Notes on the Intel 8086 processor's arithmetic-logic unit

[kens]

Author here for all your 8086 questions...

[gruturo]

Awesome article Ken, I feel spoiled! It's always nice to see your posts hit HN!

Out of curiosity: Is there anything you feel they could have done better in hindsight? Useless instructions, or inefficient ones, or "missing" ones? Either down at the transistor level, or in high level design/philosophy (the segment/offset mechanism creating 20 bit addresses out of 2 16-bit registers with thousands of overlaps sure comes to mind - if not a flat model, but that's asking too much to 1979 design and transistor limitations I guess) ?

Thanks!

[bcrl]

Thanks for publishing your blog! The articles are quite enlightening, and it's interesting to see how semiconductors evolved in the '70s, '80s and '90s. Having grown up in this time, I feel it was a great time to learn as one could understand an entire computer, but details like this were completely inaccessible back then. Keep up the good work knowing that it is appreciated!

A more personal question: is your reverse engineering work just a hobby or is it tied in with your day to day work?

[rogerbinns]

Did it make things simpler or more complex for the byte order they picked? It is notable that new RISC designs not much later all started big endian, implying that is simpler. Can you even tell the endianess from the dies?

[unixhero]

I dod not know thr 8086 had microcode. What was in it?

[kens]

It had the micro-instructions for most of the machine instructions, but a few simple instructions were implemented directly in hardware.

https://www.reenigne.org/blog/8086-microcode-disassembled/