Halting the Vic with Sally(6502c)

[GreyGhost]

Looking at the pin outs of the 6502 and 6502c, i see their are 2 pins that are exchanged (pins 34 & 36) and an extra pin halt (pin 35). Am i correct in assuming that the 6502c could be used in a Vic with the use of a single jumper? Or as I read I see that pin 35 on the 6502c has to be pulled high for the chip to work.

What does "pulled high" mean? And also could this halt feature be used also as say a click on/click off switch on the Vic?

Just wondering.

[wimoos]

I have a 65C02 in my VIC and it works.

No need for jumpers or soldering, just replace (mind the orientation of course).

Regards

[RobertBe]

I have a 65C02 in my VIC and it works.

Oh, that's interesting. I have a 65C02 lying around. What would be the benefits, if any, of putting in a 65C02 into the VIC?

Truly,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
The Other Group of Amigoids
http://www.calweb.com/~rabel1/
Southern California Commodore & Amiga Network
http://www.sccaners.org

[6502dude]

What would be the benefits, if any, of putting in a 65C02 into the VIC?

- lower power consumption
- runs cooler

[sjgray]

Looking at the pin outs of the 6502 and 6502c, i see their are 2 pins that are exchanged (pins 34 & 36) and an extra pin halt (pin 35). Am i correct in assuming that the 6502c could be used in a Vic with the use of a single jumper? Or as I read I see that pin 35 on the 6502c has to be pulled high for the chip to work.

What does "pulled high" mean? And also could this halt feature be used also as say a click on/click off switch on the Vic?

Just wondering.

If I understand correctly, Atari needed to halt the processor so the custom video chips could read memory and so they commisioned the 6502C (which is different than the 65c02). The 6502C can tri-state it's address and databus, which basically "disconnects" the CPU from the system, allowing a coprocessor to take over. The standard 6502 can not be tristated without adding some additional chips between it and the bus.

"Pulled high" means connecting to +5V (logic "1").

I suppose with a little rewiring the 6502C could work in the VIC and the halt line could be used to freeze the computer (program execution), but the VIC chip would still operate.

Steve

[RobertBe]

If I understand correctly, Atari needed to halt the processor so the custom video chips could read memory and so they commisioned the 6502C (which is different than the 65c02).

Ah, you are correct.

Truly,
Robert Bernardo
Fresno Commodore User Group
http://videocam.net.au/fcug
The Other Group of Amigoids
http://www.calweb.com/~rabel1/
Southern California Commodore & Amiga Network
http://www.sccaners.org

[Pedro Lambrini]

What would be the benefits, if any, of putting in a 65C02 into the VIC?

- lower power consumption
- runs cooler

Can I hijack this thread and ask; would changing the CPU, therefore, increase the lifespan of a machine in general? If it would this would surely be a great 'upgrade'.

[wimoos]

There are no specific advantages. Difference in powerconsumption for the VIC as a whole is marginal. The lifespan of a VIC also depends on other components (such as the 6561 VIC chip).
The reason that I replaced it, was my interest in using the extra instructions that are available in the 65C02, such as BRA, JMP (abs,X), SMB, etc. etc.

Regards.

[carlsson]

How about any undocumented op-codes? I realize very few VIC-20 programs may use those, but beware if you find one that does but which the new CPU doesn't support.

[Pedro Lambrini]

How about any undocumented op-codes? I realize very few VIC-20 programs may use those, but beware if you find one that does but which the new CPU doesn't support.

That's kind of what I thought. Wouldn't some demos suffer from such things and therefore not work on the newer chip?

[wimoos]

Try it.

Another big difference between the 6502 and the 65C02 is, that when there's an instruction that would otherwise "hang" the 6502, on the 65C02 the machine keeps going. On a demo, this could turn out handy.

[Wilson]

Another big difference between the 6502 and the 65C02 is, that when there's an instruction that would otherwise "hang" the 6502, on the 65C02 the machine keeps going. On a demo, this could turn out handy.

That's probably a bad thing, isn't it? Shouldn't a demo run on the regular 6502 present in the vast majority of Vic-20's? I'm also interested in a processor replacement, but even I use the occasional undocumented opcode. The 65C02 seems like a poor option. Has anyone tried the FPGA route with success? FPGAs have been successfully used as a PLA and other custom chip replacement.
http://fpgaarcade.com/parts.htm
I could be overlooking something, but it seems to be fairly basic as 6502 cores already exist.
Then again... I really don't have a clue when it comes to hardware.

[wimoos]

That's probably a bad thing, isn't it?

That is matter of perception. From where I stand, using undocumented opcodes is a bad thing.
It all depends on the application and what you want to demonstrate.

(I'm sorry, I don't mean to be rude )

[Wilson]

From where I stand, using undocumented opcodes is a bad thing.

This I understand.
What confused me is why you would want an instruction that causes the 6502 to "hang" to do nothing. To me that just seems to mask a problem. Sorry if I'm not following you.

[carlsson]

Well, it appears some undocumented op-codes are more stable and useful than others. While I haven't found use of any myself, I wouldn't disqualify anyone who did for producing a "dirty" program that deserves not to be executed anymore. In any case, it is a good point to make for anyone looking to replace the CPU with a similar one. People asked for the benefits of doing so and should probably also hear about the possible drawbacks.