Engineer application - pishleback

Minecraft name: pishleback

What’s a thing you have made which demonstrates sufficient engineering knowledge?:
An 8 bit CPU designed to run general purpose programs with a focus on ease of use.

Overview of specs:

  • 256 nibbles of program memory
  • 8 bit
  • 16 registers
  • 27 bytes of data stack
  • 16 bytes of call stack
  • 6 tick clock per nibble of program (excluding jumps and IO)
  • ALU instructions include and, or, xor, add, not, bitshifts
  • 14 (non-trivial) branch conditions
  • error detection (stack overflows, and IO error when going too fast for the protocol)
  • unlimited number of IO devices with serial protocol

What engineering work went into designing this device?:

This CPU is something I have wanted to build for almost a decade. Learning how a computer works and building my first CPU was the first step and I learnt how to build RAM, registers, and an ALU. Since then I have made over 10 others, each making slight improvements over the previous one and trying new things. I list some of the key steps here:

  1. using CCA adders and switching to vertical buses: this allowed for much smaller, faster, and more precisely timings in my CPUs.

  2. tick-perfect timing and pipelining: By making sure every redstone wire was timed down to the tick, I got big speed improvements as well as reliability.

  3. deciding that usability is more important than compactness and speed alone: I realised that I was making fast CPUs which I didn’t enjoy using once they were complete - this changed my approach so that usability became the top priority above both speed and compactness.

  4. “unlimited” IO addresses and IO protocol: I often found that I either didn’t have enough IO addresses, or that there were far too many. This led to annoyance and/or clutter. To fix this I switched to using a single pair of wires which connected to every IO device along with an IO protocol: I won’t explain exactly how it works here but the result is a system with no wire clutter and no limit on the number of IO devices. The downside is a loss in speed however as stated earlier I was more concerned with usability and the ability to add and remove IO devices without worrying about wiring or address space came before speed.

The CPU I present for this application is the culmination of all these factors and almost 10 years of experience.

Image/s and/or video/s of the device:

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A sample program for computing fibonacci numbers is the following:
Each character is a hexadecimal digit representing 4 bits.

This can be broken up into separate instructions as:

where the instructions are:

  • push 1
  • push 0
  • duplicate [loop start]
  • output
  • pop to reg 0
  • pop to reg 1
  • push reg 0
  • add reg 1
  • push reg 1
  • jump to [loop start]

and in pseudocode

a = 0;
b = 1;
while True{
output a;
a, b = b, a + b;

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Accepted for an interview!

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