Yeah, that's the third option I've heard of. I also saw one using two bitwise XORs, and that confused me

EDIT: Currently I'm using the one you just posted, but with % 0x10 instead of & 0xF. Should be the same, but took a bit to get my head around.

EDIT2: Wow, guess there are several people doing emus in JS...

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I don't think the Z80 is modern enough to have all that prefetching and junk messing with timing. Things were a lot simpler in the 80's Tongue

Heres something to look at: http://www.ftp83plus.net/Tutorials/z80optiA.htm . You are correct, there is no prefetch on the z80. In the document he mentions the intel 8088 and I think that maybe he got confused. My information did not come from that file though, I had read it in an assembly programming book years ago. For the past hour I have been trying to find out exactly what the chip is doing. Ive read that Zilog has never released the internal sequencing to the z80 so I dont think that I can verify anything on paper. I remember reading about how if the same registers or instructions have already been fetched, the z80 will not have to fetch them for the next instruction, thus saving at least one clock cycle. Unfortunately, I dont have the book anymore.

Well, after doing a small bit of research, it seems that the Z80 has a "fetch/execute overlap" for some instructions. Source. Though, this doesn't mean that the timing will change from case-to-case. The thing is, timings are not calculated best-case scenario depending on whether the instruction has been "prefetched" or not. More likely, the timing is the number of cycles between the fetch of the current instruction and the fetch of the next instruction. That way, there is no room for debate on cycle times.

Edit: removed duplicate quote

Ah ok. My big concern is getting JS to work fine in all browsers. I heard it was one hard task. In firefox, I think even good javascript threw full of errors

If anyone take on such project, good luck! Btw, do you think it could be made so it runs at a decent framerate? To save bandwidth, Netham45 had to set his emu to about 3 frames a second.