MFM - Revision history

Arnoldemu at 18:00, 13 January 2019

2019-01-13T18:00:18Z

Arnoldemu at 17:59, 13 January 2019

2019-01-13T17:59:44Z

Arnoldemu at 14:43, 12 August 2018

2018-08-12T14:43:13Z

Arnoldemu: /* Syncing */

2018-07-29T14:11:50Z

‎Syncing

Arnoldemu: /* Syncing */

2018-07-29T14:04:31Z

‎Syncing

Arnoldemu at 13:58, 29 July 2018

2018-07-29T13:58:08Z

Arnoldemu at 18:48, 28 July 2018

2018-07-28T18:48:32Z

Arnoldemu: Created page with "=MFM= MFM is an acronym for "Modified Frequency Modulation" and is a data encoding used on floppy discs. It is used by the NEC 765. MFM is stored as flux transitions on..."

2018-07-28T18:35:44Z

Created page with "=MFM= MFM is an acronym for "Modified Frequency Modulation" and is a data encoding used on floppy discs. It is used by the NEC 765. MFM is stored as flux transitions on..."

New page

=MFM=

MFM is an acronym for "Modified Frequency Modulation" and is a data encoding used on floppy discs. It is used by the [[NEC 765]].

MFM is stored as flux transitions on the disc. The flux transitions are converted by the read/write head into a 1 or 0.

MFM stores data as a stream of bits with clock bits and data bits interleaved. Clock bits are transmitted first and their value is the NOR of the previous data bit and the current data bit. Data is transmitted starting with bit 7 down to bit 0.

e.g.
<clock for bit 7> <bit 7> <clock for bit 6> <bit 6> <clock for bit 5> <bit 5>...

<clock for bit 5> = <bit 6> EOR <bit 5>

Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).

← Older revision		Revision as of 18:00, 13 January 2019
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−	[[Category:Hardware]]	+	[[Category:Hardware]][[Category:DATA Storage]]

← Older revision		Revision as of 17:59, 13 January 2019
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	Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).		Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).
		+
		+
		+	[[Category:Hardware]]

← Older revision		Revision as of 14:11, 29 July 2018
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	Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).		Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).
−
−	~~===Syncing===~~
−
−	~~In order to read the data correctly and to ensure the bits are in the expected order a specific bit pattern is used for syncing. This bit pattern is specific to the format used by the FDC.~~
−
−	~~When this pattern is recognised then all data following is synced and will always read the same.~~
−
−	~~This special value used by the NEC765 is 4489 and has the following properties.~~
−	* 4489 is an encoded version of the data byte A1 and has bit 7 set. It doesn't matter what the value of the previous byte is, the first clock of 4489 will always be 0. Therefore the pattern 4489 can't be influenced by data around it.
−	* To make the value special, it has a "missing clock". What this means is that one of the clocks is forced to 0. The equivalent A1 byte encoded to MFM without the missing clock is different, therefore it is unique.

@@ Line 16: / Line 16: @@
 ===Syncing===
-In order to read the data correctly and to ensure the bits are in the expected order a specific bit pattern is used for syncing.
+In order to read the data correctly and to ensure the bits are in the expected order a specific bit pattern is used for syncing. This bit pattern is specific to the format used by the FDC.
 When this pattern is recognised then all data following is synced and will always read the same.
-This special value is 4489 and has the following properties.
+This special value used by the NEC765 is 4489 and has the following properties.
 * 4489 is an encoded version of the data byte A1 and has bit 7 set. It doesn't matter what the value of the previous byte is, the first clock of 4489 will always be 0. Therefore the pattern 4489 can't be influenced by data around it.
 * To make the value special, it has a "missing clock". What this means is that one of the clocks is forced to 0. The equivalent A1 byte encoded to MFM without the missing clock is different, therefore it is unique.

@@ Line 14: / Line 14: @@
 Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).
-There is a specific bit pattern which is used for syncing. i.e. to ensure the data stream following it is always the same.
+===Syncing===
-This special value is 4489. This is the pattern for clock and data, with a "missing clock" (in reality a single clock bit is forced to 0) and assumes that the data bit proceeding this has a value of 0 so that the first clock bit (bit 15) is 0. The data byte value of 4489 is 0x0a1. It is unique because the corresponding value computed from 0x0a1 has all the correct clocks and bits will not match this value.
+In order to read the data correctly and to ensure the bits are in the expected order a specific bit pattern is used for syncing.

@@ Line 1: / Line 1: @@
 MFM is an acronym for "Modified Frequency Modulation" and is a data encoding used on floppy discs. It is used by the [[NEC 765]].
 MFM is stored as flux transitions on the disc. The flux transitions are converted by the read/write head into a 1 or 0.
-MFM stores data as a stream of bits with clock bits and data bits interleaved. Clock bits are transmitted first and their value is the NOR of the previous data bit and the current data bit. Data is transmitted starting with bit 7 down to bit 0.
+MFM stores data as a stream of cells which contain a data bit and a clock bit. The data separator synchronises to a continuous pattern of zeros, and when synced, will transmit data and clock bits to the FDC as separate signals.
 e.g.

← Older revision		Revision as of 18:48, 28 July 2018
Line 13:		Line 13:

	Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).		Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).
		+
		+	There is a specific bit pattern which is used for syncing. i.e. to ensure the data stream following it is always the same.
		+
		+	This special value is 4489. This is the pattern for clock and data, with a "missing clock" (in reality a single clock bit is forced to 0) and assumes that the data bit proceeding this has a value of 0 so that the first clock bit (bit 15) is 0. The data byte value of 4489 is 0x0a1. It is unique because the corresponding value computed from 0x0a1 has all the correct clocks and bits will not match this value.