The Commodore 8250 Dual Drive Floppy Disk

The Commodore 8250 was the high-end 5.25″ disk system for the Commodore PET, with two drives that could each fit 1 MB (4133 blocks free) on a standard disk. Here are some high-res pictures.

4133 blocks free!

The front says “CBM Model 8250 dual drive floppy disk”.

On the back, there is power and an IEEE-488 port. The model sticker says “DUAL FLOPPY 8250M”. The sticker next to it warns against opening it, but…

…unscrewing just two screws allows the top to be lifted up, similarly to the PET. The digital board is on the top side, and the transformer, the two drives and the analog board on the bottom.

This is drive 1. It’s the original SA-400 height – the later standard 5.25″ bay, still used today by CD drives in PC towers, is half of this1.

Drive 0 is below the analog board, which is responsible for both drives. The two black cables go to the two drive heads, and the wires in the two red plugs go to the mechanics of the two drives.

The digital board is really two computers in one, with two CPUs:

  • The DOS controller (IEEE-488 interface and filesystem) consists of one 6502 CPU, the DOS ROM chips 901887-01 and 901888-01 (8 KB each), and the two RIOTs2 for the IEEE ports.
  • The drive controller consists of a 6502 CPU, the ROM chip 901467-01 with the GCR tables, the 901885-04 RRIOT with the Micropolis controller code and drive I/O, and the 6522 VIA, also for drive I/O.

The 4 KB of RAM are shared by both systems.

Historical Context

The first drive systems for the PET (2040/4040, 1978) were dual-drive, single-sided, with 170 KB per side. The 2031 was a low-cost version with only a single drive and a single CPU. The well-known 1541 is a slightly modified 2031 with a Commodore Serial Bus instead of IEEE-488.

In 1980, Commodore introduced the 8050, which fit 3x as much data on a single side by using twice as many tracks and a higher bitrate. The 8250 from 1982 was a dual-sided version of the 8050. A year later, a “low-profile” version (8250LP) was released that was only about half the height.

The SFD-1001, a single-drive version of the 8250LP was introduced in 1985. It looked just like a 1541, but fit 1 MB on the same disks, and had an IEEE-488 port instead of Serial.

Drives with an IEEE-488 connector could be used with a C64 through one of several adapter cartridges.


  1. There were later “LP” (low profile) drives (8250LP, 2031LP) with mechanics half the height.

  2. In this device, the RIOTs have the markings “CO 10750”, which is an Atari designation. It seems like these chips were produced for Atari devices like the 2600, but ultimately used by Commodore themselves.

5 thoughts on “The Commodore 8250 Dual Drive Floppy Disk”

  1. My school had an 8250 (not 8520 as you wrote in the title). Great machine! It spoiled me and I thought the 360K of the double sided IBM PC drives was pretty puny (let alone the 1541 drive which added insult to injury by being insanely slow).

    However i think you’re rewriting history when you say that these drives were twice as high as what would become the standard. This height WAS the standard, and what came later was known as “half height” for a long time until full height disks started disappearing.

  2. I have 2 units of these, a bit rust outside (being metal…)
    I think I can get the original Pet that were used with both units.

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