Category: Neutron

53 20 65 20 72 20 69 20 61 20 6c 0d 0a 43 20 6f 20 6d 20 6d 20 75 20 6e 20 69 20 63 20 61 20 74 20 69 20 6f 20 6e

If you decoded the title (and if you really did that instead of reading the permalink, you’re awesome), you’ll know what this post is about, at least generally. (Also, I was going to do binary, but I figured that would be too long).

Throughout computer history, we always needed a way to connect peripherals and stuff to our computers so we can do more stuff. Want to store your data? Connect up a cassette player, floppy drive, optical drive, (floptical drive??), or memory card reader. Want to print out that essay you just wrote? Connect a printer. You get the point, right? The thing is, there are many ways to connect things together.

Generally, there are two approaches towards connecting things to your computer: parallel (sending multiple bits at a time) and serial (sending one bit at a time). Then there are many different capabilities you can add onto that, such as daisy-chaining (think FireWire, or IEC/CBM bus; you connect one thing to your computer, then you connect something else to that thing and both can talk to the computer), multi-master (multiple computers on one bus), multi-slave (multiple peripherals on one bus), plug and play (you can rip the cable out of your computer and it won’t explode), etc.

For the Neutron, I think a daisy-chaining, multi-master, multi-slave serial connection would be useful, as such a connection could be used for multiplayer games and for sharing multiple peripherals between computers. The IEC bus almost fits our needs, with the only problem being that it does not support multiple computers on the same bus. I2C is also close, but still no cigar as it does not support daisy-chaining. So, we’ll have to modify one of them and/or combine them in some way. How to do that is beyond me at the moment.

0

Up down up down up down

I was watching Retro Game Mechanics Explained. It was a video about the Game Genie and how it’s codes worked. It was interesting.

I have a project I’ve been working on, at least in concept. It’s a computer based around the 65816 processor (I’m calling the computer the Neutron), and one of the things it will have is a cartridge port. These cartridges can be up to half a megabyte large without the need for banking hardware. So, I was thinking about how a Game Genie-like device could work for that project.

The 8-letter code entered (yes it had to be 8) would encode 1) a 24-bit address and 2) an 8 bit value. For the 24-bit address, the first 8 bits would tell which 64K bank was going to be accessed. Cartridge space runs from bank F4 to bank FB, so the first two letters would have to encode the hex digits F and 4-B.

As for the letters? It’s kind of simple. Here’s what I came up with:

  • A = 0000
  • Z = 0001
  • B = 0010
  • Y = 0011
  • C = 0100
  • X = 0101
  • D = 0110
  • W = 0111
  • E = 1000
  • V = 1001
  • F = 1010
  • U = 1011
  • G = 1100
  • T = 1101
  • H = 1110
  • S = 1111

Basically, the even numbers are assigned to A through H, while the odd numbers are assigned to Z through S. Simple!

Now for how those letters (and their assigned bits) are mapped to the address and value. For each set of 4 bits, we’ll say the first two will be represented by uppercase letters and the last two will be represented by lowercase letters. The 8 letters will be labeled A through H.

To map each letter’s bits to the address and value, take each pair of letters, say A and B, with bits AAaa and BBbb. The upper case bits (AA and BB) are unchanged. If the mapped address and value are below the bits for the letters, we’ll say that AA and BB go straight down. For the lower case bits, swap the pairs. You’ll end up with AAbb and BBaa. Do this for all other pairs of letters, with the addition that with every other pair of letters, invert the lowercase bits. (I.e CCcc and DDdd become CCdd and DDcc, with italics indicating an inverse bit). Then, take the pairs of sets of four bits (called nibbles), and going from the outside in, swap the first nibbles. So, if you start out with the bits AAaa BBbb CCcc DDdd EEee FFff GGgg HHhh, you will end with the bits GGhh BBaa EEff DDcc CCdd¬†FFee AAbb HHgg. Using those bits, the first six nibbles make up the 24-bit memory address and the final two nibbles make up the 8-bit value that should be returned when that address is accessed.

0