ftp.barnyard.co.uk/2015.02.ftp.barnyard.co.uk.tar/ftp.barnyard.co.uk/cpm/walnut-creek-CDROM/ENTERPRS/CPM/UTILS/A/BIOS-R62.LZH/ZPM3.TXT

Text File | 2000-06-30 | 19KB | 436 lines

                    Z P M 3   by Simeon Cran
                    ========================

          A Z80 coded CP/M 3.0 compatible BDOS replacement.

               The first public release: 27/3/92
                    This document dated: 24/4/92

          Distributed at: Z-Node 62 (Perth, Western Australia)
                     V21,V22,V22bis 09 450 0200


WELCOME TO ZPM3
~~~~~~~~~~~~~~~
Welcome to the best CP/M compatible operating system for Z80
based computers with banked memory. The best? Yes, we believe so.
CP/M 3.0 has had bad press, but the fact is that it is faster
than CP/M 2.2 ever was, and it offered more integrated
facilities. Perhaps it was all the Z80 replacement BDOSes for
CP/M 2.2 which stole the limelight from CP/M 3.0, or was it just
that few computers had the required banked memory?

Whatever the reason for CP/M 3.0's lack of success in the
marketplace, there are still plenty of users who will stand by
its wonderful facilities and speed. For those users ZPM3 provides
the long awaited Z80 coded update.

ZPM3 offers all the good things that CP/M 3.0 does, and then it
offers more. Because ZPM3 is written in Z80 code rather than the
8080 code of CP/M 3.0, it can do everything that CP/M 3.0 does,
but in much less space. With the extra space recovered, ZPM3
packs in a number of new facilities. Yet the whole package fits
in exactly the same space as CP/M 3.0 so you can directly replace
your old CP/M 3.0 BDOS with ZPM3 without a worry.

ZPM3 is also fast. Faster, in fact, than CP/M 3.0. This is
possible because the rich Z80 instruction set allows many
algorithms to be implemented more efficiently. In addition, the
extra space available in ZPM3 has been put to use to further
optimise the code. Lots of small optimisations smooth the
execution flow, so ZPM3 becomes the fastest operating system on
most banked CP/M computers.


THE FEATURES
~~~~~~~~~~~~
ZPM3, in addition to complete CP/M 3.0 compatibility, offers the
following features:


Random Read Bug fixed.
++++++++++++++++++++++
Maybe you didn't know, but CP/M 3.0 has a bug. It affects random
reads under very specific circumstances, and can result in a
program thinking that you don't have some pieces of data in a
file when in fact you do. The bug would occur very, very rarely,
but it is real. ZPM3 finally squashes it.


Protected SCB User code
+++++++++++++++++++++++
The System Control Block of CP/M 3.0 was a revolution at the
time. ZCPR has a system environment and most other operating
systems have other similar structures, but the SCB of CP/M 3.0
was one of the very first.

Unfortunately, Digital Research never properly documented it, and
some programmers found things out about it that weren't quite
true and started programming accordingly. As well, because it is
available in the TPA bank, runaway programs can overwrite it
causing problems.

Mostly though, the SCB will survive, or at least any problems
will be so obvious that the user will realise that a crash has
occurred and will reboot. A real problem exists with the CP/M 3.0
code however when the user value is written over with a value
above 15. Many programs now directly write to this byte, and if
they put a value in that is above 15, all sorts of havoc can
happen with the disk system. Actually, CP/M 3.0 will handle user
areas above 15 with this method, and all seems ok until the
operating system mistakes one of these directory entries as an
XFCB. Simply put, user areas above 15 must not be used with CP/M
3.0.

ZPM3 has code which prevents these problems, making the system
even more stable.


Obsoleted Trap system.
++++++++++++++++++++++
One of the problems of the banked operating system was that it
was possible to redirect the BIOS to code below common memory, in
which case the banked BDOS could not access it. One solution is
to call all BIOS code from common memory, but this involves a
bank switch for every BIOS call, and this slows things down
considerably.

