Changes
/* Datasheets */
µPD765 - Floppy Disc Controller (used in CPC 664, CPC 6128, 6128 Plus and [[DDI-1]] and CPC 664/6128expansion).
Note2: The following data seperators [[Vortex Disc Drives|Vortex disc interface]] uses other ports. See its dedicated wiki page. <br> == Motor On/Off Flip-Flop ==Writing 00h to Port &FA7E turns all disk drive motors off, writing 01h turns all motors on. It is not possible to turn on/off the motor of a specific drive separately. An exception are the Vortex F1-S, F1-D, M1-S and M1-D drives. (How are they different?) Another exception is the Gotek drives. They don't take the motor flip-flop into account and are always on. [https://64nops.wordpress.com/2021/07/04/a-la-decouverte-du-fdc/ Source] Some FDC commands don't require the motor to be on. For example, the seek or recalibrate commands, that move the floppy drive head, work fine with the motor off. And seek/recalibrate also work with an empty drive (ie. without a floppy disk inserted). [https://64nops.wordpress.com/2021/07/10/a-la-decouverte-du-fdc-episode-2/ Source] The floppy disk rotates at a nominal speed of 300rpm, with some tolerance. This tolerance of the FDC has been measured by [[Roudoudou]] to be ±12% (it worked from 220 kbits/s to 283 kbits/s for a reference of 250 kbits/s). [https://64nops.wordpress.com/2021/09/02/a-la-decouverte-du-fdc-episode-4/ Source] The FDC speed is more impressive when you realize the Amstrad CPC’s tape loads at only 2 kbits/s in fast mode. Note: tape loading can be way faster than that if an MP3 player is usedinstead of a real physical tape, as it has been demonstrated here: https://youtu.be/MAIsOIwgJWA
== Accessing the FDC 765 ==
{| class="wikitable"|+ Read ID (0Ah)|-! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0|-| command byte 0 || x || MF || x || 0 || 1 || 0 || 1 || 0|-| command byte 1 || colspan= FDC Status Registers "5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US |-| Execution || colspan="8" | The first correct ID information on the cylinder is stored in data register|-| result byte 0 || colspan="8" | ST0: status register 0|-| result byte 1 || colspan="8" | ST1: status register 1|-| result byte 2 || colspan="8" | ST2: status register 2|-| result byte 3 || colspan="8" | C: cylinder number|-| result byte 4 || colspan="8" | H: head number|-| result byte 5 || colspan="8" | R: sector number|-| result byte 6 || colspan="8" | N: bytes per sector|}
{| class="wikitable"|+ Sense Drive Status Register (04h)|-! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0|-| command byte 0 || colspan="3" style="text-align: center;" | x || 0 || 0 || 1|| 0 || 0|-| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US |-| result byte 0 || colspan="8" | ST3: status register 3|}
=== FDC Status Registers === The Main Status register can always be read through Port &FB7E. The other 4 Status Registers cannot be read directly, instead they are returned through the data register as result bytes in response to specific commands. Main Status Register (Port &FB7E): b0..3 DB FDD0..3 Busy (seek/recalib active, until succesful sense intstat) b4 CB FDC Busy (still in command-, execution- or result-phase) b5 EXM Execution Mode (still in execution-phase, non_DMA_only) b6 DIO Data Input/Output (0=CPU->FDC, 1=FDC->CPU) (see b7) b7 RQM Request For Master (1=ready for next byte) (see b6 for direction) Status Register 0: b0,1 US Unit Select (driveno during interrupt) b2 HD Head Address (head during interrupt) b3 NR Not Ready (drive not ready or non-existing 2nd head selected) b4 EC Equipment Check (drive failure or recalibrate failed (retry)) b5 SE Seek End (Set if seek-command completed) b6,7 IC Interrupt Code (0=OK, 1=aborted:readfail/OK if EN, 2=unknown cmd or senseint with no int occured, 3=aborted:disc removed etc.) Status Register 1: b0 MA Missing Address Mark (Sector_ID or DAM not found) b1 NW Not Writeable (tried to write/format disc