Date   : Mon, 22 Nov 2010 21:39:23 +1100
From   : awilliams@... (Alan Williams)
Subject: BBC file formats

> Date: Sat, 20 Nov 2010 21:57:50 +0000
> From: kranser@...
> To: bbc-micro@...; mfirth@...
> Subject: Re: [BBC-Micro] BBC file formats
> 
> --- On Fri, 19/11/10, Michael Firth <mfirth@...> wrote:
> > What he said :-)
> > 
> > Plus, the reason it can be done is that games usually have
> > a startup 
> > screen in Mode 7, which only needs 1K of RAM
> > (HIMEM=&7C00), but run the 
> > actual game in one of the high graphics and colour modes
> > (usually Mode 1 
> > or Mode 2), which need 20K of RAM (HIMEM=&3000).
> > 
> > Thus, with DFS you load the game above PAGE while in Mode
> > 7, then switch 
> > to TAPE, move the code down, and then you can switch to the
> > high memory 
> > mode.
> 
> Thanks for the detail Michael.
> 
> The simple loop to move the code down in memory doesn't do any address
translation for JSR/JMP - so I presume that the code was developed to
run at &E00 (using O%)?
> 
> So, this begs the question as to how the programmer(s) tested the
work, seeing that they would need DISC access and the code would need to
run at &E00 and use a high-memory requirement MODE. So any ideas on how
they would test the Machine Code program?
> I guess they would run the source BASIC program to create the machine
code on disk and then transfer the disk to another machine to run the
machine code, and then transfer the disk back to update and save the
source program!

There is a thread on *. in which on programmer admits to having a
modifed OS rom that bypasses the copy protection system he was writing
(so it wouldn't clear the memory on break if I recall). Never assume
they didn't have some less than obvious solution.

It would be easy enough to  write a reverse the process, by switching
back to mode7 and relocating the code back so DFS can be used again. 
 
I remember talking to a C64 developer who said they used a Beeb to DMA
their code into the C64, since this was the fastest way they could do
it.  The C64 disc drive was a disturbing article even for the day.
Alan