CP/M 3.0 got around the problem by providing special code just
below the SCB. If you redirected the BIOS, you also had to change
this code which caused a bank switch when your new BIOS routine
was called. When you removed the redirection, you also had to
restore the special code.

This system has major drawbacks. For a start, if you redirect the
BIOS, then another program redirects your redirection, then you
remove your first redirection (along with the special code), the
bank switch won't happen for the second redirection and the
system will crash.

If a CP/M 2.2 program tried to do the redirection, it would know
nothing about CP/M 3.0 and would not adjust the special code, so
a crash would result in that case too.

The special code was called the "Trap System" as it was meant to
trap redirection (as long as you set the trap). ZPM3 has
eliminated the need for the traps. They are still there, and
programs can still fiddle with them, but it doesn't matter how
they are set, they are ignored. There is simply no need for them
anymore. And this has been achieved without a performance
penalty. In fact, in the case of a program which sets the traps
but forgets to restore them, performance is now much better.


Semi-Permanent Read Only status for drives.
+++++++++++++++++++++++++++++++++++++++++++
In recent years, a trend in CP/M 2.2 is to make drives which have
been set read only to remain that way until explicitly changed by
function 37. ZPM3 now adopts this logic. Previously a control-C
would return a read only drive to read write. The advantage is
that a program can now make a drive read only for a session and
know that it will stay that way.


ZCPR compatible function 152
++++++++++++++++++++++++++++
Function 152 is the CP/M 3.0 parser. It was a great innovation at
the time as parsing is one of the more tedious aspects of
programming for CP/M. Unfortunately, almost as soon as it
appeared, it was made obsolete by the fact that it didn't handle
references to user number (DU references). A line such as
A:FILE.TYP would be correctly parsed, but A3:FILE.TYP would not.
CP/M 3.0 programs would often parse the drive and user
separately, then give function 152 the line without the DU:
reference. All this extra work should not have been necessary if
CP/M 3.0 had included user number parsing.

ZPM3 parses the user number, and goes even further by handling
named directories for ZCPR. This is possible as long as you set a
special word in the SCB which tells ZPM3 where to find the ZCPR
system environment descriptor. ZCCP, a companion CCP for ZPM3,
handles this automatically, but for Z3PLUS users, a special
utility is available which automatically sets this word.

The result is that CP/M 3.0 programs will not balk at DU:
references and ZPM3 aware programs can use the full DU: and DIR:
facilities of function 152. It has also made the brilliant ZCCP
code possible.


New Functions 54 and 55
+++++++++++++++++++++++
Datestamps in CP/M 3.0 are wonderful, but difficult to
manipulate. Two new functions make them easier to handle and at
the same time give compatibility to Z80DOS aware programs.

Function 54 (Get Stamp) returns a Z80DOS compatible datestamp.
Any program (such as many directory programs) which recognise the
Z80DOS standard can make use of function 54. There is only one
slight difference between Z80DOS datestamps and ZPM3's which you
should be aware of. Z80DOS will return a correct datestamp after
any successful open or search of any extent. ZPM3 can only return
a correct datestamp after a successful open or search of the
first extent of the file. This is because CP/M 3.0 datestamps are
only saved for the first extents of each file, in order to
provide the highest performance.

Even more interesting is Function 55 (Use Stamp) which provides a
mechanism for changing datestamps on files. Trying to do this
with CP/M 3.0 was virtually impossible because it involved direct
sector writes. With Function 55 you can simply set the stamp and
then write.


Wheel protected files
+++++++++++++++++++++
If you are using a ZCPR system (ZCCP or Z3PLUS), ZPM3 has access
to the wheel byte and supports wheel protected files. Such files
act normally if the wheel is set (signifying a priveleged user),
but if the wheel is not set, the files can not be changed. This
is of most benefit to BBS systems. The implementation is
virtually the same as most current Z80 CP/M 2.2 compatible
BDOSes.


Better error messages
+++++++++++++++++++++
CP/M 3.0 introduced the best error messages that CP/M had ever
had. ZPM3 goes further. The main difference you will notice is
that the user number as well as the drive is shown in the error
message. This is invaluable in helping you identify which file
might have caused a problem.


