Classic Computer Magazine Archive ANTIC VOL. 3, NO. 4 / AUGUST 1984


The Atari 1050 does the trick


The 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

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.

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.

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.

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":

   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

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.

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.