The SFD 100x line ( S uper F loppy D rive) is a range of disk drives ranging from Commodore was made for the CBM 8xxx series. The then very high storage capacity of just under one megabyte was achieved per diskette - on the cheap DD disks customary at the time. The HD floppy disks, which were still very expensive at the time, were not needed and do not even work in these drives because they require more powerful electromagnets on the read / write heads. The models are compatible with the CBM 8250 . The VC1541 was adopted as the housing . The floppy contains two CPUs of type 6502 , one of which is responsible for file management and bus control, the other manages the diskette controller. The drive contained DOS 2.5 or 2.7.
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. 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.
For more details on the built-in software and hardware, see Commodore DOS .