DO MORE WITH DOS 2.0
The Atari 1050 does the trick
By RICHARD KRUSEThe Atari 1050 disk drive has a lot to offer:
excellent performance, economical size, and the ability to increase disk
storage with DOS 2.0.
The 1050 is smaller than the 810 (3 1/2-in. high by 7
1/2-in. wide by 12-in. deep), and its brown and white colors match those
of the Atari XL line. In place of the 810's snap-down disk-door latch,
the 1050 features a horizontal slot and an easy-to-use door mechanism,
To insert a disk, you simply turn the latch bar clockwise ninety degrees.
To release the disk, turn the latch counterclockwise.
The 1050's power supply is the familiar wall-transformer
type used on all Atari peripherals. Two serial-bus connectors (a
cable is included with the unit), the power-cord receptacle, and a drive
number-selector switch are located at the back of the 1050.
Overall, the 1050 is better designed than the 810,
and its performance so far has been rock solid. It features a single
printed circuit board. The 810, on the other hand, had two, three,
or more circuit boards, depending on the manufacture date. The simple
design of the 1050 has resulted in greater manufacturing efficiency, lower
cost and higher reliability.
The unit's drive mechanism is the Tandon TM-50 half-height
drive. A belt drive connects the motor and disk spindle. A
single read/write head (single-side recording) is positioned by the split-band
mechanism, a design favored by most drive manufacturers.
The 1050's circuit board is mounted horizontally beneath
the drive mechanism, and extends most of the length and width of the box.
Its most important components include the 6507 microprocessor, the 6532
I/O-timer device, a 6810 RAM chip, a ROM chip (with a large Tandon copyright
notice affixed) and a 2793 disk-controller chip. The 2793 is a state-of-the-art
FM/MFM disk controller that contains both analog and digital circuitry
(including a data separator). There are no pots or adjustments of
any kind on the board.
There are no conventional screws or fasteners in the 1050.
Instead, plastic clips molded into the case hold the circuit board.
Plastic pins, posts and bumpers also secure the drive mechanism.
If you shake the box, you can hear the circuit board and drive mechanism
rattle. Treat this unit carefully
TABLE 1.
Capacity
and Performance:
DOS 2.0
DOS 2.0 Modified
40 Tracks
40 Tracks
18 Sectors/track 26 Sectors/track
720 Total sectors 1040 Total sectors
128 Bytes/sector 128 Bytes/sector
92160 Free bytes 133120 Free bytes
FORMAT AND COMPATIBILITY
The 1050 drive can operate in either a single-density, 810-compatiblc
mode or in the new enhanced-density mode. Mode selection is automatic
and transparent to the user.
The key to this ingenious mode-selection process is the
way the disk "whirs" for a few moments when a new disk is inserted.
The first sector of the track is read at this time, and the drive determines
the new disk's density and sets its internal parameters accordingly.
The 1050 accepts two "Format diskette" commands.
One (SIO command code $21) produces a format that is identical to the 810
drive (see Table 1). The second command (SIO command code $22) is
available only on the 1050. It creates an enhanced-density format
on the disk.
Using standard DOS 2.0 and the 1050 drive, you can read,
write and even format disks and then swap them freely with the 810 drive.
The 1050 drive is able to read all currently available Atari-format disks.
WHAT'S WRONG WITH THE 1050?
Not much, actually. I have not experienced any serious problems
with the drive during several months of use. I'm a little disappointed
with the 1050's track-seek speed, however-it's much slower than that of
the 810. Also, the 1050's head positioning is noisy. Finally,
I've heard of at least two incidences of actual failure of the track-seek
mechanism. It is not clear yet whether this will turn out to be a
common problem. (We are not aware of any problems of this type. -ANTIC
ED)
All in all, I'm favorably impressed with the Atari 1050
disk drive. Compared to its predecessor, the 810, it's definitely
a winner.
EXPANDING DOS 2.0'S STORAGE CAPACITY
If you have a 1050 disk drive, you may want to try the following modification
to DOS 2.0 while you're waiting for Atari's new DOS 3.0 Operating
System to be introduced. This procedure is very simple, and nets
you 256 additional data sectors per disk-or 32,768 extra bytes. And
you don't need an assembler, since I've listed the changes using BASIC's
POKE command.
