TECH: 2600 Froggo Releases

[Supergun]

I have a few questions about the 2600 Froggo games. Obviously, I'm well aware that they are all re-issues of previously released games; most of them with only minor changes implemented.

 

What I'm confused about is their rom sizes. Have they all been properly dumped & absolutely confirmed to be only 4kb games? (in fact, many websites don't know)

 

And bearing that, if they are just 4k in size, why do they use an 8K rom? And, why do they all have a hex inverter?

[CPUWIZ]

A.) Not all use an EPROM.

B.) 2764 OTP's were cheaper at the time, only 4K are used.

C.) You won't find EPROM's in the label variations from the east.

[tschak909]

The hex inverter is required because the chip select lines are inverted on 2764 EPROMs versus e.g. a typical masked ROM from MOS back in the day. (And yeah, only one of the inverters is used, but the part is dirt cheap)...

 

-Thom

[Supergun]

Interesting information.

But I don't completely understand it.

 

So is substituting the one time programmable 27c64 rom, with a 27c64 EPROM possible? Are they a pin for pin match, and compatible?

 

And even if the 2 are compatible, can I assume it's usefulness would be limited to only 4kb worth of data anyhow, as the design of the board is the limiting factor?

[alex_79]

So is substituting the one time programmable 27c64 rom, with a 27c64 EPROM possible? Are they a pin for pin match, and compatible?

If they have the same part number (27c64) then yes, they are the same thing internally. The only difference is that the OTP rom has a plastic package without window that prevents it to be erased using UV light. The plastic package is cheaper than the ceramic with quartz window used on standard eproms and it was used for small production run where a mask rom was too expensive (Mask roms have an high upfront cost and a low per-unit cost and are only economically viable for high production runs).

Mask roms might or not be pin compatible with eproms, it depends on the specific model. For example a CXK3864 mask rom is compatible with 2764 and 27c64.

And even if the 2 are compatible, can I assume it's usefulness would be limited to only 4kb worth of data anyhow, as the design of the board is the limiting facto

Yes, or the board would have additional chips to implement the bankswitching which wouldn't make sense if the game only required 4k. One of the address lines is tied to either ground or +5v on the board making half of th eprom inaccessible.