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8086

The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting ICs), and is notable as the processor used in the original IBM PC design.

Reverse-engineering the register codes for the 8086 processor’s microcode

“Like most processors, the Intel 8086 (1978) provides registers that are faster than main memory. As well as the registers that are visible to the programmer, the 8086 has a handful of internal registers that are hidden from the user …

How the 8086 processor determines the length of an instruction

“The Intel 8086 processor (1978) has a complicated instruction set with instructions ranging from one to six bytes long. This raises the question of how the processor knows the length of an instruction.1 The answer is that the 8086 …

Reverse-engineering the ModR/M addressing microcode in the Intel 8086 processor

“One interesting aspect of a computer’s instruction set is its addressing modes, how the computer determines the address for a memory access. The Intel 8086 (1978) used the ModR/M byte, a special byte following the opcode, to select …

Silicon reverse-engineering: the Intel 8086 processor’s flag circuitry

“Status flags are a key part of most processors, indicating if an arithmetic result is negative, zero, or has a carry, for instance. In this post, I take a close look at the flag circuitry in the Intel 8086 processor …

Reverse-engineering the conditional jump circuitry in the 8086 processor

“Intel introduced the 8086 microprocessor in 1978 and it had a huge influence on computing. I’m reverse-engineering the 8086 by examining the circuitry on its silicon die and in this blog post I take a look at how conditional …

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 …

The 8086 processor’s microcode pipeline from die analysis

“Intel introduced the 8086 microprocessor in 1978, and its influence still remains through the popular x86 architecture. The 8086 was a fairly complex microprocessor for its time, implementing instructions in microcode with pipelining to improve performance. This blog post explains …

Inside the 8086 processor’s instruction prefetch circuitry

“The groundbreaking 8086 microprocessor was introduced by Intel in 1978 and led to the x86 architecture that still dominates desktop and server computing. One way that the 8086 increased performance was by prefetching: the processor fetches instructions from memory before …

How the 8086 processor’s microcode engine works

“The 8086 microprocessor was a groundbreaking processor introduced by Intel in 1978. It led to the x86 architecture that still dominates desktop and server computing. The 8086 chip uses microcode internally to implement its instruction set. I’ve been reverse-engineering …

A bug fix in the 8086 microprocessor, revealed in the die’s silicon

“The 8086 microprocessor was a groundbreaking processor introduced by Intel in 1978. It led to the x86 architecture that still dominates desktop and server computing. While reverse-engineering the 8086 from die photos, a particular circuit caught my eye because its …