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Upgrading the 16KB Coco2 to 64KB


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I just got from eBay and old 16KB Coco2 computer. I got eight 4164 (64Kb x 1, which is 8KB) DRAM chips (single 5V voltage) to replace the eight 16Kb x 1 (2KB) chips.


I followed the below instructions found on the net. My Coco2's model is 26-3134.


To upgrade the 26-3134 or 26-3136 to 64 KB of RAM, it is necessary to replace the eight 8040517 chips with 4164 chips of 200 ns or better access time, and to solder a wire between the two pads labeled J1. J1 is located next to IC7.


Things boots up after the upgrade, per the above instructions. I type in "? mem" from BASIC to check the memory size, and the 31015 number is printed out.

It certainly bigger than roughly 14KB reported with the original 16KB my Coco2 comes with. But, I thought it would be bigger with 64KB RAM upgraded, no?





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My memory is failing me but I believe that is correct. Just because you have 64k doesn't mean you will see anywhere near 64000 bytes free.


If when he had 16KB it reported around 14KB I would think if he had 64KB it would then show around 62KB as long as nothing new is being loaded into memory when he is doing the check. I don't where all the extra mem is being used up. That is over half of what he put in not showing up. I think it would be something else causing it not to show up.

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I found the below link/discussion.




One entry is:


"Most 16K CoCo 2’s had socketed RAM to facilitate upgrading to 64K. The 16K RAM in the CoCo 2’s were also 5V only chips (in contrast to the 4116’s of the earlier machines). 64K CoCo 2’s, at least the later ones, had soldered in RAM – no upgrade was anticipated since it already had 64K. There are always exceptions I suppose – I’ve seen some CoCo 2’s that were upgraded to 64K using a memory board like the CoCo 3 upgrade.

The CoCo 3 RAM is probably good – BASIC, in order to retain backwards compatibility only uses 32K. Software such as Big BASIC and 512K BASIC allowed BASIC to use more RAM.


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You will see the same ram size on a factory 64k coco2


Sent from my LG-H830 using Tapatalk


So, I digged out the factory 64KB coco2 *with* extended BASIC, it reports 24817 via the "? MEM" BASIC command. Pretty strange. I think it has to do with the fact that my 16KB Coco2 does not have extended BASIC ROM installed?

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  • 2 weeks later...
  • 3 weeks later...

Since it has an 8-bit data bus, the 6809 processor has a maximum address space of 64K. This includes RAM and ROM together.


Roughly half of the 64K space is, by default, mapped to ROM -- Basic, Extended Basic, and the cartridge space. This leaves the other half of the memory inaccessible to the CPU.


There exists software to out the the ROM with RAM, but it is first necessary to copy the BASIC into RAM (as the Coco has no operating system beyond BASIC). There exists at lease one custom version of BASIC that would provide slightly more free memory; it was published in 80-Micro magazine back in 1983.

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