The original version used a ‘Naughty but nice’
method by which, on exiting to CP/M, the program
altered the BDOS pointer at 0006H to point to one
of its own routines. When the CCP gained control
this routine would intercept all BDOS calls and pass
them on to the BDOS as normal until a ‘Read
console string’ came along (Function 10). When
this happened the custom routine would immediately
return having placed the name of the chain
file or command in the CCP command buffer. This
let the CCP do all the work which meant that a
program could be called using a command tail.
However this program has now been modified, and
the new version (2.0) uses a method similar to the
one that I have outlined above.
I must admit that I have scratched my head a little
on where to put the stack pointer before the chain
program is executed. (Sensible answers only
please.) Keychain V2.0 places it at 80H, and I have
put it 1 byte below the BDOS. As the CCP is only
required to point the SP at an 8 level stack I
suppose 80H would be OK. But in the case of a
general purpose chainer the 16 bytes below 80H
may be occupied with the second filename in the
command tail and thus confuse the issue. I
suppose that the ideal answer is to write a chain
function to suit your particular purpose at the time
and manage the SP accordingly.
And finally, why can’t I just read a filename into the
CCP command buffer and then call the CCP? Both
Steve and I had a look at this a while back, Steve
being more conversant with what’s what and
where it is in CP/M. Well we tried it a few ways
calling both the first and second entry into the CCP
and neither worked. It seems that the CCP pointer
to the command buffer start byte is changed during
the process of reading and executing the program.
It is not, it seems, restored back to the start of the
CCP buffer prior to the command being executed.
Thus, the next command that is read into the buffer
is either truncated or lost altogether. Regarding this
concept do not forget that the previous program
may have overwritten the CCP and so the failure of
this approach to chaining is guaranteed if this is the
Program overlays operate on the principle that a
controlling module is always present in memory,
this is usually called the Root module. If an overlay
section is required then the root module will read
the contents of the overlay file (which is executable
code) into the memory area allocated for overlays.
The root module then passes control to the start of
the overlay code. On completion the overlay
passes control back to the root module.
If, while you are reading all this gumph, you would
like to refer to the diagram illustrating overlay
techniques, you may find this text more palatable.
1. Will run faster than a suite of program
modules chained together.
2. Will allow the overlay routines to access
common variables and utility routines in the
3. You should be able to pass arguments to
4. In some cases you are able to nest
5. The overlay loaders can also be used to
load data files into memory if modified.
1. The initial setting up of the overall
program is rather complex and can be time
2. The individual modules cannot be run as
separate programs as chained programs can.
Methods of overlaying
Now this bit can require a little thought. I think that
an efficient overlaid system should have all the
common variables and most used utility routines in
the root module. This means that they are only
read in once at the beginning. This will keep the
size of the overlay files down and thus speed up the
system. So you are going to have to decide what is
going to go into the root. I know that this has little
to do with the creation of the files but I put it in to
emphasize that there is more to it than hacking the
Once again I must state that probably the best way
of doing it is to begin programming in a language
that supports overlays. However this is not always
practical and/or efficient. I know of a few languages
that will do it, namely CIS COBOL, DR PL1 and
Aztec ‘C’. There are certainly more but I have not
had the pleasure of yet. Overlaying with these is
just a matter of reading the instructions.
Assuming that all the files have to be created by
hand, and that we are dealing with programs that
pass through the .REL stage on their way to the
executable .COM stage, here is how we do it.
The whole process depends upon which linking
loader you are using at the moment. The Microsoft
LINK-80 (L80) is the most popular, it seems, but
unless I’m wrong it does not allow you to directly
create overlay files. The equivalent Digital Research
product which also seems to go under the
name of LINK-80 does give you this facility.
Due to this confusion in names I will call the
Microsoft product L80 and the DR loader LINK.
These seem to be the more commonly used