In the version of the assembler posted above, the assembler itself occupies memory from $0240 to $0FDC and the assembler source text can use memory from $0FDD to $1FFF. The end of memory value of $1FFF is set up in line 1140, and the start address for the assembler of $0240 is set up in line 1160 ...

The manual for the original version of the software is at

http://www.decayaitch.co.uk/manual.pdf

This differs from the version posted above; for details of the locations used to define the workspace used for text and symbol table storage, see the end of the assembler listing.

Here is a version of the UK101 assembler/editor which occupies memory from $0240 to $1200.
There is an assembler source listing, so it can be reassembled at any address.

In the absence of a manual, here are the commands available from the command-line:

‘P’ lists lines of source code:
P
P a–
P ...

Many thanks for the links, Ed. Managed to recover a copy of the assembler/editor from the disk image, although it's a bit different from the UK101 version and hence required some modification. The OS-65D manual only contains a couple of generic pages about the assembler/editor, and it refers to a ...

It is possible to relocate the OSI assembler editor, however to do this you need to realise that it's not written in 6502 assembly language. It is written in a kind of intermediate code (a sort of pseudo-code) which is executed by an interpreter written in 6502 machine-code. So in order to relocate ...