ACNews #63 26 декабря 2016

Review - PQ-DOS operating system

                    PQ-DOS operating system
                  by Vadim from Star Software

1) PQ-DOS - what is it? 
2) Why it was made? 
3) What are the new features? 
4) How to use it? And what for? 
5) Compatibility? What software works and what doesn't? 

                    1) PQ-DOS - what is it?

In short, PQ-DOS is an operating system for Profi that is almost
compatible with old systems for this computer (versions of
MicroDOS), and with canonical CP/M, and with MSXDOS-1/MSXDOS-2 *
- so you can run software for them. This OS combines features of
CP/M and more powerful MS-DOS. From programmer's point of view, 
OS is an intermediate layer of code and data around a program,
to allow it access files and devices such as display, keyboard,
serial interface, real-time clock etc.

OS can boot either from floppy disk or from HDD. It consists of
the following main parts:

1) BIOS - resides in ROM; **
2) boot procedures in MBR/boot sector;
3) main system loader and interpreter
config.sys;
4) BDOS module;
5) Concurrent BIOS module;
6) device drivers;
7) command processor (shell).

* some of MSXDOS-2 functions are not implemented, and there are 
no mapping procedures, so MSXDOS-2 software with them won't 
work. 

** In fact, BIOS is a separate system residing in the computer 
ROM where tests were before. BIOS call can be used for any 
system or a program. Moreover, if a used doesn't want to change 
his ROM, PQ-DOS can cope without ROM BIOS - it will load its 
emulator while booting. This way OS can't boot from HDD but it 
can "see" HDD if booted from floppy. 

                      2) Why it was made?

I'll tell why I started a new OS for Profi. When I first saw a
Profi advertisement (around 1993), I was mostly interested in
its extra mode with loaded OS. And when I tried it at home, on
self-made Profi v.3, I was shocked:

1) Files have names and extensions, and they can be fragmented; 
2) File size can exceed 64 KB; 
3) We can route program's output into a file; 
4) File commander CopyK can read and write MS-DOS disks; 
5) OS loaded from disk, not built in ROM like TR-DOS, so we can 
update it; 
6) Loaded program doesn't spoil all the RAM and returns to OS 
when finishes; 
7) There are text processors, programming languages etc, etc... 

Slight modifications to classic Spectrum schematics allowed
almost the same as PC in the early 90's! This was fantastic
breakout. Pros of this approach were visible, as were cons.

The main flaw of "Profi CP/M" was speed. It was very very slow,
and many people blamed the computer and OS for that and left the
platform. They were wrong. The computer and the OS were not
guilty. The slowdowns were provided by people who adapted this
MicroDOS to Profi. They made a large and thankful job running a
real OS on a ZX. Sadly they stopped at once - most of work was
done in a year, then the system remained mostly unchanged. The
people that ported MicroDOS to Profi even hadn't the sources of
BDOS, and they haven't tried to re-create them to fix bugs in
BDOS and speed up disk access. ***

In addition, Condor in 1995 made a version of the OS with an HDD
driver, named CBIOSS. Because of this absence of sources, the
author of this version (Pismenny D.G.) joined floppy disk driver
with HDD driver, and made disk error handler as a resident
running at any read/write error without a chance to cancel. OS
could work with no more than two logical partitions on HDD. If
you had more, you should mount and unmount logical disks that
was uneasy. And no subdirectories. And HDD partitioning system
was incompatible with PC. So the system became worse while being
nicer from user's view (for example, boot menu with
configurations, with merry sound). From my point of view, it was
disappointing.

Around 1997 I obtained documentation files and pieces of
MSXDOS-1 sources (not working disassembly). Having studied all
this, I decided that that's possible to make an MSXDOS-1 like
system compatible with Profi MicroDOS software. And it was
obvious that the OS must support FAT file system on HDD with
subdirectories. ****

*** slowdowns in Profi MicroDOS are a big topic so I won't 
discuss it there further. 

**** stages of writing the system are also skipped, ask for 
them. 

                 3) What are the new features?

