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8k games into a 4k CHIPS???? How is it possible??? (ATARI 2600)

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Hi friends.
I have a huge question and I'm very curious about how it is possible.
I own an Atari 2600 cartridge that contains four 8K games. 

They are:
PRESS COOKER, E.T., ROBOT TANK and PIGS IN SPACE. (4 x 8K = 32K)
All of it is 8K games each one so the minimum chip to fit that four games, I THINK it would be a 27C256 but as you all can see the chip used is 27C128 , that I think fits only a 16K program!
How is it possible?????
I would like to understand that magic. I have interest on doing my own 8k cartridges.

Could any friend explain to me?

 

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According to the datasheet: "The NMC27CP128 is a partial NMC27CP256..." This was common practice in the day, to sell memory chips which passed production tests on only half the capacity. Often those chips would function correctly on slower systems like the 2600 at full capacity.

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So you say that a 32K program fits in a 27C128 if I plain to use it in a slow system as ATARI 2600????

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What is more interesting is the bankswitching is done using just two logic chips. I'm assuming it's not F8, and the games have been modified for some other, as yet unknown method.

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18 minutes ago, batari said:

What is more interesting is the bankswitching is done using just two logic chips. I'm assuming it's not F8, and the games have been modified for some other, as yet unknown method.

It is a very very interesting thing indeed!!!!!!!!!!!!!!

 

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Can you photograph the back of the board so we can trace the banking circuit?

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I'm surprised it needs any ICs for banking.  If it's a 32K chip, it only needs 2 lines connected to switches.  The rest of them connect as usual to the Atari.  So it would be interesting to find out what the ICs are there for.

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1 hour ago, ChildOfCv said:

I'm surprised it needs any ICs for banking.  If it's a 32K chip, it only needs 2 lines connected to switches.  The rest of them connect as usual to the Atari.  So it would be interesting to find out what the ICs are there for.

The max size without bankswitcing is 4k (any address with A12 low will cause bus contention with TIA/RIOT), so you need bankswitching for the 8k games in this cart.

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Clever banking circuit. Only 5 gates.

 

They saved money buying 27CP128s and programming them as 27C256s. The lower half of each chip might have a few bad bits, stuck high or stuck low. So if they had a programming error they could erase and try different game binary files until it passed. Higher labor costs, but maybe labor was cheap back then in Brazil.

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11 hours ago, alex_79 said:

The max size without bankswitcing is 4k (any address with A12 low will cause bus contention with TIA/RIOT), so you need bankswitching for the 8k games in this cart.

I see (now).  So I guess the choice of games is limited to using a common banking scheme then.  Unless you patch the ROM.  Anyway, the banking is almost certainly different from the game selection.

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On 8/8/2020 at 1:15 AM, ClausB said:

Can you photograph the back of the board so we can trace the banking circuit?

I Will do that!!!
I will take a pic and post it here!

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TRS80.jpg.b4b856ecf72298e2843032ab94e185da.jpg

Interesting thread! There were home computers with chips like this too, advertised as 32K but really having 64.

 

The supposedly half-bad chips worked great; the Brazilian version was called the Prologica.

 

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Hello friends, I need the dump of these four 8k games or other dumps (8k games) with the same bankswitch to use on a pcb identical to this post.

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18 hours ago, Pilot99 said:

Hello friends, I need the dump of these four 8k games or other dumps (8k games) with the same bankswitch to use on a pcb identical to this post.

Sent you a message.

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1 minute ago, pcrock said:

Sent you a message.

Hello my friend, I need games with the same bankswitch of this cartridge, have you done a dump? know where I can find 8k games with the same bankswitch?

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On 8/8/2020 at 6:53 AM, alex_79 said:

Looks like the same board as the carts shown in these threads:

 

 

 

The pcb schematics and pictures of the first thread, with the games of the second one is exactly what I have, I think the dumps you send me will not work on this pcb/bankswitch, but I will test anyway.

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35 minutes ago, Pilot99 said:

The pcb schematics and pictures of the first thread, with the games of the second one is exactly what I have, I think the dumps you send me will not work on this pcb/bankswitch, but I will test anyway.

Please... test it and I will remove the chip from my cart to do the dump for you too.

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Good afternoon people.

I would like to start recording my 2kb and 4kb Atari 2600 games, but I can't find GERBER for 28-pin Eprom, only for 24-pin and no content teaching how to do it, but this is already turning me on.

I just found this file attached but I can't get the eprom for dip 24.

Could anyone here help me with these doubts?

I would be very grateful, because in Brazil people who know do not share knowledge are sad.

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Good afternoon people.

I would like to start recording my Atari 2600 2kb and 4kb games, but I can't find GERBER for 28-pin Eprom, only for 24-pin and no content teaching how to do it, but this is already turning me on.

I just found this file attached but I can't get the eprom for dip 24.

Could anyone here help me with these doubts?

I would be very grateful, because in Brazil people who know do not share knowledge are sad.

pp_atari2600_2K_4K_gerbers.zip

Edited by Robson Zandoná

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23 hours ago, pcrock said:

Please... test it and I will remove the chip from my cart to do the dump for you too.

I start the tests with the 4 in 1 "27C256_Phoenix_Hero_Kangaroo_Ms pac man" and only Hero don't worked, so I separeted the 4 games and replaced "Hero" with other "Hero" from the 4x1 dump of the thread above, so the 4 games work.

Edited by Pilot99

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