Function 10 history buffer and improved editing.
++++++++++++++++++++++++++++++++++++++++++++++++
Function 10 is used by the CCP to input command lines. Many other
programs use function 10 for input.

CP/M 3.0 introduced a history buffer for function 10. You press
control-W and you were returned the last command. It is a great
facility, but because it only remembers one command it is rather
limited. There have been RSXes written which give a much larger
history buffer, but RSXes take up extra program memory so are
undesirable.

ZPM3 gives a large (approximately 250 bytes) history buffer which
can store multiple commands. It also makes very intelligent use
of the buffer so that identical commands are not stored twice,
and commands of less than three characters are not stored. The
history buffer takes up no additional memory, and is always
available.

For security, it is possible to clear the history buffer so that
other users can not see what commands you have used.

The ZPM3 history buffer feature is so good, that for many users,
the ZPM3 upgrade is completely justified by it.

As part of the history buffer system, ZPM3 also offers a facility
called Automatic Command Prompting. This can be disabled, or can
be made switchable from the keyboard. When it is on, ZPM3 tries
to fill in the rest of your command based on what commands you
used most recently. It is like magic, and can save you typing out
complicated commands many times. In effect, it looks through the
history buffer for you and finds the command it thinks you want.
As you keep typing, if it turns out that the command doesn't
match anymore, it will try to match another command, and if it
can't, it lets you make the command by yourself. This facility is
quite amazing to watch.

And to integrate the history buffer and the automatic command
prompting, function 10 has the best command line editing you'll
find anywhere. Most of the control keys do something when you are
editing a function 10 line, and for the most part they mimic the
standard WordStar/NewWord/ZDE functions. You can jump to
different words in the command, delete individual words, delete
individual letters, insert letters, and a whole lot more.


Here is a list of what the various control keys do for function
10:

A    Move left one word
B    Go to the beginning or end of the line
C    Warm boot if at start of line, otherwise nothing
D    Go right one character
E    Go backwards one command in the history buffer
F    Go right one word
G    Delete current character
H    Destructive backspace
I
J    Enter line
K    Delete all to the right
L
M    Enter line
N
O
P    Toggle printing
Q    Toggle automatic command prompting (if enabled)
R
S    Go left one character
T    Delete current word
U    Add current line to history buffer
V    Clear line and delete from history buffer
W    Go forwards one command in the history buffer
X    Delete all to the left
Y    Clear the whole line
Z


CPMLDR.REL bug fixed.
+++++++++++++++++++++
If you have ever tried to use the CPMLDR.REL code supplied with
CP/M 3.0 to load a CPM3.SYS file larger than 16k, you have
probably come across the CPMLDR.REL bug. The computer probably
crashed, and you were left wondering what you did wrong in your
bios.

Well CPMLDR.REL has a bug. To solve this for you ZPM3 comes with
ZPM3LDR.REL which directly replaces CPMLDR.REL. It is also
somewhat better in that all the messages, and the fcb for loading
CPM3.SYS, are at the start of the file along with plenty of spare
room. As a result you can easily patch the signon and error
messages to say whatever you like and even change the FCB to load
a file called something other than CPM3.SYS.




All About the Random Read Bug.
==============================
Never heard of it? Well it's there in CP/M 3.0. I spent a lot of
time trying to work out what it was and just why it was
happening, and if you are interested, here are the details.

CP/M 3.0 uses the Record Count byte of an active FCB a little
differently from the way CP/M 2.2 does. It is mentioned in the
CP/M 3.0 manuals that the record count may contain numbers
greater than 128, but in such a case it implies that the record
count is really 128. CP/M 2.2 would not return record counts
greater than 128.

The reason for the use of the record count in this way is to help
speed up some of the logic used to find records in a file. It
works very well for sequential access. When it comes to random
access, the system has some failings.