with wprot_tab=on) b2 ND No Data (Sector_ID not found, CRC fail in ID_field) b3,6 0 Not used b4 OR Over Run (CPU too slow in execution-phase (ca. 26us/Byte)) b5 DE Data Error (CRC-fail in ID- or Data-Field) b7 EN End of Track (set past most read/write commands) (see IC) Status Register 2: b0 MD Missing Address Mark in Data Field (DAM not found) b1 BC Bad Cylinder (read/programmed track-ID different and read-ID = FF) b2 SN Scan Not Satisfied (no fitting sector found) b3 SH Scan Equal Hit (equal) b4 WC Wrong Cylinder (read/programmed track-ID different) (see b1) b5 DD Data Error in Data Field (CRC-fail in data-field) b6 CM Control Mark (read/scan command found sector with deleted DAM) b7 0 Not Used Status Register 3: b0,1 US Unit Select (pin 28,29 of FDC) b2 HD Head Address (pin 27 of FDC) b3 TS Two Side (0=yes, 1=no (!)) b4 T0 Track 0 (on track 0 we are) b5 RY Ready (drive ready signal) b6 WP Write Protected (write protected) b7 FT Fault (if supported: 1=Drive failure) <br> === C, H, R, N values at result phase === If the processor terminates a read (or write) operation in the FDC, then the ID information in the Result phase is dependent upon the state of the MT bit and EOT byte: {| class="wikitable"|-! rowspan="2" | MT !! rowspan="2" | HD !! rowspan="2" | Final Sector Transferred to Processor !! colspan="4" style="text-align: center;" | ID Information at Result Phase|-! C !! H !! R !! N|-| 0 || 0 || style="text-align: center;" | Less than EOT || - || - || R + 1 || -|-| 0 || 0 || style="text-align: center;" | Equal to EOT || C + 1 || - || 0 || -|-| 0 || 1 || style="text-align: center;" | Less than EOT || - || - || R + 1 || -|-| 0 || 1 || style="text-align: center;" | Equal to EOT || C + 1 || - || 0 || -|-| 1 || 0 || style="text-align: center;" | Less than EOT || - || - || R + 1 || -|-| 1 || 0 || style="text-align: center;" | Equal to EOT || - || LSB || 0 || -|-| 1 || 1 || style="text-align: center;" | Less than EOT || - || - || R + 1 || -|-| 1 || 1 || style="text-align: center;" | Equal to EOT || C + 1 || LSB || 0 || -|} * An empty cell means it is the same value as the one at the beginning of command execution* LSB (Least Significant Bit): The least significant bit of H is complemented <br> === Notes ===
Before accessing a disk you should issue a ''recalibrate'' command to the drive to move the head backwards until the ''track zero'' signal from the drive is sensed by the FDC. The FDC will also set its track counter for that drive to zero.
Despite the name, a sector with a ''Deleted data Address Mark'' (DAM) is not deleted; the DAM-flag is just another ID bit. 'Deleted' sectors can be read/written just like normal data sectors and if that ID bit is specified correctly in the command.
<br>
== Standard floppy disk formats ==
[[AMSDOS]] has been designed to complement CP/M, not to compete with it. They share the same structure and can read and write each other's files.
There are 3 standard floppy disk formats in AMSDOS. They are all single-sided and double density (MFM), with the following characteristics:
{| class="wikitable" style="text-align: center;"
! !! DATA !! VENDOR or SYSTEM !! IBM
|-
| Sector size || colspan="3" | N=2 (512 bytes)
|-
| Number of sectors || colspan="2" | 9 || 8
|-
| Sector names || &C1 to &C9 || &41 to &49 || &01 to &08
|-
| Sector interleave || colspan="2" | Yes || No
|-
| Formatting byte || colspan="3" | &E5
|-
| Starting track || colspan="3" | 00
|-
| Ending track || colspan="3" | 39
|-
| Catalog position || Track 0: sectors &C1 to &C4 || Track 2: sectors &41 to &44 || Track 1: sectors &01 to &04
|-
| Catalog size || colspan="3" | 2 KB
|-
| Catalog entries || colspan="3" | 64 max
|-
| Size per side || 178 KB || 169 KB || 154 KB
|-
| Diskette size || 356 KB || 338 KB || 308 KB
|}
<br>
=== DATA format ===
It is the most commonly used format on Amstrad CPC as it offers the most available space among the standard formats.
