Ok, got that far. Map to mem can be an issue for large ISO files as you already know. Will this work with just a map (OxFF) command without the --mem option or have I missed something?
Yes, you are missing something

(but not a problem

, though it has been already stated/explained).
The concept is more general, it is about the possibility of have a number of !BAT grub4dos commands be executed by invoking (from grub4dos command line or from an entry in menu.lst) just the name of the "container" (which is nothing but either a HD image or a .ISO image).
The difference is not so light, as for all .iso's that right now you can boot as 0xFF mapping, typically with a menu.lst entry like:
title foo
find --set-root /foo.iso
map /foo.iso (0xFF)
map --hook
root (0xFF)
chainloader
or:
title foo
find --set-root /foo.iso
map --mem /foo.iso (0xFF)
map --hook
root (0xFF)
chainloader
or:
title foo
find --set-root /foo.iso
map /foo.iso (0xFF) || map --mem /foo.iso (0xFF)
map --hook
root (0xFF)
chainloader
You can have a menu.lst entry like:
title foo
foo.iso
And in the actual first sector of the .iso something like:
find --set-root /%~0
map / %~0 (0xFF) || map --mem /%~0 (0xFF)
map --hook
root (0xFF)
chainloader
boot
and this latter code is the same for any and all .iso's that can be booted through plain (0xFF) mapping.
But as said it is a more general concept, as an example, say that you want a FAT12/16/32 ONLY filesystem defragger working from grub4dos.
You prepare a COSMIAS "container" with in it:
- a "basic" FreeDOS system
- the FreeDOS defrag.exe
- a suitable autoexec.bat
and name this container (say) qdefrag or qdefrag.g4b. (see here about the .g4b "tentative" extension:
http://reboot.pro/to...l-for-grub4dos/ )
The !BAT code in the container first sector will take care of everything, and on grub4dos command line you just type qdefrag or qdefrag.g4b followed by a partition id, say (hd1,0) and the filesystem on that partition will be defragged.
Same goes for a number of grub4dos "native" utilities, as an example the above referenced MBRVIEW.g4b !BAT needs WENV, which may or may not be available in the "final user" root directory or grub4dos path, so you just put the mbrview.g4b and WENV in a COSMIAS container with something like this in it's first sector:
!BAT
find --set-root /%~0
map --mem /%~0 (hd9)
root (hd9,0)
mbrview.g4b %~1
and name the container "mbrview".
When you boot to grub4dos you simply type on command line (say)
mbrview (hd0)
and you can check the MBR partition table.

Wonko