CoCo ROM to RAM (64K test) – part 1

See also: part 1 and part 2.

Due to reasons I discussed at length in an earlier article, the Microsoft BASIC in the Color Computer recognizes no more than 32K of RAM, minus the memory used for BASIC and screen memory.

When folks figured out how to upgrade the original Color Computer to 64K (by piggybacking 32K of RAM on top of the existing chips and running some wires), BASIC could not see or use that extra memory.

Enter the Dragon

Dragon Data, which sold the CoCo “clone” Dragon computers in the U.K., improved on this limitation with their Dragon 64 computer. It would boot up in a compatibility mode with the same memory limitation as the earlier Dragon 32 machines, but allowed entering a new mode that moved things around to allow more memory for BASIC.

If you start up a cassette-based Dragon 64 in the XRoar Online emulator, you get 24871 bytes available to BASIC:

If you then typed EXEC (to execute a machine language routine), the default routine it executed would do some magic to relocate BASIC and restart it with 41241 bytes available. (Both of these values will be less if using a disk system.)

Oh how I wish we had gotten that feature added to the CoCo’s BASIC when 64K machines first came out. (There were rumors that the Deluxe Color Computer 2 would have had something like this as part of it’s new features, but that machine never came to exist beyond some mentions in the BASIC manuals.)

CoCo RAM: Don’t ask, don’t tell.

On the CoCo, once you got to 32K, the machine would show the maximum amount of memory available to BASIC — the same 24871 seen by a Dragon 32 machine.

On startup, a cassette-based CoCo has 24871 bytes available for BASIC.

If you had a 64K machine, it would still show the same value since that is all BASIC knew about. The rest of the 64K was hidden.

“Assembly Language or OS-9”

In those early years, it seemed the official line from Radio Shack was that 64K was usable only from assembly language or the OS-9 disk operating system.

The same was true when the CoCo 3 came out in 1986. It came with 128K but could be expanded to 512K. Even though BASIC was enhanced to support the new graphics modes of the machine, it still only saw 32K available for BASIC programs. On a 512K CoCo 3, free memory for BASIC was reported as 24872:

…and now I am very curious to see where that extra byte came from.

Show me the mem’ry!

And here we are in 2022, over four decades after the first CoCo was sold in 1980, and folks are still wondering how to tell how much memory is in their machine.

One of the most suggested ways of determining if a CoCo 1 or 2 has 64K is to try to run the game Saylor Man from Tom Mix Software. It is notable for being the first CoCo game to require 64K, so if it runs … you have 64K and not just 32K.

But loading up a large program just to do a memory test isn’t the efficient way to do a memory check (even if it is the most fun way).

The second method is to run a short BASIC program that attempts to copy the BASIC ROMs in to RAM. If it works, and you can POKE to the BASIC ROM locations and change things, it proves you have 64K.

This test is convenient because it can be done by typing in a very short program in BASIC, rather than needing a way to download and transfer a tape or disk image of Sailor Man over to a CoCo.

In part 2, I’ll discuss several implementations of this “ROM to RAM” program, and then present a super simple 64K test program.

Until then…

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