<br>
=== SYSTEM format ===
The SYSTEM format is a VENDOR format with the addition of information to boot the CP/M on:
* Track 0: sectors &41 (boot sector), &42 (configuration sector), &48, &49
* Track 1: sectors &41, &46, &42, &47, &43, &48, &44, &49, &45
Type |CPM and you will find yourself under CP/M 2.2.
By the way, all the |CPM command does is load the 512 bytes of track 0 sector &41 into memory at address &100 and then execute it.
<br>
=== IBM format ===
The IBM format is only usable on early IBM PCs as it uses 8 sectors per track and the CP/M filesystem. This was the format used in MS-DOS 1.x. [https://www.os2museum.com/wp/dos/dos-1-0-and-1-1/ Source]
With MS-DOS 2.0 (1983), PC floppies switched to 9 sectors per track and the FAT12 filesystem. [https://minuszerodegrees.net/5150/early/5150_early.htm Source]
And MS-DOS 4.0 (1988) removed compatibility with MS-DOS 1.x formatted floppies. This is one of the reasons why MS-DOS 4.0 faced significant criticism.
Also, [[CP/M|CP/M]] 2.2 supports the IBM format but CP/M Plus does not.
<br>
=== Catalog structure ===
In CP/M (and AMSDOS), files are stored on disks in a flat structure and can only be organized using USER numbers. CP/M does not have a hierarchical directory structure (with folders and subfolders) like newer operating systems.
The catalog structure supports up to 64 entries of 32 bytes. A file larger than 16KB will require multiple entries in the catalog.
{| class="wikitable"
|-
! Byte offset !! Description !! Comment
|-
| 0 || User Number || USER goes from 0 to 255. USER 229 (&E5) is for deleted files. RSX only gives access to USER 0 to 15.
|-
| 1-8 || Filename (excluding full stop) || Always in CAPS and maximum 8 characters long.
|-
| 9-11 || Extension || Always in CAPS and maximum 3 characters long. Read Only flag on byte9 bit7. Hidden flag on byte10 bit7. Archive flag on byte11 bit7.
|-
| 12 || Current Extent || 0 is first extent. There can be up to 128K bytes (8 logical extents) directly addressed by a single directory entry. [http://www.cpm.z80.de/manuals/cpm22-m.pdf Source]
|-
| 13 || Reserved ||
|-
| 14 || Extent High Byte || Not used.
|-
| 15 || Record Count (Number of 128 Byte Blocks) || Goes up to 128 (&80).
|-
| 16–31 || Block IDs where to find the file data || 1 byte is used per 1KB block.
|}
<br>
==== CAT'art ====
The catalog structure can be hacked by using ascii control characters (codes 0 to 31) to create decorated catalogs (CAT'art).
CAT never loads and executes a boot sector. It just displays the chars of the files using "TXT OUTPUT" (#bb5a) which will execute control codes as well.
There were already tools from the 80ies for modifying a directory in a way that will display colourful chars placed at any location on the screen by adding dummy file entries with 0KB sizes. Or you hardcode it directly by yourself, which allows much more chars and effects.
The trick is to disable text output at the end of a filename to prevent printing file sizes/line feeds and enable it again at the beginning of the next filename; and you also have to use one hidden char for sorting them in the correct order.
See here for more details: [http://cpc.sylvestre.org/technique/technique_catart1.html All about CAT'arts creation (FR)]
<br>
==== CP/M terminology ====
'''Record''': The basic unit of data access in CP/M, 128 bytes in size. This was the standard size for transferring data between memory and disk.
'''Block''': A group of records, 1 KB in size. Blocks were used to improve disk access efficiency by reading/writing multiple records at once.
'''Extent''': A group of blocks, 16 KB in size. CP/M used extents to manage file allocation, with each extent corresponding to a portion of a file on disk.
'''Sector''': The smallest physically addressable unit on a disk, 512 bytes in size on Amstrad CPC. CP/M accessed the disk at the sector level, though users typically worked with records.
