponding external interrupt is enabled. For example, if TCON.0 is set to 1, INT0 interrupt is enabled.

2. Interrupt enable:
The interrupt enable feature of the 8051 microcontroller is used to globally enable or disable interrupts. The IE (Interrupt Enable) register is an 8-bit register that controls the interrupt enable/disable status. The IE register has two bits, i.e., IE.4 and IE.5, which are used to enable or disable the external interrupts. When IE.4 is set to 1, INT0 interrupt is enabled, and when IE.5 is set to 1, INT1 interrupt is enabled.

3. Priority register:
The priority register in the 8051 microcontroller is used to set the priority of the interrupts. The IP (Interrupt Priority) register is an 8-bit register that controls the interrupt priority levels. The IP register has two bits, i.e., IP.0 and IP.1, which are used to set the priority of the external interrupts. When IP.0 is set to 1, INT0 interrupt has high priority, and when IP.1 is set to 1, INT1 interrupt has high priority.

Conclusion:
In conclusion, the external interrupts of the 8051 microcontroller can be enabled by the 4 LSBs of the TCON register, the interrupt enable feature, and the priority register. These features provide flexibility in controlling and responding to external events or signals in embedded systems applications.