ROM Chip Line Usage

[rallyrabbit]

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.

[+SpiceWare]

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.