6502 Programmable Logic Array

[beannaich]

Hello all,

Has any work been done on reverse engineering the decode ROM of the 6502? visual6502.org, 6502.org, and these forums/wiki don't seem to have anything concrete. I am fully aware that deciphering what the decode ROM actually does for each line is a very taxing process, but I would have thought someone would have done it by now.

Before anyone suggests that I look at visual6502 or visual2a03's simulator and pick away at it myself, I have to say that I've considered it numerous times, but trying to glean any information from the simulators is as fruitful as staring at spaghetti for me. I don't get what I'm looking at when I see all the lines and colors, then my ADD kicks in and before I know it, it's 2 am and I've been on a random video kick on youtube.

That said, any response or documentation would be awesome

Thanks

[lidnariq]

The lookup table has been fully decoded, and as you've noticed, it's part of Visual6502. Just click one by one through the columns of the PLA to get the names they've assigned.
Alternatively, you can dig around on their site (google:site:visual6502.org pla) and get to this page: visual6502wiki:6507 Decode ROM or visual6502wiki:650X Schematic Notes

Sometimes the signals are something intelligible, such as "node: 1601 op-sty/cpy-mem", but as often they're something cryptic like "node: 1031 op-T3-stack/bit/jmp" which tells you "when" but not anything else particularly useful. Getting from this "branch of tree" level view up to the forest requires more than just looking at the node names and when they fire.

[beannaich]

Sometimes the signals are something intelligible, such as "node: 1601 op-sty/cpy-mem", but as often they're something cryptic like "node: 1031 op-T3-stack/bit/jmp" which tells you "when" but not anything else particularly useful. Getting from this "branch of tree" level view up to the forest requires more than just looking at the node names and when they fire.

Yeah, I've seen the 6507 decode ROM thing on Visual 6502's site, and I've poked around the decode ROM and looked at the signal names. I was more wondering if someone had done the legwork of getting the view of the forest already. I guess I'll keep dorking around until I figure out what enough signals do to start emulating the PLA.

Thanks for the reply

[org]

Do you really need it ?

Its just bunch of wires (130 to be exactly).

[beannaich]

This is fantastic Do you have the original Logisim file?

[org]

I'm doing it right now)

Will be released when ready.