<br>
==== File Headers ====
Cassette files are divided into 2KB blocks (then subdivided in 256-byte segments + CRC), each block being preceded by a 64-byte header.
AMSDOS files are usually stored with a 128-byte [[AMSDOS Header]] (based on cassette header) but can also be headerless, depending on the contents of the file. Unprotected ASCII files do not have header.
CP/M files do not have AMSDOS headers.
<br>
=== Random File Access ===
One of the major computing flaws of the Amstrad CPC machines has been the inability to handle simple Random Access Filing even when equipped with disc drives. The only way random access data handling was possible was if the program was written entirely in CP/M. [https://cpcrulez.fr/applications_bureau-random_accesss_database.htm Source]
Nowadays, files can be random accessed in SymbOS, so you can always set the current file pointer to any position in a file and load any amount of data in one step from this position (in CP/M this is at least possible in 128byte steps, in Unidos also byte-based). [https://www.cpcwiki.eu/forum/games/micro-machine-for-symbos-g9k/msg248806/#msg248806 Source]
<br>
== Custom floppy disk formats ==
Usually single sided 40-track 3" disk drives are used in CPCs. For practical purposes, 42 tracks could be used — the limit is specific to the drive and some support more tracks but 42 is a good maximum.
RSX programs exist on top of AMSDOS to get up to 208 KB of usable space per side, by using 42 tracks with 10 sectors per track.
The problem with that is there is not much gap between sectors anymore, and so the diskette becomes very sensitive to rotation speed.
The FDC controller can be used to control 80-tracks and/or double sided drives, though AMSDOS doesn't support such formats. To use 80-track drives and/or dual-head drives, you need to use another CPC DOS instead of AMSDOS. The most popular one is [[ParaDOS]]. It can handle up to 796 KB instead of 178 KB for AMSDOS and it supports 22 different disk file formats, including the three standard AMSDOS ones.
{| class="wikitable"
|+ Supported Formats in ParaDOS
! Format !! Capacity !! Catalog !! Type !! Sectors/Track !! Sectors
|-
| PARADOS 80 || 396k || 128 || SS || 10 || &91-&9a
|-
| PARADOS 41 || 203k || 64 || SS || 10 || &81-&8a
|-
| PARADOS 40D || 396k || 128 || DS || 10 || &a1-&aa
|-
| ROMDOS D1 || 716k || 128 || DS || 9 || &01-&09
|-
| ROMDOS D2 || 712k || 256 || DS || 9 || &21-&29
|-
| ROMDOS D10 || 796k || 128 || DS || 10 || &11-&1a
|-
| ROMDOS D20 || 792k || 256 || DS || 10 || &31-&3a
|-
| ROMDOS D40 || 396k || 128 || SS || 10 || &51-&5a
|-
| S-DOS || 396k || 128 || SS || 10 || &71-&7a
|-
| DATA (SS 40) || 178k || 64 || SS || 9 || &c1-&c9
|-
| DATA (DS 40) || 356k || 64 || DS || 9 || &c1-&c9 E
|-
| DATA (SS 80) || 356k || 64 || SS || 9 || &c1-&c9 E
|-
| DATA (DS 80) || 716k || 64 || DS || 9 || &c1-&c9 E
|-
| SYSTEM (SS 40) || 169k || 64 || SS || 9 || &41-&49
|-
| SYSTEM (DS 40) || 346k || 64 || DS || 9 || &41-&49 E
|-
| SYSTEM (SS 80) || 346k || 64 || SS || 9 || &41-&49 E
|-
| SYSTEM (DS 80) || 704k || 64 || DS || 9 || &41-&49 E
|-
| IBM (SS 40) || 169k || 64 || SS || 8 || &01-&08
|-
| IBM (DS 40) || 346k || 64 || DS || 8 || &01-&08 E
|-
| IBM (SS 80) || 346k || 64 || SS || 8 || &01-&08 E
|-
| IBM (DS 80) || 704k || 64 || DS || 8 || &01-&08 E
|-
| ULTRAFORM || 203k || 64 || SS || 9 || &10-&19
|}
Legend: SS = Single-Sided, DS = Double-Sided.
