CBM floppy drives
The Commodore computers of the PET 2001 to CBM 8000 series, with the exception of the CBM 8296D , had no built-in floppy disk drives . All drives had to be connected externally via the IEC bus . The floppy disk drives of home computers ( VC-20 , C64 etc.), such as the VC1541 , were derived from these drives. The differences are in the interface (serial CBM bus ) and a cost-saving design.
hardware
With regard to the basic properties of the hardware structure, the information here also applies to the CBM 9060 and CBM 9090 hard disks .
In typical computer systems of that time, and in principle until today, floppy disk drives were connected almost directly via the processor bus. The data was encoded either by a simple controller chip or by the operating system software , and the file system was also managed by the computer's operating system.
With the computers of the Commodore 8-bit series, a different approach was chosen. In order to minimize the costs for the computer, which was initially often used without a floppy disk drive, and to keep the connection of drives as flexible as possible, the computer only had very simple and general, character-oriented (not block-oriented) software - and hardware - Interfaces to external devices, while the complete logic for encoding / decoding sectors as well as the file system and DOS command logic was implemented in the floppy disk drive.
Commodore 8-bit floppy disk drive units ("Units") were therefore independent computers with one or two processors that were independent of the actual computer. A unit had one or two drives (“drives”). Several units could be connected to one computer. The drives were connected via the parallel IEC bus provided for this purpose . After the start of the first PET, Commodore still needed some time before it could offer the appropriate floppy disk drives.
software
The operating system that was used in these drives was Commodore DOS . In contrast to the DOSes of most other companies at the time, it was not stored on a floppy disk, but in ROM chips ( firmware ) inside the drive.
The DOS was not yet aware of any subdirectories, which was not a major limitation given the limited capacity. Compared to the competing CP / M and IBM PC worlds, file names with 16 characters were allowed to be considerably longer and were not restricted to the 8 + 3 pattern.
In addition to the table of contents, there were other organizational structures in the normal, "sequential" files (and also with the program files) at the beginning of each data sector (usually called a block ) with a total of 256 bytes, two bytes with a reference to the track and sector number of the following block. So if a floppy disk z. B. holds 644 data blocks and each of these blocks contains 2 pointer bytes and 254 usable data bytes, that results in a net capacity of 170 KB. This structure was simpler than the CP / M table of contents or MS-DOS FAT, but did not allow parts of a file to be read without first reading all of the preceding parts (sequential access). If such direct access to any point in a file was required - for example in database applications - a special file type called relative file with separately stored pointers ("pointers") to the data blocks of the file was used.
In order to speed up the average data access, the directory track was placed in the middle of the floppy disk so that from there only a maximum of half the floppy disk radius had to be traveled to the beginning of the file.
The commands from the computer to the floppy disk drive were practically written in plain text abbreviations that were sent over the IEC bus ( parallel or serial ).
Structure of the drives
With the exception of the rather rare 8060/61/62 and 8280 devices that used 8-inch drives, all devices on offer were based on 5.25-inch drives.
In order to utilize the data capacity as efficiently as possible, two measures were taken: On the one hand, the GCR writing method (instead of MFM ) was used, which means that 5 gross bits have to be written for 4 net bits, whereas it would have been 8 with MFM. On the other hand, more data was packed onto the outer, longer tracks by increasing the writing speed (the writing clock) there. This allowed the speed of rotation to be kept constant, which did not complicate the mechanical construction. To keep the write clock switching simple, it was switched between four different speed levels for certain track areas.
These measures could on 5¼-inch floppy easier Density ( "single density", SD) that have been used on one side, 170 KB are stored on the later drives with double-operation of higher-density (but still with SD floppy material) even up to 1MB.
The original models for the Commodore PET series had two symmetrical CPUs (also from the 6502 family), 2 KB RAM and 16 KB ROM. CPU 1 implemented the logic to read and write sectors, while CPU 2 took care of the file system logic as well as communicating with the computer. The communication between the two processors took place via the main memory: CPU 2 wrote command tuples (action, drive, track, sector, memory address) in a defined memory area, which CPU 1 read out, executed and overwritten the command tuple with the result code. The two CPUs could use the same memory at the same time by making use of a special feature of the 6502 processors - they only access the system bus in one of the two halves of each clock cycle . The CPUs were now interconnected in such a way that CPU 1 used the bus in the first half of the clock and CPU 2 in the second half of the clock, just like in the Commodore computers themselves the processor and the video chip shared the bus.
Later devices only had one drive, and the second CPU was saved for reasons of cost. The individual CPU took on both tasks, but the original conception of the system remained unchanged: The tasks of CPU 2 (file system) ran as the main program, while the tasks of CPU 1 (hardware control) were performed in an interrupt routine. The CPU continues to communicate with itself via command tuples.
Model development
The first models were all 5.25-inch double floppy disk drives with a parallel IEC bus interface. The CBM 2020 should match the PET 2001 . It was a typical first run with various program errors. It is uncertain whether it was ever sold in Germany. It was superseded by the CBM 2040 , with revised ROMs, DOS 1 and 670 blocks for user files. A format similar to VC1541 was used, the floppy disks it describes are readable on the later models CBM 3040, CBM 4040, CBM 2031, CBM 4031, VC 1540, VC 1541, VC 1551, VC 1570 and VC 1571. The first model The CBM 3040 , still based on the CBM 2040, was produced in significant numbers to match the CBM-3xxx series .
For the CBM 4000 series came the CBM 4040 , with revised DOS 2.5 / 2.7, 664 blocks for user files, "relative files" (which allow both sequential and random access), read and write compatible with CBM 4031, CBM 2031 , VC1541, VC1551, VC1570 and VC1571, capacity 2 × 170 kB.
The track density in all of these models mentioned so far corresponded to the industry standard of 48 tpi, so that SD disks could be used as disk material .
For the CBM-8000 series , they wanted to offer more capacity and developed the CBM 8050 - writing on one side, four times the write density, 100 TPI (more than twice the previous models), capacity thus 2 × 500 kB. It was not backwards compatible, so it could not read the disks of the previous models. A little later it was surpassed by the CBM 8250 - writing on both sides, four times the write density, i.e. capacity 2 × 1 MB. This drive was then mainly available as the CBM 8250LP - like the CBM 8250, but in "Low Profile" - a new, flatter design, suitable for the CBM-Pet-II series. Despite their increased write density, all of these drives were still designed for SD disks (only now in double-sided operation, Double Sided, DS , i.e. DS SD ) and literally failed with HD disk material because it would have required higher write currents. This has been a source of numerous misunderstandings and errors among users. The track density of 100 TPI was the reason why no competing computer could even read such disks physically, because their drives worked with a standard track density of 96 TPI. This parameter is determined by the mechanical structure of the drive and can therefore not be adapted by skillful driver programming.
In addition to these standard versions with double drives, parallel models were also offered as 5¼-inch single disk drives with an IEC bus interface, such as the CBM 4031 (CBM 4040 in 1 × 170 kB), CBM 2031 (CBM 4031 in "low profile" - in new design like VC1541 , like this one only with a microprocessor 6502) and the SFD 1001 (CBM 8250LP in 1 × 1 MB, see also SFD100x ).
To exchange data with IBM large and mini computers, an 8-inch double floppy disk device with an IEC bus interface was also released, the CBM 8280 - with 2 × 1 MB or 512 kB. But only a very small number existed.
Web links
Remarks
- ↑ a b c In contrast to hard disks , the capacities of floppy disks are noted in powers of two. The unit "KB" stands for the unit of 2 10 = 1024 bytes, also known as Kibibyte .
