Why does the 8 bit C= run at 1MHz and the A8 run at 1.7+MHz

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Why does the 8 bit C= run at 1MHz and the A8 run at 1.7+MHz

Postby WelshWizard on Sat Nov 19, 2011 8:26 pm

As put forward by Tezza as a post in its on right.

The one thing I find interesting as of yet its still a question of what known against what I have erad about the C64 is why does the Atari run at 1.7 odd and the C64 seems to run at 1 yet they both use an 8 bit CPU ?
6502B (most 400/800 machines)
Atari SALLY 6502 (late 400/800 machines and all XL/XE machines)

1.7897725 MHz (NTSC machines)
1.7734470 MHz (PAL/SECAM machines)
The C64 uses an 8-bit MOS Technology 6510 microprocessor. This is a close derivative of the 6502 with an added 6-bit internal I/O port that in the C64 is used for two purposes: to bank-switch the machine's ROM in and out of the processor's address space, and to operate the datasette tape recorder.
which I have be told and have read some where on the internet only give a speed of about 1 MHz

But the VIC11
The VIC-II was manufactured with 5 micrometer NMOS technology[2] and was clocked at either 17.73447 MHz (PAL) or 14.31818 MHz (NTSC). Internally, the clock was divided down to generate the dot clock (about 8 MHz) and the two-phase system clocks (about 1 MHz; the exact pixel and system clock speeds are slightly different between NTSC and PAL machines). At such high clock rates, the chip generated a lot of heat, forcing MOS Technology to use a ceramic DIL package (called a "CERDIP"). The ceramic package was more expensive, but it dissipated heat more effectively than plastic.

Carcenomy wrote:Yeap they do only run at 1MHz. Not sure why... you'd need to ask the designers. Could be to do with the timing of the memory modules they were using I guess?

Good grief, five posts and we're already back to computing as a topic! :)
Been looking around its seems the Commodore runs at more than one speed and can be 1MHz 2MHz or even 3MHz

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Re: Why does the 8 bit C= run at 1MHz and the A8 run at 1.7+MHz

Postby Carcenomy on Sun Nov 20, 2011 10:49 pm

The Commodores that run above 1MHz typically did so in specific modes; for instance, the 128 in native 128 mode.
I'm guessing that it'll be down to the way the Commodore machines are engineered, and how they interface the 6502/6510 to the VIC and the CIAs etc. Those chips will be looking for a reference clock, so running the 6502 at that reference clock would keep everything on the level. I'm only guessing, I'm no electrical engineer :)
Just the local Commodore hobo and middle-aged PC hoarder.
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