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What’s Inside the Atari 1050 (and the US Doubler)

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  • REPRINT
  • ATARI COMPUTER ASSOCIATION ORANGE COUNTY
  • MARCH 1985
  • AUTHOR: JEFF REID

This article explains a bit of the inner workings of the Atari 1050 and Atari’s orphaned “enhanced” density.

The 1050 is an Atari 5 1/4 inch floppy disk drive. It performs all of the necessary functions to support Atari DOS (disk operating system) functions: reading, writing, formatting, and so on. It is an intelligent controller, as are all Atari peripherals; the Atari computer sends simple commands to the 1050, which in turn performs the necessary steps. This requires a little “computer” inside the 1050.

Boxed Atari 1050 Disk Drive set including the drive, power adapter, and connection cable in original styrofoam packaging. The retail box in the background features branding and a yellow label with 'New DOS Power.'

The 6507 is similar to the 6502 microprocessor used in the Atari, but with fewer address lines. The 2793 is similar to the controller chip used in the ATR8000 and is Western Digital’s top-of-the-line floppy disk controller chip. It is capable of driving 5 1/4 or 8-inch floppy drives, single or double density, and single or double-sided.

So why doesn’t the 1050 support true double density? Well, the problem is memory, or actually, lack of it. There are only 256 bytes of memory, some of which is needed for program control. This limits the 1050 to 128-byte sectors and is OK for single density. However, double density sectors are normally 256 (or more) bytes long. Atari’s answer to this problem is enhanced density. Enhanced density actually uses the double density recording technique but only 128-byte sectors are implemented. This ends up producing 26 sectors per track, or over 133,000 bytes of storage.

It makes you wonder why Atari used an expensive floppy controller chip and then didn’t give it enough memory to support double density. Well, somebody else did. You can enhance your 1050 with an upgrade like the US Doubler and get “true” double density.

The US Doubler

The US Doubler, available for around $60.00, adds another 128 bytes of RAM memory and a new software ROM chip. These modifications support true double density with 256-byte sectors. In addition, the drive can communicate with the Atari computer about three times faster than normal.

Included with the US Doubler 1050 enhancement is SpartaDOS, a totally new operating system for the Atari. This new operating system is similar to MS-DOS as used on the IBM PC. The main features are date and time support, including file date/time stamping, and tree-structured directories. Since the directory structure is not like the Atari DOS standard, a special copy utility is provided that will transfer files to/from SpartaDOS from/to Atari DOS-type diskettes.

Atari 1050 disk drive with power supply and connecting cable, shown on a white surface. A handwritten note on the drive says 'Remove Before Power On.'

SpartaDOS also takes advantage of the US Doubler speed enhancement by utilizing a more efficient “interleave.” Interleave describes the physical ordering of the sectors on each track of a diskette; an interleave of 17 means that the sectors are arranged so that sector two is located 17 sectors past sector one, sector three is 17 past sector two, and so on, so that each higher-numbered sector is the 17th sector from the current one. Another way to look at this is to realize that if all the sectors on a track are read in order, the number of revolutions required to read the track is the same as the interleave. For example, since the 1050 spins at 288 rpm, it takes 3.54 seconds to read a track with an interleave of 17, and 1.46 seconds with an interleave of seven. The normal 1050 uses an interleave of 17, although it is capable of using an interleave of 16. The high-speed US Doubler mode uses an interleave of seven, about 2.4 times faster than a normal Atari 1050. 🕹️

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