µPD765 - Floppy Disc Controller (used in CPC 664, CPC 6128, 6128 Plus and [[DDI-1]] and CPC 664/6128expansion).
The ports used by Besides the Amstrad CPC, this chip equipped the [[PCW|Amstrad PCW]], the [[ZX Spectrum|ZX Spectrum +3]], the [[SC-3000|Sega SC-3000]], and compatible interfaces use:the [[PC|IBM PC]] (including [[Amstrad PC]]).
* Port &FA7E - Floppy Motor On/Off Flipflop* Port &FB7E - FDC 765 Main Status Register (read only)* Port &FB7F - FDC 765 Data Register (read/write)The recommended ports used by Amstrad and compatible interfaces are:
The [[Vortex Disc Drives{|Vortex disc interface]] uses other ports. See its dedicated wiki page.class="wikitable"|-!I/O port address!Function|-|&FA7E||Floppy Motor On/Off Flipflop|-|&FB7E||FDC 765 Main Status Register (read only)|-|&FB7F||FDC 765 Data Register (read/write)|}
<br>Note: Bit b10 of the address port is reset as the FDC is seen as an expansion, even if it is an internal chip. Bit b7 is reset to select the FDC. Bits b8 and b0 are used to select the specific mode of operation. All other bits should be set to 1 to avoid conflict. [https://www.cpc-power.com/cpcarchives/index.php?page=articles&num=48 Source]
== IC Models used in CPC == More than one manufacturer made 765 compatible ICs. These are the ones known to be used in the CPC by looking at pictures of CPC mainboards. All should operate almost identically. * NEC D765AC [httpsNote2://www.cpcwiki.eu/imgs/3/3c/CPC664_Z70205_MC0005B_PCB_Top.jpg Source]* NEC D765AC-2 The [https://www.cpcwiki.eu/imgs/4/45/CPC6128_PCB_Top_%28Z70210_MC0009A%29.jpg Source]* UMC UM8272A [[Media:Amstrad cpc 6128 azerty (f) placa2.jpgVortex Disc Drives|SourceVortex disc interface]]* Zilog Z765APS [https://wwwuses other ports.cpcwiki.eu/imgs/f/fb/CPC6128_Z70290_MC0020C_PCB_Top.jpg Source]* [[Zilog]] Z0765A08PSC [https://www.cpcwiki.eu/imgs/6/67/CPC6128_PCB_Top_%28Z70290_MC0020F%29.jpg Source] The following data seperators are used: * FDC9216 [https://www.cpcwiki.eu/imgs/4/45/CPC6128_PCB_Top_%28Z70210_MC0009A%29.jpg Source]* SED9420C [https://www.cpcwiki.eu/imgs/3/3c/CPC6128_PCB_Top_%28Z70290_MC0020A%29.jpg Source] The CPC464, CPC472, 464 Plus and GX4000 are not equipped with a FDC chipSee its dedicated wiki page.
<br>
== Accessing the FDC 765 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.
The Main Status Register An exception are the Vortex F1-S, F1-D, M1-S and M1-D drives. (Port &FB7EHow are they different?) signalizes when the FDC is ready to send/receive the next byte through the Data Register.
The Data Register (Port &FB7F) Another exception is used to write Commands the Gotek drives. They don't take the motor flip-flop into account and Parameters, to readare always on. [https:/write data bytes, and to receive result bytes/64nops. These three operations are called Commandwordpress.com/2021/07/04/a-, Executionla-, and Resultdecouverte-Phase.du-fdc/ Source]
<br>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]
=== Command Phase ===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]
A command consists 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 used instead of a command byte (eventually including the MT, MF, SK bits)real physical tape, and up to 8 parameter bytesas it has been demonstrated here: https://youtu.be/MAIsOIwgJWA
<br>
=== Execution Phase =Accessing the FDC 765 ==
During this phase, The Main Status Register signalizes when the actual data FDC is transferred (if any). Usually that are ready to send/receive the data bytes for the read/written sector(s), except for next byte through the Format Track Command, in that case 4 bytes for each sector are transferredData Register.
During data transfers between the FDC The Data Register is used to write Commands and the processorParameters, the FDC must be serviced every 26µs (for MFM mode with CPC timings) or the FDC terminates the FDC commandto read/write data bytes, and to receive result bytes.These 3 operations are:
<br>* Command Phase: A command consists of a command byte (eventually including the MT, MF, SK bits), and up to 8 parameter bytes.
=== Result * Execution Phase ===: During this phase, the actual data is transferred (if any). Usually that are the data bytes for the read/written sector(s), except for the Format Track Command, in that case 4 bytes for each sector are transferred. During data transfers between the FDC and the processor, the FDC must be serviced every 26µs (for MFM mode with CPC timings) or the FDC terminates the FDC command.
