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Simple 8k supercart process

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    Chopper Commander

  • 104 posts

Posted Thu Jan 17, 2019 4:59 PM

Can someone plz explain the process of how the CPU sets up to write or read >6000 to a ram chip that thinks it's first address is 0000? In other words, how did the ram chip end up writing to address 0 when it received an address of 6000 to write to instead of writing to address 24576 of the memory chip which cannot happen cuz it's only biggest address is he 8000? I understand the first available address at hex 6000 would be written to but I don't understand how the ram chip ends up writing to address is 0 when it hasn't gotten an address to send it hex 0. Confused. Thanks for clarifying in advance

#2 FarmerPotato OFFLINE  



  • 272 posts
  • Location:Austin, TX

Posted Thu Jan 17, 2019 5:19 PM

How it works


The top 3 bits of the address are decoded into 8 enable lines by a 74LS138 decoder. Not all of these are needed, but the one that corresponds to A0,A1,A2 equal to 0,1,1 or addresses >6000 to >7fff is labeled ROMS* or cartridge select. This goes to the module port, where it is connected to a CE1* or chip enable pin on the cartridge ROM. The effect is that the cartridge ROM only 'wakes up' when one of those addresses is present on the address bus.


The rest of the address lines A3-A15 go into the ROM  or RAM chip to select a byte.



In a similar way, the line responding to 0,0,0 goes to the CS* on and activates the console ROMs (16 bits wide). The line for 1,0,0 goes to yet another 74LS138 chip enable which decodes A3,A4,A5 to subdivide the >8000 - >9fff space into 8 blocks.



    Chopper Commander

  • Topic Starter
  • 104 posts

Posted Thu Jan 17, 2019 5:25 PM

Great!! The 138 is at it again, like in my ramdisk lol.. now that makes better sense than what I was struggling with, which would have been, first writable device found flipped the switch somehow..but regardless, I appreciate that info..I just couldn't put my finger in the explanation!!

Edited by GDMike, Thu Jan 17, 2019 5:25 PM.

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