IBM System 34 double density format - Revision history

Arnoldemu: /* Data separator syncing */

2018-08-12T19:36:38Z

‎Data separator syncing

Arnoldemu at 08:54, 12 August 2018

2018-08-12T08:54:19Z

Arnoldemu at 08:53, 12 August 2018

2018-08-12T08:53:01Z

Arnoldemu: /* Syncing */

2018-08-03T07:04:38Z

‎Syncing

Arnoldemu: /* Syncing */

2018-07-29T14:13:36Z

‎Syncing

Arnoldemu at 14:12, 29 July 2018

2018-07-29T14:12:10Z

Arnoldemu at 18:57, 28 July 2018

2018-07-28T18:57:30Z

Arnoldemu at 18:56, 28 July 2018

2018-07-28T18:56:08Z

Arnoldemu: Created page with "The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when writing in MFM mode. The data has this form: The track starts with thi..."

2018-07-28T18:40:53Z

Created page with "The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when writing in MFM mode. The data has this form: The track starts with thi..."

New page

The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when writing in MFM mode.

The data has this form:

The track starts with this:

Gap 0 (80 bytes of data 4E)
Sync (12 bytes of 0)
Index mark IDAM which contains 3 bytes of C2 (this has a missing clock) followed by 1 byte FC.
Gap 1 (50 bytes of data 4e)

@@ Line 49: / Line 49: @@
 The pre-amble is 12 bytes of 0 which are used to prime and synchronize the data separator with the data bits and clock bits.
 encodes as AAAA and has clock bits of '1' and data bits of '0' interleaved. The data separator expects a continuous stream of 0 bits to sync and maintains a count of them. If the separator sees a 1 data bit during this time then it will re-sync and reset it's internal count to 0. When a certain count is reached then the data separator is synced and it will output separate clocks and data to the FDC.
 ===FDC syncing and Address Mark ===

@@ Line 38: / Line 38: @@
 NOTE:
 * A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.
 ===CRC===

@@ Line 1: / Line 1: @@
-The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when writing in MFM mode.
+The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when reading and writing in MFM mode.
 The data has this form:
@@ Line 6: / Line 6: @@
   Gap 0 (80 bytes of data 4E)
-  Sync (12 bytes of 0)
+  Pre-amble (12 bytes of 0)
   Index mark IDAM which contains 3 bytes of C2 (which have a missing clock) followed by 1 byte FC.
   Gap 1 (50 bytes of data 4e)
@@ Line 12: / Line 12: @@
 Then follows the data for each sector:
-  Sync (12 bytes of 0)
+  Pre-amble (12 bytes of 0)
   ID mark IDAM which contains 3 bytes of A1 (which have a missing clock) followed by 1 byte FE.
 byte 'C' (defined by FDC format command)
@@ Line 21: / Line 21: @@
   Gap 2 (22 bytes of 4E)
-The CRC for the ID field is computed from the 3 bytes of A1, the 1 byte FE, C,H,R and N. When verifying include the CRC and if the result is 0, then the  CRC is correct.
+The CRC for the ID field is computed from the 3 bytes of A1, the ID mark and C,H,R and N. When verifying include the CRC bytes and if the result is 0 then the  CRC is correct.
 Then comes the data field for the sector:
-  Sync (12 bytes of 0)
+  Pre-amble (12 bytes of 0)
   Data mark IDAM which contains 3 bytes of A1 (which have a missing clock) followed by 1 byte FB for Data or F8 for Deleted Data.
   'n' bytes of sector data (defined by format when formatting, or by ID field when reading/writing)
 bytes CRC (stored big endian)
   Gap 3 ('n' bytes of data 4E) (defined by FDC format command)
 After the last sector:
@@ Line 35: / Line 37: @@
   Gap 4 ('n' bytes of 4E all the way to the end of the track).
-NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.
+NOTE:
 ===CRC===
-The CRC is initialised to FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written to the track high byte and then low byte.
+The CRC is initialised to FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written after the ID and data field high byte and then low byte.
-===Syncing & Address Mark ===
+The pre-amble is 12 bytes of 0 which are used to prime and synchronize the data separator with the data bits and clock bits.
-The sync is 12 bytes of 0 which are used to prime and synchronize the data separator with the data bits and clock bits.
+===FDC syncing and Address Mark ===
-encodes as AAAA.
-The address mark itself effectively syncs the FDC with the data marking where the expected data structures start.
+The address mark sync is for the FDC and marks where the data structure starts. The FDC expects this within a certain time after the data separator has synced.
-When this pattern is recognised then all data following is synced and will always read the same.
+When this special pattern is recognized 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 cell which encodes the clock and data bit has no clock. The equivalent A1 byte encoded to MFM without the missing clock is different, therefore it is unique.

@@ Line 41: / Line 41: @@
 The CRC is initialised to FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written to the track high byte and then low byte.
-===Syncing===
+===Syncing & Address Mark ===
-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.
+The sync is 12 bytes of 0 which are used to prime and synchronize the data separator with the data bits and clock bits.
 When this pattern is recognised then all data following is synced and will always read the same.
@@ Line 49: / Line 52: @@
 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.
+* To make the value special, it has a "missing clock". What this means is that cell which encodes the clock and data bit has no clock. The equivalent A1 byte encoded to MFM without the missing clock is different, therefore it is unique.

← Older revision		Revision as of 14:13, 29 July 2018
Line 36:		Line 36:

	NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.		NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.
		+
		+	===CRC===
		+
		+	The CRC is initialised to FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written to the track high byte and then low byte.

	===Syncing===		===Syncing===

← Older revision		Revision as of 14:12, 29 July 2018
Line 36:		Line 36:

	NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.		NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.
		+
		+	===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 19: / Line 19: @@
 byte 'N' (defined by FDC format command)
 bytes CRC (stored big endian, i.e. high 8 bits, then low 8 bits).
 The CRC for the ID field is computed from the 3 bytes of A1, the 1 byte FE, C,H,R and N. When verifying include the CRC and if the result is 0, then the  CRC is correct.
@@ Line 33: / Line 34: @@
   Gap 4 ('n' bytes of 4E all the way to the end of the track).