[Coco] [Color Computer] .os9 file format

Robert Gault robert.gault at worldnet.att.net
Tue Jun 17 21:36:41 EDT 2008

Walt wrote:

> Can anyone tell me the format/layout of .OS9 files? I belive they are 
> disk "images" produced by OS9Max. I have one that I would like to use 
> in an Keil's Coco3 Emulator, but no tool (nor the emulator) seems to 
> recognize the format of this file.

I don't know about OS9Max whatever that is, but PC .os9 files are all 
the data portions of the disks tracks strung end to end. The only 
emulator that requires the extension .os9 to work correctly with the 
disk image is MESS. Other emulators may or may not know what that 
extension means, so just change it to .dsk

You may or may not know that there is much more information stored on a 
Coco disk than exists within the 256 byte data of each sector. There are 
track headers, sector headers, and sector footers. You can look at the 
format in the technical section of the Tandy "Color Computer Disk 
System" owner's manual or a Western Digital products handbook.
Most emulators omit all of this control data (with the exception of the 
Kiel emulator) and just contain the sector data. If the disk image is 
single sided, the image is T0S1-T34S18 or T0S1-T39S18. If the image is 
double sided, the tracks, front and back, alternate in the image.

If your image contains one of the tricky copy protected programs, then 
it may need to have the track and sector system control bytes as well as 
the data portions of the sectors or it won't work.
The Kiel emulator should come with documentation describing how the 
extra control bytes are used in the emulation. Regardless, you don't 
need to do anything except change the extension from .os9 to .dsk

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