I'm trying to find out if any of the Tandy CoCos could boot from disk. I'm thinking the answer is no but I'm having trouble confirming it.

I believe that non-disk machines booted from the standard ROM and that machines with disks had another ROM on the disk controller, which had an enhanced version of Basic which included extra commands for accessing the disk.

But was this the only way to boot them? Was there no boot track / boot sector / boot "granule"?

I know there were alternative OSes for the CoCo, so how did they start up?


2 Answers 2


Thanks to RichF's answer I looked for information specifically on booting OS-9 and found that you definitely did have to manually launch even an alternative operating system.

From OS-9 Level Two Operating System page 2-2, "Booting OS-9": scan of page

However, this DOS is not a filename and this is not how you load or run any other program. These are the normal ways:

To run a BASIC program "XXXX.BAS" in drive 0


To run a machine language program "ZZZZ.BIN" on drive 0


DOS is actually a command to manually boot an operating system from the boot sector of a floppy. So there is code in the ROM to boot from a disk, but unlike on some computers this process must be initiated by the user.

Here is the process documented by Boisy G. Pitre who worked it out in 1989 in order to make an alnternative ROM that booted directly into OS-9 without user intervention:

Examining the OS-9 Boot Process
A second prerequisite in completing the project is to understand how the process of booting into OS-9 from BASIC works. The most common way to boot into OS-9 from BASIC is through the DOS command. How does the DOS command work? It seeks to track 34 of the floppy disk and copies the 18 256 byte sectors into RAM location $2600 and JMPs to $2602. The code there sets up the CoCo’s screen and memory then jumps into a module called OS9, which brings up the rest of the operating system. In order to get OS-9 to work from ROM, there must be some method devised to emulate what the DOS command does.

I don't know if such customized OS-9 boot ROMs went on to become popular in the CoCo scene.


The Dragon 32 and Dragon 64 computers, which are very similar to the CoCo but not clones, use a BOOT command rather than a DOS command for "autorun" disks. From the XRoar emulator manual:

If a disk image is autorun, XRoar types “BOOT” (Dragon) or “DOS” (CoCo).

  • 2
    You'll most likely find it here: NitrOS-9
    – scruss
    Commented Jun 5, 2020 at 13:09
  • @scruss: Thanks. I got some original ones and some NitrOS ones after some searching. Now I'm trying to figure out what a "system disk" is after finding several references to them having a $55 flag in the first byte of Track 17 Sector 1. Which none of my disk images have. Commented Jun 5, 2020 at 13:14
  • 1
    Some people did create alternative ROMs which could boot OS-9 (from disk) directly and automatically, such as CoCoBoot 2. I've seen a couple of others but I can't find them again right now. Commented Jun 5, 2020 at 13:22
  • Apparently the article I quoted was about booting diskless and running RAM-only using a modified ROM to duplicate the OS-9 boot process. Not sure if the OS was running from the ROM or copied into RAM and then used the bank switching to keep the OS-9 RAM and no ROM. I'm mainly working on disk formats so I didn't go down that rabbit hole though it made the CoCo look way more interesting than the impression I had of if when it first came out. Commented Jun 5, 2020 at 13:26
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    And some people created OS-9 in ROM... And some people created OS-9 packs that make the whole system more accessible - lcurtisboyle.com/nitros9/nitros9.html
    – Brian H
    Commented Jun 5, 2020 at 13:28

IIRC there was no need to boot the operating system. It was in the ROM as part of the machine's BASIC. There were actually two choices, with Extended Color Basic being in the more advanced machines. A disk would simply provide access to files, sort of like a fast, random access tape.

There was at least one alternative operating system which would load from disk, OS-9. This was a unix-like OS developed for 6809 processors.

  • Did OS-9 boot from the disk or did the user have to boot it manually from BASIC? Commented Jun 5, 2020 at 3:35
  • @hippietrail I never had it, but I'm not sure. My guess is you would load it like any other program, except that it would take control once loaded.
    – RichF
    Commented Jun 5, 2020 at 3:36
  • 1
    This was the purpose of Extended Disk Basic's "DOS" command.
    – Brian H
    Commented Jun 5, 2020 at 14:03

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