Date   : Fri, 28 Jun 2013 11:06:49 +0100
From   : dl.harper@... (David Harper)
Subject: Importing files to B-Em 80x86

On 27/6/13 07:47, Jon Welch wrote:
> A Master 512 running in BeebEm will not recognise a 360kb or 720kb *.img
file. (I have no idea why. An actual 512 will work perfectly well with 360kb
or 720kb floppy disks.)
>
> The Mac version of BeebEm supports 720Kb discs images when in Master 512
> mode.

Intriguing.

I presume we are talking about the same thing here. In one sense, the 
Windows version does recognise 720kb disk images; it just thinks they 
are CP/M disks rather than DOS.  DOS (nor Windows) will not read or 
write CP/M disks and the (emulated) M512 will not read or write the 
files on a DOS disk, because it thinks the disk is CP/M format.

Robin Burton found out that when M512 DOS-Plus accesses a disk it starts 
by looking through the various disk formats it supports until it finds 
what it assumes is the right one, starting with the Acorn 800kb format 
and finishing with the Acorn 640kb bootable format (which is one reason 
why these are the fastest and slowest formats, respectively).

DOS-Plus always gives priority to CP/M when it can, so I presume it 
checks for CP/M formats before it looks for DOS ones. I would guess that 
there must be some difference between physical CP/M and DOS disks which 
doesn't show up in disk images. It might be something like sector size 
or the presence of a sector numbered 0. (DOS disk tracks start with 
Sector 1.) This would mean that the emulated M512 looks at the disk 
image, first rejects it as an Acorn 800kb format disk, but then accepts 
it as a CP/M disk. A physical system would reject the CP/M option, and 
move on to looking at it as a DOS disk.

As I say, though, that is a guess. If a version of the emulator causes 
the M512 to recognise 720kb disks as DOS rather than CP/M, then there 
must be some other cause lying in the emulator program rather than in 
DOS-Plus.

David Harper