Commodore 1581

The VC 1581 is a 3.5 -inch floppy disk drive for the Commodore home computer C64, C16, C116, Plus / 4 and C128. The MFM format used by the drive allows storage of 800 KB (compared to MS- DOS 3.5 " DD disks 720 KB, Amiga DD disks 880 KB).


Physical & General

The 1581 has a beige housing similar to the 1541 II and how these a separate power supply. The device is slightly wider than and is approximately twice as high as the actual drive mechanics. It works faster and quieter than previous models.

The drive has a rear two DIP switches for setting the device address on the CBM - bus in the range 8 to 11 It has, as the VC1571, a burst mode for fast data transfer, a C128 or a C64 with a hardware requires modifications and enables up to 30 times faster data transfer than with the VC1541.


Like all floppy drives for their Commodore 8 -bit computer is the 1581 stand-alone computer, here with a 6502 processor ( clocked at 2 MHz ) and 8 KB RAM ( see below). The capabilities of the drive as the computer at the time were significantly greater than those of many Commodore computers (eg speed of the C64 about 1 MHz, the memory of the VC20 5 KB). Because of this and because of the for a drive quite generous storage facilities there were even programs that shifted computationally intensive tasks in the floppy drive.

In addition to the control board is the 1581 hardware in principle of a double density drive that can be easily replaced by an Amiga HD drive. However, this works then only with DD disks, since the operation -ROM in 1581 would otherwise have to be changed.

Early models used the WD1770 controller from Western Digital, but it came with this (also in 1571 used ) chip sometimes cause problems, so that later in 1581 have a WD1772. The conversion can be done easily afterwards.

Used the 1581, unlike the other floppy drives Commodore, always a Modified Frequency Modulation format ( MFM) for data recording. MFM is also used by PC floppy controllers; with the help of special software, a PC can read and write the specific MFM format in 1581. This provides an easy way of data exchange between Commodore computers and PCs dar. The only prerequisite is that the PC floppy controller for the CPU of the PC at the register level is approachable, what with onboard controllers or such ISA and VLB cards the case. USB floppy drives for PCs can not handle the format, however, because the PC is not possible to directly access the controller chip. In this case, the only possibility is to generate the normal PC 720 -KB MS -DOS disks and other commodore to use a special software to read this format.


Internally, the 1581 Commodore DOS version 10, despite fundamental backward compatibility of the documented commands noticeably different from all previous versions. Discs are formatted with the license plate "3D" and offer 3160 free blocks and 288 Directory entries ( per directory, see 154x/157x 144 entries). The " Save with Replace" bug is corrected, there are also over older versions any more problems with relative (REL ) files that can take up all the free space in 1581. Overall, the DOS is more stable than many older versions, but there are also commands with serious bugs that can cause data loss. There are two versions of the firmware, but only minimally different.


The only version of CBM -DOS has this some kind of sub-directories, but there are in fact partitions ( " Validate" ) can be used to protect sectors against overwriting or unsubscribe through the DOS when cleaning up the BAM. These partitions can be a minimum of one block in size ( about the protection of a C128 boot block ) and at most as large as continuous free space available on the disk, ie at a blank disk about half of the disk, since the on the logical track 40 Directory is located. A partition is large enough ( at least 3 tracks or 120 blocks) can be formatted and used as a " folder" with a fixed data capacity. Partitions are marked in the directory above the file type " CBM ".


Located at power on or resetting the drive a USR file on a diskette inserted, the " 86 COPYRIGHT CBM " bears the specified name and a specific internal format has, it will be read and executed automatically in a specific memory area of the drive RAM. From Commodore this was used for the CP / M disks for the C128, it could be so if programmed but also fast charger contribute directly to the drive's RAM without having to explicitly load them first on the computer.

This ability is a pure drive- side and is not to be confused with the ability to boot the C128, in which computer page loaded from a sector of the floppy disk into the computer RAM and executed depending on the content.

Parallel routines

Inexplicably exist in ROM also routines for parallel data transmission, although this had been on the hardware side is not provided and not announced - possibly planned CBM to use the same DOS in later drives, such as the no longer published in 1591.

Easter Eggs

There are the undocumented DOS commands " B *" and " B? ", The output on the channel error credits the programmer and a dedication.

Data Organization

  • Physics (from the perspective of WD177x controller ) is a 1581- disk from 2 × 40 tracks of 20 sectors to 512 bytes.
  • Logically, however ( from the user, computer, or higher levels of the 1581- DOS view) is a 1581- disk from 80 tracks of 40 sectors of 256 bytes.

The smaller logical sector size (one could speak of a cluster size of 0.5 sectors) is created for the sake of compatibility with older drives, also because the DOS would otherwise strongly need to be restructured (with a size of 256 bytes 1 byte is sufficient as a pointer ). To nevertheless ensure an efficient data management, an entire track is buffered (all sections of a track on top or bottom ) in memory. For this purpose, the drive has 8 KB RAM.

On a 1581- formatted disk thus fit 800 KB of data ( " gross "). Since two bytes are used per sector as a pointer to the next logical sector and a track (10 KB ) for the Directory and other internal information (eg, BAM, Block Availability Map) are reserved, remain 79.40.254 = 802 640 bytes ( 3160 block or just under 784 KB) of memory for user data ( "net" ), provided that no additional partitions are set up.

Quick charger, copy protection, third-party DOS

Due to the fundamental differences in timing, format and firmware, most C64 rapid charger or programs do not work with copy to the 1581st Even games or general programs that access the drive, eg to reload more parts to run only if they only use the official I / O routines of the computer and DOS commands and does not operate on low-level hardware level. Since accumulated such problems with later drives ( CMD FD, HD, RAMLink etc.) exist for many programs (mostly unofficially created ) drive- independent versions, which of course also ideal for 1581.

A quick charger designed for 1581 is included on the supplied Test-/Demodiskette; also works such as the Fastloader the Action Replay 6- module (but not of the Final Cartridge 3) with the drive together.

Because of the low compared to the 5.25 "drives spread not very many exchange operating systems from third parties for the 1581 appeared. However, there are, for example, JiffyDOS for this drive, which eliminates the well-known software problems with almost complete compatibility.


  • Diskette type: 3 ½ inches
  • Recording Format: MFM, double- sided, double density
  • Capacity: 1 MB unformatted, 790 KB formatted (in theory), or approximately 784 KB ( practical)
  • CPU: MOS Technologies 6502 @ 2 MHz
  • RAM: 8 KB
  • ROM: 32 KB
  • Transfer protocols: standard (such as 1541) and fast serial (such as 157x, with C128 ); Burst mode ( C128 ); theoretically parallel ( via hardware modifications in the self-made)
  • Interface: CBMs " serial IEEE - 488" (CBM bus, via DIN connector )