Unlikely most contemporary (or trendy) disk system, the file system software was burned in ROM and thus couldn’t ever be enhanced. CP/M did not have this and thus it was only a software convention to one way or the other sign the last meaningful byte of the file, eg -Z in a textual content file. If the software program sees about 5 byte occasions of mark (1), it assumes it is an intersector hole and begins looking for a begin bit. Each sector begins with about 10 byte occasions (one hundred bits, or 130 us) of gap — nothing however mark (‘1’) state and no transitions. The disk has 41 tracks, however monitor 0 contains nothing and is not used by the system. Physical observe 1 is the first logical monitor. When reading a sector, the software program discards the first few few bytes is would possibly learn till it sees the 0x5A sync byte. Which means when reading from or writing to the floppy, the serial port is disconnected and any transfers during that time are lost. If the disk speed is off by a few %, especially if the system which wrote the sector was completely different than the system studying it, bits could possibly be dropped or inserted. Most drives have a PLL which permits the info stream to be recovered even with disk speed variations, as there is guaranteed to be a transition at the very least every bit (FM) or every few bits (GCR).
Because it used the raw 5501 serial knowledge stream, in the worst case there might be eight ‘0’ information bits following the ‘0’ begin bit before the final ‘1’ stop bit. To save money, Compucolor bought the uncooked drive mechanism from Wangco (later purchased by Siemens) and used their own drive electronics. The file model byte is incremented on every save. FCS views the disk as 400 sectors, sector zero by means of sector 399. Sector 0 begins with a small descriptor indicating what number of sectors are to be used for the disk listing, and in addition contains a 10 byte volume label. Because there are forty tracks in use and every monitor has 10 sectors per observe, there are four hundred sectors complete. Note there isn’t a sensor for reporting track 0, and no write protect notch sensor. However, by placing all the FCS in ROM, there was no have to waste treasured disk house on every floppy to hold it. The unit didn’t shield the drive at all, placing the r/w head and associated analog amplifier and sign conditioning circuitry inches away from a high voltage CRT and in addition the switching power supply. To address the primary drawback, the software program blindly steps the r/w head out forty two times when it needs to residence the head to trace 0. The source code for FCS literally calls this the POUND routine.
Once other overheads are factored in, the disk format saved 10 sectors of 128 bytes per sector, or 1280 bytes per monitor. The reply is to have a big hole between sectors with none transitions at all. Once a hole is discovered, the ID block is learn. If there’s a learn error, the only recovery mechanism is to retry the read. There is no refined error restoration scheme. Because it takes various cycles to determine that the best sector was discovered and to switch the state of the write line, there is some ambiguity of when the minimize-over will occur. Once the correct monitor and sector are discovered, the software program switches on the write allow and then writes the data block. The 3-section floppies had one step per track, however the 4-section floppies had half the step measurement, requiring two steps per track. The 5501 has a easy RS-232 fashion serial interface: one begin bit, eight knowledge bits, one cease bit, costing 10 bits per byte. With none simple sync token, how can the system inform the place a sector begins? It means the data stream might need lengthy-term DC content material, but the recording medium is incapable of monitoring it; it also signifies that there are no unlawful bit encodings, such that there are not any straightforward to differentiate encodings to make use of to sync to; it additionally implies that the learn logic cannot use AGC (automated achieve control) and should have an absolute threshold for distinguishing a one bit from a zero bit.
No modulation scheme is employed in recording to the disk (such as FM, MFM, or GCR), which is problematic. Most different system have a tiny ROM routine to read a boot sector from the disk, which then hundreds in a 2nd degree disk working system, permitting for bug fixes and enhancements after the actual fact. The floppy disk system is just barely extra subtle than that utilized by the 8-monitor drive. FCS (the File Control System) routines treat the disk as 400 sectors, quite than a observe/sector addressing scheme. The Compucolor II was ahead of the microcomputer curve in including a floppy disk as a normal function. There was another aspect which made the inner floppy drive on the Compucolor II troublesome. This same logic is used to serialize/deserialize knowledge to/from the floppy disk. As a way to see a coherent character, the identical character and attribute bytes needed to be saved in corresponding positions of a good/odd row pair. Being a single sided drive with forty used tracks, the full capability of the drive was 51,200 bytes. To solve this, the formatter writes three dummy 0xFF bytes between the ID block and the data block. If a couple of sector is allotted for the directory (the default is three sectors), every subsequent block contains six file directory entries.
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