

                    ZPM3LDR.REL
                    ===========

     A CPMLDR.REL replacement for CP/M 3.0 and ZPM3 systems.


CPMLDR.REL, as supplied by DRI, has a bug on some systems which 
prevents the loading of CPM3.SYS files larger than 16k. This is a 
significant problem especially if you intend to enlarge your bios 
or increase the number of buffers allocated to your system.

ZPM3LDR.REL was developed primarily to overcome this bug. 
ZPM3LDR.REL is able to load CPM3.SYS files up to the maximum 
possible system size without any problem. ZPM3LDR.REL also offers 
some convenient enhancements.


The usual way to use ZPM3LDR.REL is exactly as you would use 
CPMLDR.REL: link it to your loader bios and SCB.REL files to make 
the loader program which must be installed onto your system 
tracks. Before you install the program however, you may choose to 
patch the file at locations provided for in ZPM3LDR.REL.

The messages issued by ZPM3LDR can be changed. They take a 
standard '$' terminated form (as used by BDOS function 9). Using 
a debugger such as SID.COM, you should be able to view these 
messages and note that they have extra '$' terminators at then 
end of each. This is the room in which you may expand or alter 
the ZPM3LDR messages. Just remember not to overwrite the next 
message.

The CPM3.SYS FCB will be visible there too, allowing you to 
change it so that ZPM3LDR will load a file of a different name 
instead.

The copyright message is there but not to show that DRI has 
copyright on ZPM3LDR.REL (which it doesn't!).  That is part of an 
advanced feature of ZPM3LDR.REL which allows it to check for 
valid CPM3.SYS files.  CPMLDR.REL would attempt to load any file 
called CPM3.SYS, even if it wasn't really a CP/M 3.0 system file. 
The results could be catastrophic.  ZPM3LDR.REL will always check 
for the 112 byte signature at the start of the file, and will 
refuse to load CPM3.SYS unless the signature is correct.

This has another advantage. You may patch this signature to 
whatever you wish.  Then, after generating your CPM3.SYS file 
(using GENCPM.COM) you should patch it too. The patch might be to 
put in the version of your BIOS or some such thing.  On MYZ80, I 
use this system whenever I change the MYZ80 80x86 bios in such a 
way that the CPM3.SYS files won't work properly anymore. That 
way, any old CPM3.SYS files that are not valid anymore, won't get 
loaded accidentally.

For your information, the first 128 bytes of CPM3.SYS always 
begin with 6 bytes which tell CPMLDR where to load each section 
and where the cold boot entry is. After that there are 10 bytes 
of 0. ZPM3LDR does not check these bytes against anything so you 
can patch them with whatever you like.

The next 112 bytes would normally contain the DRI copyright 
message, the serial number of your system, then a fill of 0 bytes 
to the next record. Because ZPM3LDR looks for the copyright 
message as a signature of a valid CPM3.SYS, if it is changed, you 
will have to change ZPM3LDR as well.