* Result Phase: Returns up to 7 result bytes (depending on the command) that are containing status information. The Recalibrate and Seek Track commands do not return During the result bytes directlyphase, instead all the program result bytes must wait until the Main Status Register signalizes that the command has been completed, and then it must (!) send be read. The FDC will not accept a Sense Interrupt State new command to 'terminate' until all the Seek/Recalibrate commandresult bytes are read.
During the Note: The Recalibrate and Seek Track commands do not return result phasebytes directly. Instead, all the result bytes program must be readwait until the Main Status Register signalizes that the command has been completed. The FDC will not accept And then it must (!) send a new Sense Interrupt State command until all to 'terminate' the result bytes are readSeek/Recalibrate command.
<br>
== FDC Command Table (= The 15 commands) FDC Commands ===
{|
|
{| class="wikitable"
|+ Read Data Scan Equal (06h11h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT || MF || SK || 0 1 || 0 || 1 0 || 1 0 || 01
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
| command byte 7 || colspan="8" | GPL: gap 3 length
|-
| command byte 8 || colspan="8" | DTLSTP: data length scan test (if command byte 51=scan contiguous, 2=0scan alternate)
|-
| Execution || colspan="8" | Data-transfer from compared between the FDDand main-system
|-
| result byte 0 || colspan="8" | ST0: status register 0
| result byte 6 || colspan="8" | N: bytes per sector
|}
|
{| class="wikitable"
|+ Read Deleted Data Scan Low or Equal (0Ch19h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT || MF || SK || 0 || 1 || 1 || 0 || 0|| 1
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | C: cylinder number
| command byte 7 || colspan="8" | GPL: gap 3 length
|-
| command byte 8 || colspan="8" | DTLSTP: data length scan test (if command byte 51=scan contiguous, 2=0scan alternate)
|-
| Execution || colspan="8" | Data-transfer from compared between the FDDand main-system
|-
| result byte 0 || colspan="8" | ST0: status register 0
| result byte 6 || colspan="8" | N: bytes per sector
|}
|
{| class="wikitable"
|+ Write Data Scan High or Equal (05h1Dh)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT || MF || x SK || 0 1 || 0 1 || 1 || 0 || 1
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
| command byte 7 || colspan="8" | GPL: gap 3 length
|-
| command byte 8 || colspan="8" | DTLSTP: data length scan test (if command byte 51=scan contiguous, 2=0scan alternate)
|-
| Execution || colspan="8" | Data-transfer to compared between the FDDand main-system
|-
| result byte 0 || colspan="8" | ST0: status register 0
|-
| result byte 6 || colspan="8" | N: bytes per sector
|}
|}
{|
|
{| class="wikitable"
|+ Write Deleted Read Data (09h06h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT || MF || x SK || 0 || 1 || 0 || 0 1 || 1|| 0
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | C: cylinder number
| command byte 8 || colspan="8" | DTL: data length (if command byte 5==0)
|-
| Execution || colspan="8" | Data-transfer to from the FDD
|-
| result byte 0 || colspan="8" | ST0: status register 0
| result byte 6 || colspan="8" | N: bytes per sector
|}
|
{| class="wikitable"
|+ Read Track Deleted Data (02h0Ch)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || x MT || MF || SK || 0 || 0 1 || 0 1 || 1 0 || 0
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | C: cylinder number
| command byte 8 || colspan="8" | DTL: data length (if command byte 5==0)
|-
| Execution || colspan="8" | FDC reads all data fields Data-transfer from index hole to EOTthe FDD
|-
| result byte 0 || colspan="8" | ST0: status register 0
| result byte 6 || colspan="8" | N: bytes per sector
|}
|
{| class="wikitable"
|+ Read ID Write Data (0Ah05h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || x MT || MF || x || 0 || 0 || 1 || 0 || 1 || 0
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| Execution command byte 2 || colspan="8" | The first correct ID information on the C: cylinder is stored number|-| command byte 3 || colspan="8" | H: head number|-| command byte 4 || colspan="8" | R: sector number|-| command byte 5 || colspan="8" | N: bytes per sector|-| command byte 6 || colspan="8" | EOT: end of track (ie. last sector in track)|-| command byte 7 || colspan="8" | GPL: gap 3 length|-| command byte 8 || colspan="8" | DTL: data registerlength (if command byte 5==0)|-| Execution || colspan="8" | Data-transfer to the FDD
|-
| result byte 0 || colspan="8" | ST0: status register 0
| result byte 6 || colspan="8" | N: bytes per sector
|}
|
{| class="wikitable"
|+ Format Track Write Deleted Data (0Dh09h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || x MT || MF || x || 0 || 1 || 1 0 || 0 || 1
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | NC: bytes per sectorcylinder number
|-