Half of the original AMSDOS rom is occupied by [[Dr. Logo]] from Digital Research. If you replace AMSDOS with ParaDOS, you will lose access to Dr. Logo.
There is also a special version of ParaDOS, called [[VaraDOS]], that supports the [[Vortex Format]].
[[UniDOS]] goes beyond and supports various mass-storage expansions, but also includes support for AMSDOS/ParaDOS floppy discs and tape.
<br>
== FDC Track Format ==
The 765 FDC only supports the single-density IBM 3740 track format and the double-density IBM System/34 track format, which are the defacto standards for floppy disks in most computer systems. These formats have been created by an IBM engineer named Alan Shugart (not to be confused with Amstrad's CEO, Alan Sugar).
[[File:FDC765 - FM and MFM.png|900px]]
On Amstrad CPC, the MFM MODE pin of the FDC is not connected. So the FM mode is unusable.
On Amstrad CPC, an MFM track contains about 6250 raw bytes: 200 ms per track (at 300rpm) / 32 µs per byte (with a bit cell of 4µs). Fun fact: You can squeeze a few more bytes on disk by using a floppy drive that spins a little slower, and counting on the tolerance of the FDC to make that disk readable on unmodified floppy drives.
Notes:
* IAM signifies the beginning of a track.
* IDAM marks the beginning of a sector's header.
* DAM (or DDAM) marks the beginning of the actual data in a sector. Deleted data sectors are marked by an F8 byte instead of an FB byte.
* In MFM encoding, IDAM and DAM are always preceded by three A1 bytes to help the FDC lock onto the data stream after a gap and accurately read the following datas. This is needed because MFM is more compact and harder to read than earlier encoding methods.
* Gaps are necessary to accommodate variations in rotation speed between different drives and avoid overlapping.
<br>
=== Gaps Protection ===
Gaps was a common method of protecting floppy disks against copying on the Amstrad CPC, particularly with French software houses such as [[Infogrames]] and [[Loriciels]]. It consists of writing specific values (other than the standard &4E) in the separation area between 2 consecutive sectors.
When the protected program is launched, it checks if these special values (most often a string signature) are present. If they aren't, the program can then crash or reset the computer or format the disk – or in the case of some games, the player can play the game, but it will crash or freeze at some point.
The trick is that the 765 FDC can read the custom byte values in the gaps, but it cannot write them. This made it hard for people to make working copies of protected disks.
<br>
=== Weak Sectors Protection ===
Another common copy-protection scheme was using (fully or partially) unmagnetized sectors. The unmagnetized data would then appear as random values when read by the FDC.
When the protected program is launched, it reads these weak sectors multiple times. Due to their unstable nature, these sectors will return different data on each read. If the program detects this changing data, it recognizes the disk as an original and continues to run. If not, the program can then crash, reset the computer, etc.
The trick is that the FDC cannot unmagnetize portions of a sector or leave portions of a sector unmagnetized to recreate this effect on another floppy disk.
<br>
=== Error Detection ===
The FDC765 happens to use the exact same algorithm for error detection as the one the Amstrad firmware uses to check each 256-byte tape segments.
The CRC error-detecting code is initialised to &FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written after the ID and data fields of each sector in big-endian format (high byte first and then low byte).
<br>
=== FM Encoding ===
FM (Frequency Modulation) encoding was one of the earliest methods used to store data on floppy disks by creating changes in the magnetic field, known as flux transitions, to represent binary data.
In FM encoding, each bit of data is divided into two distinct parts: a clock signal and a data signal. The clock signal is always present to indicate the timing, while the data signal only appears when the bit is a 1, and remains absent when the bit is a 0.
This constant clock pulse ensures that the FDC always knows where each bit begins and ends. For every bit of information written to the disk, there’s always one clock signal and, if the data is a 1, a data signal as well.
[[File:FM encoding scheme.png|600px]]
Legend: C=Clock, D=Data
<br>
=== MFM Encoding ===
MFM (Modified Frequency Modulation) was developed as a more efficient alternative to FM encoding. This method greatly reduces the number of transitions needed to represent data compared to FM.
