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ZPM3LDR.TXT
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1993-02-04
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3KB
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68 lines
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.