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ROM Chip Line Usage

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#1 rallyrabbit OFFLINE  


    Combat Commando

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Posted Sat Jan 5, 2019 8:19 AM

Hi All,


I've been reading this forum a long time and finally started working on my 2600 clone game console since my 2600 died.


If I am reading the schematics right, we have A1-A11 tied to the chips through the cartridge connector and the 8 data lines D1-D8.  This should mean we have a 2k mappable ROM space.


However, in looking at the schmatics more, we seem to have 2 Chip Selects.  1 is typically tied down to the Atari CPU and would assume it is used in a standard way to select the ROM chip when addresses A1-A11 are used.  This seems to be active low.


Curious is the other CS (EXROM) (also active low), which in all the sample cartridges that I found is tied to VCC,




I am trying to understand a bit more about how we get >2k ROM given the schematics I've seen.

#2 SpiceWare OFFLINE  



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  • Medieval Mayhem
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Posted Sat Jan 5, 2019 2:03 PM

Cartridge port pinout is A0-A12, not A1-A11.


A12 is used to select the cartridge port.  This means if A12 is 0 the internals of the 2600 are being accessed, if it's 1 then the cartridge is being accessed.


That leaves A0-A11 to select an address from within the cartridge.  That's 12 address lines, not 11, and 2^12 = 4096 unique addresses to select from.

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