If a cell contains a 1, the signal will be recorded in the middle of the cell. If a cell contains a 0, the signal will be recorded at the beginning of the cell, unless it was preceeded by a cell with a 1. If a signal is transmitted for a 0 cell, it is used as clock signal.
This clever encoding eliminates the need for separate clock signals for every bit, as they are implied by the data pattern. It allows the recording of double the density compared to FM without increasing the bit rate on the floppy material. Each bit cell has only half the size compared to FM.
[[File:MFM encoding scheme.png|600px]]
<br>
== HD Floppy Disks on CPC ==
It is theoretically possible to use HD floppy disks (1.44MB) on CPC by using either a Gotek drive or Amiga HD floppy drives spinning at 150rpm. However, nobody has ever written a CPC DOS to make use of it.
On a real floppy drive, if you want to fit more sectors on the same track, you have 2 options: you can make the FDC run faster, or you can make the disk spin slower.
Normally, HD disks use a faster FDC (with an automatic switch between the slow and fast modes).
But, for example on Amiga systems with HD drives, they used floppy drives that spin slower for HD disks (150 rotations per minutes instead of 300) and this allow to use the same controller as before. By the time the floppy has done one complete rotation, you can write 18 sectors instead of just 9.
With a real PC HD floppy drive, this requires some mechanical modifications, and you may get into trouble if the spinning speed is not fast enough.
But with the Gotek, this is not a problem. You can have as many sectors as you want, and the Gotek will generate the index pulse (simulating the floppy completing a turn) after it has sent them all. So it is essentially emulating a floppy that turns very slowly. The FDC has no problems handling that, but you may need to be a bit more relaxed than usual with the timeouts, as it will be some time before the sector you need will pass in front of the drive head, and the index pulse will also be slower than usual.
On a Gotek drive, you can even simulate fantasy floppy disks with up to 255 cylinders, and the FDC will handle them perfectly fine.
<br>
== Mass Storage via FDC ==
The Gotek drive (with a FlashFloppy or HxC firmware) has a special direct access mode that allows it to be used as a poor's man hard drive, offering access to up to 32 GB of storage space. It is accessible via the track 255. [https://hxc2001.com/download/floppy_drive_emulator/SDCard_HxC_Floppy_Emulator_Direct_Access_mode.pdf Source]
This is a great feature as it alleviates the need for another mass storage expansion. And the relative slowness of this solution (~15 KB/s) compared to a proper mass storage device (~130 KB/s with [[Symbiface II]]) is not much of an issue on Amstrad CPC.
This special Gotek direct access mode is supported in [[SymbOS]]. [https://www.cpcwiki.eu/forum/applications/symbos-with-hxc-direct-access-fat32-support/ Source]
<br>
== FDC Block Diagram ==
The FDC chip is quite different from the other CPC chips. It contains its own internal CPU, ROM and RAM.
[[File:WDC37C65 block diagram.png|700px]]
<br>
=== Internal details of the chip ===
Internally this is a microcoded part with a primative controller of NEC’s own design.
Testing microcode embedded in a part can be troublesome. The uPD765 had a few extra gates associated with the DMA Request and DMA Ack pins.
Presenting a certain illegal combination here places the part into a “test” mode and allows the sequencer microcode to be output on the normal Data pins. The sequencer microcode is responsible for high level commands such as Read Track, Recalibrate, Format Track, or Write Data.
There is a similar test mode for the nano-code array which serializes data at the floppy disk head. [https://hackaday.com/2012/08/13/taking-a-look-at-decapped-ics/#comment-734991 Source]
<br>
=== Intel 8272 FDC ===
NEC uPD765 (or just D765) is exactly the same chip as the 8272. NEC and Intel cross-licensed a bunch of IP back in the day. [https://forum.vcfed.org/index.php?threads/8272-fdc-to-8-drive-connections.45816/#post-557651 Source]
The 8272/µPD765 was the result of a cross-licensing deal between NEC and Intel. It was essentially NEC design. [https://classiccmp.org/mailman3/hyperkitty/list/cctalk@classiccmp.org/thread/MMWRWPGKWRVOLMUBIFQTZSCRRMF3YOXF/ Source]
[https://scarybeastsecurity.blogspot.com/2020/11/reverse-engineering-forgotten-1970s.html The decapped photos of Intel 8271, NEC D765 and Intel 8272 chips] prove that NEC D765 and Intel 8272 chips are identical.
