In an 8085 or 8086 style microprocessor, what happens inside the CPU when a HLT (halt) instruction is executed?

Introduction / Context:
The HLT instruction, short for halt, is provided by many classic microprocessors such as the Intel 8085 and 8086. It is often used for power saving, waiting for events or simple debugging. Understanding what HLT actually does inside the CPU helps clarify how microprocessors interact with interrupts, resets and low power states. This question focuses on the internal effect of executing a HLT instruction in such processors.

Given Data / Assumptions:

We are considering 8085 and 8086 style microprocessors that support a HLT instruction.
HLT is executed as part of a normal instruction stream fetched from memory.
External events such as hardware interrupts or reset signals can occur while the processor is halted.
The question asks what the processor does after decoding and executing HLT, not what a high level operating system might do in response.

Concept / Approach:
When the processor executes HLT, it finishes the current instruction and then stops fetching additional instructions from memory. Internally, it enters a special halt state, in which the instruction execution pipeline is idle and the address and data buses may be quiescent. However, the processor does not forget its state; registers and flags remain unchanged. The processor stays in this halted condition until a hardware event such as a maskable interrupt, non maskable interrupt or reset occurs. Upon such an event, the processor resumes operation in a defined way, either by servicing the interrupt or restarting from the reset vector.

Step-by-Step Solution:
Step 1: Recall that HLT is a one byte or short instruction that the processor decodes like any other. Step 2: After completion of HLT, the processor stops fetching further instructions. The program counter no longer advances through normal code. Step 3: In this halt state, the internal logic may reduce power consumption and the external buses remain idle, but the processor context (registers, flags and stack) is preserved. Step 4: The only way to exit the halt state is through an external event: a reset signal or an enabled interrupt request. When such an event arrives, the processor leaves the halt state and responds according to the usual interrupt or reset handling rules. Step 5: Therefore, HLT does not power off the CPU, does not clear registers and does not itself cause a jump to the reset vector; it simply stops instruction execution until an external event occurs.

Verification / Alternative check:
Microprocessor documentation for the 8085 and 8086 clearly states that the halt state is terminated by an interrupt or reset. Example programs sometimes use an infinite loop with HLT inside to wait for interrupts in simple embedded applications. When an interrupt fires, the processor wakes, executes the interrupt service routine and may return to the HLT loop. This practical pattern confirms that HLT pauses instruction fetching and relies on external signals to resume activity.

Why Other Options Are Wrong:
The processor does not clear all registers and power off permanently; low power modes still preserve state so that execution can resume correctly.
HLT does not perform a privilege level change such as user to kernel mode; such notions belong to more advanced protected mode architectures, not to the basic HLT behaviour in 8085 or 8086 real mode.

Common Pitfalls:
One common misconception is to treat HLT as a kind of software reset. In reality, it is more like a pause than a restart. Another pitfall is forgetting that interrupts must be enabled for the processor to wake up from HLT via maskable interrupts. If interrupts are disabled and no non maskable interrupt is available, only a reset signal may bring the processor out of the halt state. Designers must plan their interrupt configuration carefully when using HLT in embedded systems.

Final Answer:
When a HLT instruction is executed, the CPU stops fetching and executing further instructions and enters a halt state until an interrupt or reset occurs.