The Commodore 1571 was Commodore's

Commodore International

Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...

 high-end 5¼" floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

 drive. With its double-sided

Double-sided disk

In computer science, a double-sided disk is a disk of which both sides are used to store data.Early floppy disks only used one surface for recording. The term "single sided disk" was not common until the introduction of double-sided disks, which offered double the capacity in the same physical size...

 drive mechanism, it had the ability to utilize double-sided, double-density (DS/DD) floppy disks natively. This was in contrast to its predecessors, the 1541

Commodore 1541

The Commodore 1541 , made by Commodore International, was the best-known floppy disk drive for the Commodore 64 home computer. The 1541 was a single-sided 170 kilobyte drive for 5¼" disks...

 and 1570

Commodore 1570

The Commodore 1570 was a 5¼" floppy disk drive for the Commodore 128 home/personal computer. It was a single-sided, 170KB version of the double-sided Commodore 1571, released as a stopgap measure when Commodore International was unable to provide large enough quantities of 1571s due to a shortage...

, which could fully utilize such disks only if the user manually flipped them over

Flippy disk

A flippy disk is a double-sided 5¼" floppy disk, specially modified so that the two sides can be used independently in single-sided drives...

 to access the second side. (However, the two methods were not interchangeable; disk which had their back side created in a 1541 by flipping them over would have to be flipped in the 1571 too, and the back side of disks written in a 1571 using the native support for two-sided operation could not be read in a 1541).

The 1571 was released to match the Commodore 128

Commodore 128

The Commodore 128 home/personal computer was the last 8-bit machine commercially released by Commodore Business Machines...

, both design-wise and feature-wise. It was announced in the summer of 1985, at the same time as the C128, and became available in quantity later that year. The later C128D had a 1571-compatible drive integrated in the system unit. A double-sided disk on the 1571 would have a capacity of 340 KB

Kilobyte

The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...

 (70 tracks, 1,360 disk blocks of 256 byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s each); as 8 KB are reserved for system use (directory and block availability information) and, under CBM DOS, 2 bytes of each block serve as pointers to the next logical block, 254 x 1,328 = 337,312 B

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

 or about 329.4 KB were available for user data. (However, with a program organizing disk storage on its own, all space could be used, e.g. for data disks.)

Depending on format, CP/M disks would format to 360 KB, with a mechanical maximum capacity of a 400 KB format (as with DD 5.25" drives generally).

The 1571 featured a "burst mode" when used in conjunction with the C128 (although not when used with the Commodore 64

Commodore 64

The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

 or VIC-20

Commodore VIC-20

The VIC-20 is an 8-bit home computer which was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PET...

). This mode replaced the slow bit-banging

Bit-banging

Bit banging is a technique for serial communications using software instead of dedicated hardware. Software directly sets and samples the state of pins on the microcontroller, and is responsible for all parameters of the signal: timing, levels, synchronization, etc...

 serial routines of the 1541 with a true serial shift register

Shift register

In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...

 implemented in hardware, thus dramatically increasing the drive speed. Although this originally had been planned when Commodore first switched from the parallel IEEE-488

IEEE-488

IEEE-488 is a short-range digital communications bus specification. It was created for use with automated test equipment in the late 1960s, and is still in use for that purpose. IEEE-488 was created as HP-IB , and is commonly called GPIB...

 interface to a custom serial interface, hardware bugs in the VIC-20's 6522 VIA

MOS Technology 6522

The 6522 Versatile Interface Adapter was an integrated circuit made by MOS Technology, as well as second sources including Rockwell and Synertek. It served as a I/O port controller for the 6502 family of microprocessors, providing the parallel I/O capabilities of the PIA as well as timers and a...

 shift register prevented it from working properly http://www.binarydinosaurs.co.uk/Museum/Commodore/c64/c64notes.php.

For compatibility with copy-protected software, the 1571 could closely emulate the 1541. This mode was the default when the drive was used in conjunction with a C64; while always being able to read and write the 1541's GCR format of 170 KB DD single-sided, in this mode it also would format disks single-sided and transfer data at 1541 speed. An undocumented command allowed the drive to format and use the second side of a disk, but only in single-sided mode.

The 1571 was noticeably quieter than its predecessor and tended to run cooler as well, even though, like the 1541, it had an internal power supply (later Commodore drives, like the 1541-II and the 3½" 1581

Commodore 1581

The Commodore 1581 is a 3½ inch double sided double density floppy disk drive made by Commodore Business Machines primarily for its C64 and C128 home/personal computers. The drive stores 800 kilobytes using an MFM encoding but format different from both MS-DOS , and the Amiga formats. With...

, came with external power supplies). The 1541-II/1581 power supply makes mention of a 1571-II, hinting that Commodore may have intended to release a version of the 1571 with an external power supply. However, no 1571-IIs are known to exist. The embedded OS in the 1571 was CBM DOS V3.0 1571

Commodore DOS

Commodore DOS, aka CBM DOS, was the disk operating system used with Commodore's 8-bit computers. Unlike most other DOS systems before or since—which are booted from disk into the main computer's own RAM at startup, and executed there—CBM DOS was executed internally in the drive: the DOS...

, an improvement over the 1541's V2.6.

