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Forums > CSDb Discussions > Overclock the 6502?
2017-05-18 15:55

Registered: Nov 2005
Posts: 50
Overclock the 6502?

I've been keeping an eye on the ZX Spectrum Next, and one of the features they've put in is a turbo button to run the Z80 at 7MHz rather than 3.5MHz


I never had a speccy to tell, but it seems make a difference without just playing the whole thing at twice the speed.

Would a similar thing be doable on a C64? and would it actually provide any benefit to any games or apps?

I'm also curious how their colour upgrade example in Cybernoid 2 works, unless it's a patched game
2017-05-18 16:25

Registered: Jul 2007
Posts: 323
This is already built into the C128. That's probably the easiest way to check what effect it has on games and applications.
2017-05-18 16:53

Registered: Dec 2001
Posts: 8408
its not that easy, since the entire system is based on a single clock... so you will at least need some additional logic to deal with that (like the various turbo cards do). or run most of the system at higher speed, like kisiels turbo-demo-card did. both ways have its own problems, and are likely not very compatible with existing code.
2017-05-18 16:55

Registered: Nov 2005
Posts: 50
Interesting, didn't realise that. Thanks.

Although from what I've read (quickly), if you try the 2MHz in C64 mode, you can only use it during VSync, otherwise it breaks things. But I guess if the software runs directly in C128 mode, that should be a pretty good check :)


2017-05-18 18:42

Registered: Apr 2002
Posts: 4176
what happens in c128 2mhz cpu mode is that there are no free BUS cycles for the VICII to read memory to display gfx. So you see garbage, as the VICII reads for gfx what was left there in the previous cycle by the cpu.

thats why it only makes sense to use it on top/bottom borders only. however nothing stops you from using it all the time. only thing VicII display will be garbage.

the 80 column chip is safe from this at it has its own memory that can only be accessed by the cpu trough its registers.
2017-05-18 18:45

Registered: Oct 2014
Posts: 167
On the C64 you need to overclock the VIC chip not the CPU ;) If we can get a FPGA dropin VIC and fiddle with the clock, we a good to go ;)
2017-05-18 20:36

Registered: Dec 2001
Posts: 8408
So you see garbage, as the VICII reads for gfx what was left there in the previous cycle by the cpu.

still waiting for the c128 demo that would *still* display some graphics :)
2017-05-18 21:05

Registered: Apr 2002
Posts: 4176
risen from oblivion displays gfx in 2mhz mode. that is the very first fx. idle byte and lda #gfx code, hence VICII reads the bus every 2nd cycle.
2017-05-18 21:16

Registered: Dec 2001
Posts: 8408
mmmh are you sure? iirc that works in x128 - which doesnt emulate the 2mhz idle fetch correctly :)
2017-05-18 21:44

Registered: Apr 2002
Posts: 4176
Quote: mmmh are you sure? iirc that works in x128 - which doesnt emulate the 2mhz idle fetch correctly :)

iirc graham told me on irc its done like that. but can you imagine any other way ? the vallejo pic is by somehow fucking up the VIC internal counters and it thinks it still on the middle of the screen, but thats just a wild guess, based on my experiments where I could push the top/bot border to the middle of the screen.
2017-05-18 21:56
White Flame

Registered: Sep 2002
Posts: 134
Given the high number of clock cycles per instruction on the Z80, it wouldn't surprise me if it had a ton of dead bus cycles. If that's the case, one could conceivably see increasing just the clock rate of the CPU, but still work on the same speed memory bus, just with higher density of operations. As the speccy a framebuffer machine, there's also far fewer things that require cycle-precise timing in normal bitmap games.

The 6502 has very few dead bus cycles. The 65c02 and later reduced the cycle count of some of those as well. But if you change the timing of anything in the CPU, tons of stuff breaks regarding C64 software.

(of course, I have very little clue about the z80 and the above might just be random mumbo jumbo)
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