If we compare PQ-DOS with MicroDOS + CBIOS, we have a lot of new
things:

3.1) BDOS module is now in 2 banks. In the original MicroDOS, it
occupied the memory from D400 to F800 for code, data, and
buffers. PQ-DOS has main code in the first bank (around 12?),
and some of code is moved to the second bank. Handle assigning
tables and environment variables are also in the second bank.
The main RAM contains the resident part or BDOS (under 4 KB), so
free space for user programs is 58 KB.

3.2) Driver subsystem in PQ-DOS is made alike one in MS-DOS.
Drivers are loaded in RAM banks forming a linked list. So a
driver has no install procedure as it was in CBIOS. To make a
driver relocable, do nothing. (This is science fiction :-) )

3.3) BDOS system has two sets of functions. FCB oriented
functions are for running CP/M software and old Profi software
for MicroDOS. Handle oriented functions work with ASCIIZ
strings. This module was totally rewritten in 2014.

3.4) File system format for PQ-DOS is FAT12/16. Floppy disks
supported have 40 or 80 tracks, one or two sides. IDE HDD can be
either CHS or LBA. Subdirectories limited with 64 symbols for
file path. CP/M file system is not supported, but I plan to make
a driver that will encode/decode CP/M file system for PQ-DOS
kernel, so it will see a FAT12 disk without subdirectories.

3.5) Command processor cmd.com was rewritten from scratch, and
it has more commands than in early Q-DOS where it was based on
MSXDOS-1.

3.6) Drivers are divided in two levels: logical level (high
level drivers) and physical level (low level drivers). Low level
drivers with POST procedure and a boot loader are the BIOS. High
level drivers are loadable device drivers. Some of loadable
drivers are loaded always, if OS can't work without them.
However, a user can change any of them to his driver via
config.sys file.

3.7) The system has a loader that parses config.sys, loads
standard drivers and implements boot menu.

3.8) There is a susbystem that handles extended memory. It was
here for a long time, even in MicroDOS modification, but now
it's updated. Programs can request RAM pages and return them.
They can also reserve parts of RAM starting from BDOS top.

3.9) PQ-DOS introduces 'processes' to run programs and control
occupied memory. A program can run another program as a
subroutine or initiate a child process. Also appeared residents.

3.10) Concurrent BIOS (CBIOS) is only used for backwards
compatibility. In the future it can be changed to more universal
implementation that would allow porting PQ-DOS to other ZX
Spectrum clones. The main ideas were documented in 2011, but
they are still incomplete because of the lack of time and my
lazyness)

3.11) A lot of new utilities appeared. Namely the formatter,
unique for ZX Spectrum clones, that can format floppies in FAT
with bad blocks and finetuning. HDD can be formatted as a whole
(writing a constant in all sectors), with fast format (cleaning
the title), and with fast format submode that clears sectors
faster. Again, bad sectors are marked so in the file system.

3.12) Disk drivers read/write physical sectors, as many sectors
as the caller wants. DOS read/write functions include an
optimized algorithm to find maximum possible block to speed up
disk access. MicroDOS systems always asked their disk driver for
128 bytes at once.

3.13) PQ-DOS has a built-in disk error handler that is called by
BDOS in case of error. User program can set its own error
handler.

3.14) Command processor can also be changed via config.sys.

3.15) OS loads very fast: 5-7 sec from floppy without file
shell, 1 sec from HDD (2 sec with file shell).

3.16) 'Environment variables' appeared: these are string
variables keeping text (or numbers in text format). There are
several system variables: for example, 'comspec' variable
contains path to cmd.com, 'path' variable contains the list of
default paths where to find executable if it weren't found in
the current directory.

                4) How to use it? And what for?

The system is thoroughly documented, and the docs are constantly
updated. Writing software is easy. File handling became a lot
easier than in older systems and in MicroDOS. You can write
music players for various formats, snapshot loaders for .z80,
.sna etc, to run old Spectrum software. Mounting .trd images for
a new TR-DOS ROM for Profi. There must be created utilities for
reading and writing disks to and from .trd images.

All of this is with FAT16, with directories. You can delete,
create, rename files and directories without a PC. Or you can
make an image of a floppy disk on a flash memory card, then read
it at PC. SD card driver will be created very soon)

    5) Compatibility? What software works and what doesn't?

No data so far.