Commodore 1541

The VC 1541 is a 5.25-inch floppy disk drive for the Commodore 64 home computer from Commodore. She came onto the market in 1982 and was the most successful model of VC15xx series. The 1541 has only one read / write head and can therefore describe floppy on one side only. In order to utilize the full capacity of a floppy disk, this can be provided for example by means of a disk puncher with an additional read / write notch on the left side. On one of such modified disk further 165 KB can be saved by turning.

• 2.1 slowness and floppy speeder
• 2.2 Other Problems

• 5.1 Hardware
• 5.2 Plug-in module
• 5.3 Software

General

Like the other floppy drives the Commodore 8 -bit model series is the 1541 a stand-alone computer. As was the CPU used 6502, a close relative of the employed in the C64 6510th You could both be expected in parallel with tricky programming (although this possibility was used by very few programs).

Format

The data are stored on 35 tracks, each with 17 to 21 sectors of 256 bytes (Zone Bit Recording ). The first two bytes of each sector and track 18 is responsible for managing reserved ( contents, occupancy "Block Availability Map" ) so that 664 sectors or 168 656 bytes can be used for user data.

Firmware

The operating system of the drive, CBM DOS v2.6, a stripped down version of the previous drives the CBM series, contains a number of errors. The most famous is the so-called " SAVE @ " bug. The " SAVE @ " is used to overwrite files with new content while keeping the file name. War on the disk before saving less free space than was necessary for the storage of the new version of the file, there was a destruction of disk contents. Since this error was soon known that, most Commodore programs instead the "Scratch " command to delete files and then wrote it again with the normal SAVE command.

The operating system takes over in 1541 by hardware technical control tasks because the hardware itself should be kept as simple as possible. The drive uses a GCR - encoding in which the data are written to the disk by the first bytes are recoded by the firmware writing a 10 -bit values ​​, and the reading corresponding to the reverse. The beginning (sync ) of a recorded sequence of bytes ( data block header or data block content), however, unlike, say, the Apple II, recognized the hardware.

The different length of the tracks depending on the distance of the reading head from the disk center, is used by the software. To the further outwardly disposed more data tracks are located by the bit rate is adjusted in four stages (which are generated at bit rates in the hardware, by software, however, between them is switched ). This means that the surface is radially divided into four zones that have different numbers of sectors per track.

Technical weaknesses

Slowness and floppy speeder

Proverbial slowness of the 1541 was ( therefore among users snails called Carousel ). Already reading the data from the disk to the internal memory of the drive was not optimally designed. Above all, the slowness was due to the complex programming of the data transmission between the drive and computer through the serial CBM bus. This was originally developed for matching the VC 20 VC1540, since the time used in the VIC-20 interface chip MOS 6522 VIA contained an error in the automatic serial transmission ( at irregular intervals were 9 instead of 8 bit transfer ). Therefore, the transfer was organized so that each bit to be explicitly transferred from the processor must - an extremely slow process. In the small memory of the VC -20 that did not so much like later on the C64. The interface chip of the C64, MOS 6526 CIA, this error did not include it, but for the purpose of backward compatibility with the VC1540 drives ( which also contained the erroneous 6522 ) it was decided not to develop a completely new drive for the C64.

In addition, the transmission rate in 1541 was artificially reduced in comparison to 1540 again to bypass caused by the VIC-II chip timing problems of the C64. Unlike the VIC I of the VC -20, the VIC II stops from time to time the processor for up to 40 microseconds to read graphics data from memory. Thus was necessary to ensure that the drive is every bit increased significantly longer than 40 microseconds on the bus so it could not be lost. This was accomplished by a slight modification of the firmware in the hardware structure, the early 1540 and 1541 are otherwise identical drives.

With the help of so-called floppy speeder, tricky programmed transfer programs (eg Hypra Load), which dispense with the complicated protocol on the serial line, or transmitted alternatively on a separate parallel bus, the transmission rate of 300 bytes / s to sometimes over 10 kB / s can be increased. The floppy speeder software has been partially integrated together with still other features ( function key assignment, etc.) into the operating system, resulting in a change of ROM block necessitated in C64 and in 1541. There were floppy speeder as plug-in modules for the computer ( the drive- side code was transmitted in this case at each loading into RAM of the drive) and software-only version, which were soon integrated into most commercial programs for the C64.

A parallel data transfer is realized by use of the user port on the C64 and a free 8 -bit ports on a 6522 port block in 1541. The whole completed conversion kits, such as the well-known SPEEDDOS: Two eproms with updated operating system for the C64 and the drive 1541 and a parallel data cable from the user port of the C64 to the floppy. The command exchange between the computer and floppy drive via the serial bus has been retained, in addition to the parallel cable was laid a " HiSpeed ​​bus". Since this was associated with extra costs for additional hardware, this realization was not nearly as common as the purely software-based approach.

In the course of these modifications to speed declined in the face of fallen RAM prices later even a board to be installed in the 1541, offers a complete floppy track ( and in a still later version an entire disk's contents ) in their own RAM ( ie as a cache) saved and then with could replace high speed with the computer.

Other problems

Other negative qualities are caused by the internal power supply overheating problems as well as in the first generation the characteristic chattering occurs at track 0 when you hit the read / write head. A track -0- sensor Commodore had saved, so it is formatting or write errors of the head carriage - regardless of its location - just down 40 tracks to the outside, a mechanical stop prevents it can move beyond track 0. This rattle is not only very uncomfortable, but by the mechanical stress also the main reason for frequent misalignment of the drive.

Floppy copies

Creating a diskette copy requires additional software. Although the 1541 DOS of the dual-drive devices and hence the local copy command contains almost unchanged, for lack of a second drive but this is not functional. A copy program which is executable on all Commodore home computers is included, along with some other tools and programming examples on the test / demo diskette that is included with each 1541. However, it is very slow because it uses only the regular DOS commands for reading and writing of data blocks is for compatibility with all Commodore home computers. It was therefore relatively quickly by other manufacturers and in computer magazines additional software - mostly for the Commodore 64 - offered, which used its own transfer routines for the serial bus as fast charge programs and therefore a factor of about 10 were faster. This allowed some of these copy programs, such as the Turbonibbler the individual tracks of raw disk, ie without interpretation of the sector boundaries transferred to the computer and restore it on the destination disk, and so nullify most of the common copy protection mechanisms.

At the end of the C64, some copy programs have been published, which vied with each other in carrying out floppy copies very quickly. Of particular note is the 1989 published in the 64'er magazine program " Master Copy 64 " by Frank Riemenschneider, to this day as the only one that can make a diskette copy on a VC1541 without any hardware upgrade in under a minute. Later there was a follow-up version " Master Copy Parallel", which dealt with by parallel data transmission via the user port this task in less than 30 s. It should be remembered, however, that in addition, the source and target disk had to be replaced several times because the C64 has only 64KB of RAM and thus at least three changes are required per disk side.

Versions

Types of quick chargers

Hardware

The hardware quick charger are supplied on circuit boards or chips and must be installed in the home computer and the floppy drive.

The rate factors are related to LOAD with C64 and 1541st with other hardware (HDD instead of floppy ... ) may differ greatly values ​​.

Plug-in module

Some quick charger also appeared as a plug-in module for the expansion port.

Software

A software quick charger is a short program and is first loaded and started, followed by the actual program that should usefully be significantly larger than the quick charger. Such software quick charger were installed in most commercial programs for the Commodore 64, usually closely intertwined with the copy of the disk.