Early 1571s had a bug in the ROM-based disk operating system that caused relative files to corrupt if they occupied both sides of the disk. A version 2 ROM was released, but though it cured the initial bug, it introduced some minor quirks of its own - particularly with the 1541 emulation. Curiously, it was also identified as V3.0.

Unlike the 1541, which was limited to GCR

Group Code Recording

In computer science, group code recording refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme...

 formatting, the 1571 could do both GCR and MFM

Modified Frequency Modulation

Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...

 disk formats. A C128 in CP/M mode equipped with a 1571 was capable of reading and writing floppy disks formatted for many CP/M

CP/M

CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...

 computers; specifically, the following formats:

• IBM PC
  IBM PC
  The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
  
   CP/M-86
  CP/M-86
  CP/M-86 was a version of the CP/M operating system that Digital Research made for the Intel 8086 and Intel 8088. The commands are those of CP/M-80. Executable files used the relocatable .CMD file format...
  
  
• Osborne 1
  Osborne 1
  The Osborne 1 was the first commercially successful portable microcomputer, released on April 3, 1981 by Osborne Computer Corporation. It weighed 10.7 kg , cost USD$ 1795, and ran the then-popular CP/M 2.2 operating system...
  
  
• Epson QX10
• Kaypro
  Kaypro
  Kaypro Corporation, commonly called Kaypro, was an American home/personal computer manufacturer of the 1980s. The company was founded by Non-Linear Systems to develop computers to compete with the then-popular Osborne 1 portable microcomputer...
  
   II, IV
• CBM CP/M FORMAT SS
• CBM CP/M FORMAT DS

Other MFM formats were possible if their characteristics were added to the CP/M C128-specific source code (available from Commodore) and the CP/M operating system were re-assembled. However, booting CP/M was only supported from disks in the standard Commodore GCR format; the MFM formats could only be used once the system was running.

With additional software, it was possible to read and write to MS-DOS

MS-DOS

MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

-formatted floppies as well. Numerous commercial and public-domain programs for this purpose became available, the best-known being SOGWAP

SOGWAP

SOGWAP Software, its acronymic name standing for Son Of God With All Power, was a company developing and producing Commodore home computer software from 1985 to 1996...

's "Big Blue Reader". Although the C128 could not run any DOS-based software, this capability allowed data files to be exchanged with PC users. Reading Atari 8-bit

Atari 8-bit family

The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips...

 130kB or 180kB disks was possible as well with special software, but the standard Atari 8-bit 90kB format, which used FM rather than MFM encoding, could not be handled by the 1571 hardware without modifying the drive circuitry as the control line that determines if FM or MFM encoding is used by the disc controller chip was permanently wired to ground (MFM mode) rather than being under software control.

As with the 1541, Commodore initially could not meet demand for the 1571, and that lack of availability and the drive's relatively high price (about US$300) presented an opportunity for cloners. Two 1571 clones appeared, one from Oceanic and one from Blue Chip, but legal action from Commodore quickly drove them from the market.

Commodore announced a dual-drive version of the 1571, to be called the 1572

Commodore 1572

The Commodore 1572 was a dual floppy disk drive designed by Commodore. It reached the prototype stage, and was introduced at the Consumer Electronics Show in 1985, but was never released to the public. One reason it was never released was that the code of the 1572 had compatibility...

, but quickly cancelled it, reportedly due to technical difficulties with the 1572 DOS.

The 1571 built into the European plastic-case C128 D computer is electronically identical to the stand-alone version, but 1571 version integrated into the later metal-case C128 D (often called C128 DCR, for D Cost Reduced) differs a lot from the stand-alone 1571. It includes a newer DOS, version 3.1, replaces the MOS Technology CIA

MOS Technology CIA

The 6526/8520 Complex Interface Adapter was an integrated circuit made by MOS Technology. It served as a I/O port controller for the 6502 family of microprocessors, providing for parallel and serial I/O capabilities as well as timers and a Time-of-Day clock...

 interface chip, of which only a few features were used by the 1571 DOS, with a very much simplified chip called 5710, and has some compatibility issues with the stand-alone drive. Because this internal 1571 does not have an unused 8-bit input/output port on any chip, unlike most other Commodore drives, it is not possible to install a parallel cable in this drive, such as that used by SpeedDOS, Dolphin DOS and some other fast third-party Commodore DOS replacements.

In the 1541 format, while 40 tracks are possible for a 5.25" DD drive like the 154x/157x, only 35 tracks are used. Commodore chose not to use the upper five tracks by default (or at least to use more than 35) due to the bad quality of some of the drive mechanisms, which did not always work reliably on those tracks. By reducing the number of tracks used (and thus the capacity), Commodore could further reduce cost - in contrast to the double-density drives used e.g. in IBM PCs of the day which saved 180 KB on one side (by using a 40-track format).

For compatibility and ease of implementation, the 1571's double-sided format of one logical disk side with 70 tracks was created by putting together the lower 35 physical tracks on each of the physical sides of the disk rather than using two times 40 tracks, even though there were no more quality problems with the mechanisms of the 1571 drives.

External links

• C64 Preservation Project Discusses internal drive mechanics and copy protection