8086 Multiply Algorithm Gets Reverse Engineered

March 16, 2023 by Lewin Day 10 Comments

The 8086 has been around since 1978, so it’s pretty well understood. As the namesake of the prevalent x86 architecture, it’s often studied by those looking to learn more about microprocessors in general. To this end, [Ken Shirriff] set about reverse engineering the 8086’s multiplication algorithm.

[Ken]’s efforts were achieved by using die photos of the 8086 chip. Taken under a microscope, they can be used to map out the various functional blocks of the microprocessor. The multiplication algorithm can be nutted out by looking at the arithmetic/logic unit, or ALU. However, it’s also important to understand the role that microcode plays, too. Even as far back as 1978, designers were using microcode to simplify the control logic used in microprocessors.

[Ken] breaks down his investigation into manageable chunks, exploring how the chip achieves both 8-bit and 16-bit multiplication in detail. He covers how the numbers make their way through various instructions and registers to come out with the right result in the end.

It’s a fun look at what’s going on at the ground level in a chip that’s been around since before the personal computer revolution. For any budding chip designers, it’s a great academic exercise to follow along at home. If you’ve been doing your own digging deep into CPU architectures, don’t hesitate to drop us a line!

Reverse-Engineering The Conditional Jump Circuitry In The 8086 Processor

January 27, 2023 by Maya Posch 13 Comments

The condition PLA evaluates microcode conditionals.

As simple as a processor’s instruction set may seem, especially in a 1978-era one like the Intel 8086, there is quite a bit going on to go from something like a conditional jump instruction to a set of operations that the processor can perform. For the CISC 8086 CPU this is detailed in a recent article by [Ken Shirriff], which covers exactly how the instructions with their parameters are broken down into micro-instructions using microcode, which allows the appropriate registers and flags to be updated.

Where the 8086 is interesting compared to modern x86 CPUs is how the microcode is implemented, using gate logic to reduce the complexity of the microcode by for example generic parameter testing when processing a jump instruction. Considering the limitations of 1970s VLSI manufacturing, this was very much a necessary step, and an acceptable trade-off.

Each jump instruction is broken down into a number of micro-instructions that test a range of flags and updates (temporary) registers as well as the program counter as needed. All in all a fascinating look at the efforts put in by Intel engineers over forty years ago on what would become one of the cornerstones of modern day computing.