Reverse-engineering the conditional jump circuitry in the 8086 processor #Intel #ReverseEngineering #x86 @kenshirriff

Intel introduced the 8086 microprocessor in 1978 and it had a huge influence on computing. Ken Shirriff is reverse-engineering the 8086 by examining the circuitry on its silicon die and takes a look at how conditional jumps are implemented. Conditional jumps are an important part of any instruction set, changing the flow of execution based on a condition. Although this instruction may seem simple, it involves many parts of the CPU: the 8086 uses microcode along with special-purpose condition logic.

The 8086 processor has six status flags: carry, parity, auxiliary carry, zero, sign, and overflow. These flags are updated by arithmetic and logic operations based on the result. The 8086 has sixteen different conditional jump instructions that test status flags and jump if conditions are satisfied, such as zero, less than, or odd parity. These instructions are very important since they permit if statements, loops, comparisons, and so forth. Sixteen is a large number compared to earlier CPUs: the 8080, 6502, and Z80 all had 8 conditional jumps, specified by 3 bits.

In machine language, a conditional jump opcode is followed by a signed offset byte which specifies a location relative to the current program counter, from 127 bytes ahead to 128 bytes back. This is a fairly small range, but the benefit is that the offset fits in a single byte, reducing the code size. For typical applications such as loops or conditional code, jumps usually stay in the same neighborhood of code, so the tradeoff is worthwhile.

The 8086’s microcode was disassembled by Andrew Jenner (link) from my die photos, so we can see exactly what micro-instructions the 8086 is running for each machine instruction…  In brief, the conditional jump code (Jcond) gets the branch offset byte. It tests the appropriate condition and, if satisfied, jumps to the relative jump microcode (RELJUMP). The RELJMP code adds the offset to the program counter. In either case, the microcode routine ends when it runs the next instruction (RNI).

Read the details in Ken’s blog here.

Adafruit publishes a wide range of writing and video content, including interviews and reporting on the maker market and the wider technology world. Our standards page is intended as a guide to best practices that Adafruit uses, as well as an outline of the ethical standards Adafruit aspires to. While Adafruit is not an independent journalistic institution, Adafruit strives to be a fair, informative, and positive voice within the community – check it out here:

Join Adafruit on Mastodon

Adafruit is on Mastodon, join in!

Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.

Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Join over 36,000+ makers on Adafruit’s Discord channels and be part of the community!

CircuitPython – The easiest way to program microcontrollers –

Maker Business — “Packaging” chips in the US

Wearables — Enclosures help fight body humidity in costumes

Electronics — Transformers: More than meets the eye!

Python for Microcontrollers — Python on Microcontrollers Newsletter: Silicon Labs introduces CircuitPython support, and more! #CircuitPython #Python #micropython @ThePSF @Raspberry_Pi

Adafruit IoT Monthly — Guardian Robot, Weather-wise Umbrella Stand, and more!

Microsoft MakeCode — MakeCode Thank You!

EYE on NPI — Maxim’s Himalaya uSLIC Step-Down Power Module #EyeOnNPI @maximintegrated @digikey

New Products – Adafruit Industries – Makers, hackers, artists, designers and engineers! — #NewProds 7/19/23 Feat. Adafruit Matrix Portal S3 CircuitPython Powered Internet Display!

Get the only spam-free daily newsletter about wearables, running a "maker business", electronic tips and more! Subscribe at !

No Comments

No comments yet.

Sorry, the comment form is closed at this time.