The idea behind CP/M 3.0's unusual use of the record count is to
keep the record count of the last logical extent of the current
physical extent always in the Record Count byte. When accessing
extents before the last one, bit 7 of the byte is set. That way
it will always be at least 128 for logical extents before the
last (which CP/M 3.0 translates to mean equal to 128), and the
lower 7 bits are used as convenient storage for the record count
of the last logical extent. This is particularly convenient
because it means there is no need to go and read the directory
entry again when it comes time to read the last logical extent.

I hope you have followed that! In sequential access, this scheme
is great. The problem occurs with random access. In this case it
is possible to access a logical extent which has no records in
it. This could be any logical extent past the last one. In such a
case the record count must be returned as 0 (which is correct).
If we then go back to a previous logical extent in the same
physical extent, CP/M 3.0 gets confused and assumes that there
must be 128 records in that extent because the one we just came
from had no records and we are now accessing an earlier extent.
You're probably well and truly lost by now!

Anyhow, the assumption that CP/M 3.0 makes is quite wrong. The
record count ends up being set to 128, a read is allowed to go
ahead as if nothing was wrong, no error is returned, and the
record count remains incorrectly set until a different physical
extent is opened. The result could be chaos, but mostly it just
means that a program returns the wrong information.

Remember, a logical extent is always 16k. A physical extent can
be a multiple of 16k and is all the data described by one
directory entry. If your system has physical extents which are
16k, you would never have the problem because a new physical
extent would be properly opened for every new logical extent that
was accessed.

Typically though, a physical extent is 32k, so it holds 2 logical
extents. The problem won't arise until the file grows past the
32k mark in such a case. And when the file gets over 48k the
problem can't occur again until it gets over 64k... and so on.
Even then, it can only happen if reads are attempted to
particular extents in a particular order. So you shouldn't be too
surprised if the bug hasn't been too noticeable to you.

ZPM3 squashes the bug once and for all by using the correct
logic. In the situation where the bug would normally occur, ZPM3
makes sure it gets the correct record count information, and the
reads return the correct record count every time.

If you are interested in seeing a demonstration of the bug in
action (on CP/M 3.0) and comparing it with ZPM3, there is a file
floating around various bulletin boards which contains
demonstrations for the bug and an RSX to fix it. The RSX is a
less than perfect way of overcoming the bug, although it seems to
work. However, now that you have ZPM3, you don't need to worry.




LEGALS and SUCH
===============
The ZPM3 package is supplied free of charge, on the condition
that you don't use it to make money. If you want to use it
commercially you must contact me to get the OK (and negotiate our
fee).

If you find anyone (except myself) charging money for ZPM3,
please inform me!

Nobody is making any guarantees about this software. None at all.
If it causes your house to burn down, or a divorce, or just a bad
day, this is unfortunate, regrettable, but there is nothing that
I can or will do about it. You have been warned.

The ZPM3 package must only be distributed in the form that you
found it. Do not change or add anything. Don't even change it
into a different type of archive. Just leave it alone. However
you are free to distribute it to as many places and people that
you can. Just don't charge for it.



If in using ZPM3 you find that it doesn't act as described,
please forward the details to me so that either the ZPM3 code or
the documentation can be changed. If you would like further
details, please forward your specific questions to me. SJC.




As a service to all our ZPM3 fans, the latest version of the ZPM3
package can now be ordered. At this stage we can only supply IBM
formatted 3.5 inch 720k disks, however if you are keen enough
that shouldn't matter. ZPM3 remains free, however this service
will cost you $15 Australian (for the disk, copying, postage and
packing) to most places in the Western World (others by
arrangement).

This is a good way to guarantee you have the latest version, and
to guarantee that your package has not been corrupted by some
unscrupulous person.

When we fill your order, we will make sure to include the latest
demonstration copy of MYZ80 - the fastest and best Z80 emulator
for IBM AT (and better) compatibles. MYZ80 can run ZPM3 with
ease. It also handles ZCPR and CP/M 2.2. And yes, we do mean
FASTEST.

Send your international money order to:

          Software by Simeon
          ZPM3 Package
          2 Maytone Ave
          Killara   NSW
          Australia 2071

Your order will be promptly filled.