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Magnetic Disk Storage System

1,575 bytes added, 23:05, 9 April 2019
Floppy disk storage
* Floppy Disk Drive ([[FDD]]) - the low level device for performing basic operations
* [[Floppy Disk]] - the storage medium itself
 
===Data recording===
====Data bits====
Data is recorded on the magnetic media bit by bit with magnetic flux changes. Every data bit preceded by a clock bit. A magnetic transition represents a 1, a "not transition" a 0.
 
There are several encoding schema:
* FM - Frequency Modulation - the clock bit always a 1 followed by the data bit (0 and 1 encoded as 10 and 11).
* MFM - Modified Frequency Modulation - a zero is encoded as 10 if preceded by a zero, and 00 if preceded by a one; a one is always encoded as 01
 
====Marked byte====
MF and MFM coding can mark data bytes with missing clock bit. The storage format uses this marked bytes to mark the beginning of a track, header (id) and data block.
 
====The low level format of a track====
A track contains several area:
* gaps between data blocks
* sync areas (in order to data separator circuit can "lock" the clock bits)
* marks (index, id, data, deleted data)
* data blocks
 
{|class="wikitable"
|-
! || Index hole || SYNC || IAM || GAP1 || SYNC || IDAM ||colspan="6"| ID || GAP2 || SYNC || DAM or DDAM ||colspan="3"| DATA || GAP3 || ~ || GAP4
|-
| '''FM''' ||style="text-align:center;"| O || 6x #00 || #FC* || 26x #FF || 6x #00 || #FE* || cyl||head||sec||no||crc||crc || 11x #FF || 6x #00 || #FB* or #F8* || Nx data byte || crc || crc|| 20x #FF || ... || 8-300x #FF
|-
| '''MFM''' ||style="text-align:center;"| O || 12x #00 || 3x #C2* #FC || 50x #4E || 12x #00 || 3x #A1* #FE || cyl||head||sec||no||crc||crc || 22x #4E || 12x #00 || 3x #A1* #FB or #F8 || Nx data byte || crc || crc || 40x #4E|| ... || 16-600x #4E
|}
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