There is an important caveat to remember when you're running
this modified DOS: Do not attempt to write any data to a single-density
disk, because the Volume Table of Contents (VTOC) has been altered on this
modified version of DOS.
On each DOS 2.0 disk, sector 360 stores the VTOC.
This VTOC data is organized so that each binary digit Of data indicates
the in-use status of one of the 707 data sectors. Thirteen additional
sectors are used for housekeeping functions by DOS. DOS changes the
value of these bits every time a file is added or deleted. if a bit is
set to zero, its corresponding sector is allocated to an active file.
If a bit is set to one, this corresponding sector is free, and is available
for re-assignment to a file.
Our DOS modification offsets the bit map by 32 bits and
increases the size of the VTOC. The bit that used to designate sector
one on a single-density disk, for example, designates sector 32 on the
modified-density disk. Our DOS expects this offset. In fact,
if you try to read the offset VTOC with an unmodified DOS, you'll end up
with zero bits that point to free sectors and one bits that point to assigned
sectors. If you ask an unmodified DOS to write data, it will probably
overwrite some of the sectors that are currently assigned.
THE MODIFICATION PROCEDURE
To modify DOS 2.0, enter each of the following commands in BASIC's
immediate mode (with no line numbers), and press [RETURN] after each statement:
POKE 1974,34
POKE 3363,34
POKE 3426,195
POKE 3434,3
POKE 3443,6
POKE 3450,128
POKE 3456,6
POKE 3460,51
POKE 4334,6
POKE 4359,5
POKE 4362,128
POKE 4418,6
Next, take these two additional steps: 1) Insert a blank disk into Drive 1, and format it:
XIO 254, #1,0,0,"D:" [RETURN]
2) Write your new DOS.SYS to the newly formatted disk:
OPEN #1,8,0,"D:DOS.SYS":
CLOSE #1[RETURN]
You now have a formatted, modified-density disk that contains
a single file, DOS.SYS. You should also transfer the DUPSYS. file. just
use the "O" option on a one-drive system, or the "C" option on a system
with two or more drives.
Do not use the "J" option (copy disk) to transfer files
between disks with different formats, because the j option copies both
files and the VTOC. The VTOC holds our new sector-assignment information,
and it's the VTOC that will be incompatible with a disk that's formatted
in a different way
WHAT YOU'VE DONE
You've just made a number of changes. First, you've changed DOS's
format command from $21 to $22, the 1050's enhanced-density format command
You've also caused the available-sectors bytes in the VTOC to be initialized
to the value 963, instead of 707. The "FREE SECTORS" value shows
this new capacity when you do a directory listing. In addition, you've
added 28 new sector-assignment bytes (224 new sectors) to the end of the
VTOC bit map.
Finally, you've added four new sector-assignment bytes
(32 sectors) to the beginning of the VTOC map. This offsets the whole
bit map by 32 sectors with respect to single-density disks, thus creating
the incompatibility between single and enhanced-density disks.
USING THE NEW DOS
Within the limits mentioned, your modified DOS 2.0 will work the same
way as the unmodified version. Just remember not to perform any DOS operations
with this modified DOS that writes to a single-density disk. Similarly,
do not perform any DOS operations that write to an enhanced-density disk
while you're running a single-density DOS. This includes directory
operations such as lock, unlock, rename and delete.
You can safety READ files into memory, and COPY files
to your new disk. Be sure, however, that you've booted up with your
target DOS, the one that will receive the data.
Some programs, including a number of disk utilities, as
well as commercial programs that don't directly access sectors and/or that
include their own file-management systems, will not work with this new
format. Since there is always a danger of destroying valuable data
in such cases, you should always test a copy of the program, using a "scratch"
data disk if one is required, before you proceed.
Richard Kruse, an electronics engineer living in Wichita, Kansas,
worked most recently for NCR. He currently runs a small Atari-specific
business out of his home. His company's products include a memory-expansion
board and two software packages.