[vcf-midatlantic] What limited the C64's CPU to ~ 1 MHz?

Dan Roganti ragooman at gmail.com
Thu Mar 23 13:19:03 EDT 2017

On Thu, Mar 23, 2017 at 12:53 PM, John Heritage <john.heritage at gmail.com>

> So extrapolating to the C64 it could mean:
> 1.  May have been limited due to the specific process fabbing the C64 6502
> or support chips and they couldn't communicate higher
> 2-4 - Not really relevant for the 6502 but may have limited the size of
> the support chips in the C64 limiting the max frequency they could run at
> without using dividers.
> 5.  Power distribution - good point, 6502 was pretty low power but more
> mhz required more beefy power circuits which required more cost;  the C64
> was an exercise in low cost (although so was the Atari 2600...)
> 6 - I'd actually to know more about this;  Did the 8080 require custom /
> external solutions to get it to clock higher later on?  maybe these were
> fixed by the Z80.
> 7-12 these sound like items that could complicate the routing and
> electronics layouts necessary to make the entire computer work, requiring
> additional engineering/cost to raise the clock speed of the C64.
> Am I on the right path for possible (sorry for being less technical)
> reasons for why the C64 stayed at 1 MHz while it's intended successors (C+,
> C128) and some contemporaries (A800, BBC Micro) ran at higher clock
> speeds?  Sounds like cost may have been the overall driver..
> With the 6502 executing every other clock on many instructions (rating
> 0.43 MIPs @ 1 MHz), that means RAM needs to at least be double that to
> allow the custom chips to have their share of every other access, and 4x
> that if commodore ran the cpu at 2 MHz.

​I don't know if you can extrapolate this so easily to the platform level.
Some of them are as you noted, but that list referred mainly to the VLSI
design process.
That is your bottleneck, so to speak, among the various IC's, not just the
processor, but also the peripheral IC's, and of course memory.
After that, ​the platform is designed around that, with this mixture of
components, based on the specs.
You could always try some parlor tricks to increase the system clock but
not without potentially creating some damage without enough cooling.
As I mentioned, the CPU clock, as the rest of the design, is always done
with shortcuts to minimize parts on these consumer computers.
And these system clocks are typically interlocked to some exiting xtal, via
a peripheral IC, such as the color burst xtal or something else
So you typically find some of them running at less than the desired
The vlsi process for intel 8080 never really changed, they instead went on
to next generation processors such as the 8085 which only needed +5v.
The Z80 did this straight off the bat of course, as well as the 6800 and
6502, 6809, etc.

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