VC15xx

An original 1541

The VC 15xx line is a series of floppy disk drives manufactured by Commodore for its 8-bit home computers (including VC 20 , C64 , C128 ). The original model 1540 is based on the CBM 4040 , a double drive that was reduced by one drive for use in home computers for cost reasons and provided with a serial variant of the parallel IEEE 488 bus ( CBM bus ). The specialty of these drives is that an independent computer (with CPU) works inside. That's why they were very expensive. The computers to which they were connected did not need DOS and could therefore be sold cheaper. The drives themselves contained an operating system, the Commodore DOS . Some computationally intensive programs used the computing capacity of the floppy disk drive in such a way that the processor contained there, which was just as fast as that of the C64, was involved in complex computing tasks. The results were then sent back to the computer via the serial interface and theoretically you had twice the computing capacity.

Models

• Series models:
  • 1540 : Designed for the VC 20 , one read / write head, 170 KB capacity, GCR recording method
  • 1541, 1541C, 1541-II : Designed for the C64 , can also be used on the C128 and Plus / 4 . The 1541 is a 1540 modified for use with the C64.
  • 1551 : Designed for the Plus / 4 , like the 1540/1541, but with faster parallel data transfer to the computer
  • 1570 : Designed for the C128 , a read / write head, faster data transfer, improved DOS. In addition to GCR, it also used the MFM process for data recording and could thus also be used for data exchange with CP / M and MS-DOS computers. The 1570 model was an emergency solution because the 1571 originally intended for the C128 was not finished in time. It was basically a stripped down 1571 in a 1541 case.
  • 1571 : Like the 1570 but with two read / write heads, so that the floppy disks could be written to on both sides without turning.
  • 1581 : The first 3½ "drive for the C64 and C128 , 800 KB capacity, MFM recording method

• Prototypes:
  • 1541D: 1541 with two drives in one housing.
  • 1542: Like a 1541, but intended for the Plus / 4 . Was presented at CES 1984, but never mass-produced.
  • 1543 / 1541H: 1541 for high density floppy disks.
  • 1561: Similar to the 1581 a 3½ "-inch drive, details are not known. Intended for the Commodore LCD .
  • 1563: Similar to the 1581, a 3½ "drive, details are not known. Intended for the C128.
  • 1565: A 3½ "drive for the C65 , never officially sold.
  • 1572: The 1571 with two drives in one housing.
  • 1582/1590/1591: High-density versions of the 1581 .
  • SFS 481: Was made later than 1551 .

Data transfer

By default, the drives (exception: VC1551) use the CBM bus , a bit-serial version of the IEEE-488 protocol. Originally, the data was to be transferred via a serial shift register of the input / output module VIA ( MOS Technologies 6522 ). Due to a design flaw in this chip, Commodore was forced to implement the transmission in software, which was accompanied by a greatly reduced speed. Only with the 1570/1571 was this problem partially resolved.

The drives with serial CBM bus can be connected to the parallel port of a PC with a simple cable. Building instructions can be found on the Internet. With an appropriate driver, for. B. old C64 disks can be read. Reading and writing of the 5¼ "floppy disks with a conventional PC drive is possible with alternative disk controllers (e.g. the Catweasel controller ). Another possibility for reading is the Disk2FDI tool, which, however, has special requirements for the PC hardware provides.

File system

See also : file system

A floppy disk was divided into tracks (tracks) and each track in turn was divided into sectors. Each sector formed a data block of 256 bytes, which was addressed by means of the track number (starting with 1, the zero had a special meaning) and the sector number (starting with 0). An occupied block looked like this: [TR-Next, SCT-Next, Data 1, ..., Data 254]. TR-Next indicated the track number of the subsequent data block in the file, SCT-Next indicated the sector number accordingly. If it was the last data block of a file, TR-Next was zero and SCT-Next contained the number of data bytes still valid. The remaining 254 bytes contained the actual data.
The data blocks free and occupied on the medium were noted in a bit map , the so-called B lock A viability M ap ( BAM ) (bit with value 0 for free and value 1 for occupied ).

A problem with this structure has been that data can only be processed sequentially; H. could be continuously read or written. To solve this problem there was a special file type, the REL files (REL for relative ), which had a so-called "side-sector" table (in correspondingly separate data blocks) in which the physical position of all data blocks was entered whereby the position of each data set (engl. record ) was immediately determined. Such a data record could be selected for a specific REL file with a size between 1 and 254 bytes.

Up to 144 files could be stored on a floppy disk. There were no subdirectories, only with the 1581 it was possible to create multiple partitions . The file names could be up to 16 characters long and contain special characters (even control characters). The only exceptions were the comma and the colon. A distinction was made between 4 file types: PRG for programs, SEQ for sequential files, USR (user files) and REL for files with relative (= random) access, although only the REL files differ in terms of storage (see previous paragraph). In 1581, CBM was added for the partitions.

Individual evidence

1. ↑ X1541 : Overview of adapter solutions in the C64 Wiki
2. ↑ Disk2FDI homepage . Oldskool.org. Retrieved July 11, 2010.