<br>
== Generic System Diagram ==
The Amstrad CPC and Amstrad Plus do not have a DMA controller associated with the FDC. And the clock runs at 4MHz instead of 8MHz.
[[File:FDC765 - System Diagram.png|800px]]
<br>
=== Unconnected Pins ===
The DRQ (14), INT (18), VCO SYNC (24), MFM MODE (26), US1 (28), and HEAD LOAD (36) pins are not connected. The TC (16) and RESET (1) pins are connected together. [https://www.cpcwiki.eu/imgs/4/45/CPC6128_Disk_Interface_Schematic.png Source]
At the end of a successful read or write command, the program should send a ''Terminal Count'' (TC) signal to the FDC. However, in the CPC the TC pin isn't connected to the I/O bus, making it impossible for the program to confirm a correct operation. For that reason, the FDC will assume that the command has failed, and it'll return both Bit 6 in Status Register 0 and Bit 7 in Status Register 1 set. The program should ignore this error message.
The CPC doesn't support floppy DMA transfers, and the FDCs FDC's Interrupt signal isn't used in the CPC.
== Datasheets ==
* [[Media:UPD765 UPD765_App_Note_Mar79.pdf| NEC uPD765 Datasheet OCRedpreliminary (1979)]] [[Media:D765 NEC.pdf]] [[Media:UPD765-NEC.pdf]] [[Media:Z765A datasheet.pdf]] - uPD765 disc controller* [[Media:FDC9229BT Intel 8272A Datasheetpreliminary (1982).pdf]] - FDC9229BT data separator [[Media:Datasheet.hk_d8272a_2873060.pdf|Intel 8272A Datasheet (that usually assists 1986)]] [[Media:Floppy Disk Controller(FDC) UM8272A UM8272A-4.pdf]] - Licensed clone of the uPD765 chip)* [[Media:TN6-1 9216 Floppy Disk Data Separator Jun82.pdf]] [[Media:FDC9216 datasheet.PDF]] [[Media:SED9420.pdf]] - Data separators
* [[Media:DDI Schematic.png]] - DDI-1 Schematic (disc interface for CPC464)
* [[Media:Panasonic-3 inch Floppy Drive EME-150.pdf]] [[Media:NEC_FD1035_Floppy.pdf]] - Floppy disk drives
* [[Media:Floppy User Guide.pdf]] - Floppy User Guide
<br>
== External links ==
*[http://quasar.cpcscene.net/doku.php?id=assem:fdc Quasar FDC documentation (in french)]
*[https://64nops.wordpress.com/2021/07/04/a-la-decouverte-du-fdc/ FDC blog articles (in french)]
*[https://en.wikipedia.org/wiki/History_of_the_floppy_disk History of the floppy disk] Wikipedia article
*[https://www.cpc-power.com/cpcarchives/index.php?page=articles&num=92 Floppy disk formats (in french)]
*[https://www.fvempel.nl/3bible.html The 3inch bible]
*[https://info-coach.fr/atari/hardware/FD-Hard.php Atari ST Floppy Drive hardware analysis]
*[https://map.grauw.nl/articles/low-level-disk/ MSX low-level disk storage article]
*[https://youtu.be/aoXr7Anr5DY Réparation de lecteurs Amstrad] [https://youtu.be/-A-USMg9xvE Un Gotek dans un CPC] [https://youtu.be/QPFVfgaMv68 Les disquettes : Le fonctionnement] [https://youtu.be/RMTYevdGH6I Les protections] by [[Rodrik Studio]]
*[https://github.com/dbalsom/fluxfox FluxFox] A floppy disk image library in Rust
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[[Category:CPC Internal Components]][[Category:Programming]][[Category:DATA Storage]][[Category:Electronic Component]]
