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Reverse-engineering the Intel 8086 processor's HALT circuits

The 8086 processor was introduced in 1978 and has greatly influenced modern computing through the x86 architecture. One unusual instruction in this processor is HLT, which stops the processor and puts it in a halt state. In this blog post, I explain in detail how the halt circuitry is implemented and how it interacts with the 8086’s architecture.

The die photo below shows the 8086 microprocessor under a microscope. The metal layer on top of the chip is visible, with the silicon and polysilicon mostly hidden underneath. Around the edges of the die, bond wires connect pads to the chip’s 40 external pins. I’ve labeled the key functional blocks; the ones that are important to this discussion are darker and will be discussed in detail below. Architecturally, the chip is partitioned into a Bus Interface Unit (BIU) at the top and an Execution Unit (EU) below. The BIU handles memory accesses, while the Execution Unit (EU) executes instructions. Both are stopped by a halt instruction.

Halt processing in the Execution Unit
In this section, I’ll explain how the HLT instruction is decoded and handled in the Execution Unit. The 8086 uses a combination of lookup ROMs, logic, and microcode to implement instructions. The process starts with the loader, a state machine that provides synchronization between the prefetch queue and the decoding circuitry. When an instruction byte is available, the loader provides a signal called First Clock that loads the instruction into the Instruction Register and starts the instruction decoding process.

Before microcode gets involved, the Group Decode ROM classifies instructions by producing about 15 signals, indicating properties such as instructions with a Mod R/M byte, instructions with a byte/word bit, instructions that always act on a byte, and so forth. For the HLT instruction, the Group Decode ROM provides two important signals. The first is one-byte logic (1BL), indicating that the instruction is one byte long and is implemented with logic circuitry rather than microcode.1 The second signal is produced for the HLT instruction specifically and generates the internal HALT signal. This signal travels to various parts of the 8086 to halt the processor.”

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