| command byte 3 || colspan="8" | SCH: sectors per trackhead number
|-
| command byte 4 || colspan="8" | GPLR: gap 3 lengthsector number
|-
| command byte 5 || colspan="8" | DN: filler pattern to write in each bytebytes per sector
|-
| command byte 6 || colspan="8" | EOT: end of track (ie. last sector in track)|-| command byte 7 || colspan="8" | GPL: gap 3 length|-| command byte 8 || colspan="8" | DTL: data length (if command byte 5==0)|-| Execution || colspan="8" | FDC formats an entire trackData-transfer to the FDD
|-
| result byte 0 || colspan="8" | ST0: status register 0
|-
| result byte 6 || colspan="8" | N: bytes per sector
|}
|}
{| class="wikitable"
|+ Scan Equal Read ID (11h0Ah)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT x || MF || SK || 1 x || 0 || 0 1 || 0 || 1|| 0
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | C: cylinder number|-| command byte 3 || colspan="8" | H: head number|-| command byte 4 || colspan="8" | R: sector number|-| command byte 5 || colspan="8" | N: bytes per sector|-| command byte 6 || colspan="8" | EOT: end of track (ie. last sector in track)|-| command byte 7 || colspan="8" | GPL: gap 3 length|-| command byte 8 || colspan="8" | STP: scan test (1=scan contiguous, 2=scan alternate)|-| Execution || colspan="8" | Data compared between The first correct ID information on the FDD and main-systemcylinder is stored in data register
|-
| result byte 0 || colspan="8" | ST0: status register 0
{| class="wikitable"
|+ Scan Low or Equal Read Track (19h02h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT x || MF || SK || 1 || 1 0 || 0 || 0 || 1|| 0
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
| command byte 7 || colspan="8" | GPL: gap 3 length
|-
| command byte 8 || colspan="8" | STPDTL: scan test data length (1if command byte 5=scan contiguous, 2=scan alternate0)
|-
| Execution || colspan="8" | Data compared between the FDD and main-systemFDC reads all data fields from index hole to EOT
|-
| result byte 0 || colspan="8" | ST0: status register 0
{| class="wikitable"
|+ Scan High or Equal Format Track (1Dh0Dh)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || MT x || MF || SK x || 1 0 || 1 || 1 || 0 || 1
|-
| command byte 1 || colspan="5" style="text-align: center;" | x || HD || colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | CN: cylinder numberbytes per sector
|-
| command byte 3 || colspan="8" | HSC: head numbersectors per track
|-
| command byte 4 || colspan="8" | RGPL: sector numbergap 3 length
|-
| command byte 5 || colspan="8" | ND: bytes per sectorfiller pattern to write in each byte
|-
| command byte 6 || colspan="8" | EOT: end of track (ie. last sector in track)|-| command byte 7 || colspan="8" | GPL: gap 3 length|-| command byte 8 || colspan="8" | STP: scan test (1=scan contiguous, 2=scan alternate)|-| Execution || colspan="8" | Data compared between the FDD and main-systemFDC formats an entire track
|-
| result byte 0 || colspan="8" | ST0: status register 0
{| class="wikitable"
|+ Recalibrate Seek (07h0Fh)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || colspan="3" style="text-align: center;" | x || 0 || 0 1 || 1 || 1 || 1
|-
| command byte 1 || colspan="65" style="text-align: center;" | x || HD|| colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | NCN: new cylinder number|-| Execution || colspan="8" | Head retracted to track 0is positioned over proper cylinder
|}
{| class="wikitable"
|+ Seek Recalibrate (0Fh07h)
|-
! !! D7 !! D6 !! D5 !! D4 !! D3 !! D2 !! D1 !! D0
|-
| command byte 0 || colspan="3" style="text-align: center;" | x || 0 || 1 0 || 1 || 1 || 1
|-
| command byte 1 || colspan="56" style="text-align: center;" | x || HD|| colspan="2" style="text-align: center;" | US
|-
| command byte 2 || colspan="8" | NCN: new cylinder number|-| Execution || colspan="8" | Head is positioned over proper cylinderretracted to track 0
|}
Abbreviations used:
*N = 2^(n+7) bytes, with N between 0 and 7. For example, N=2 means 512 bytes. There is conflicting info about the meaning of N=8: CPC-Power thinks it means 32768 bytes [https://www.cpc-power.com/SectorView.php?fiche=4225&slot=10&rang=0 Source]. Same for Roudoudou [https://64nops.wordpress.com/2021/09/09/a-la-decouverte-du-fdc-episode-5/ Source]. While Simon Owen thinks it means N=0 (128 bytes) [https://www.cpcwiki.eu/index.php/Format:DSK_disk_image_file_format Source]. On the Intel 82078 datasheet, it is precised that all values up to 7 are allowable.
*MT = Multi-track (continue multi-sector function on other head)
*MF = MFM mode (1 = Double Density)
<br>
=== FDC Status Registers ===
The Main Status register can always be read through Port &FB7E. The other four 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):
<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:
<br>
== Motor On/Off Flipflop =Notes =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?) The floppy disc rotates at a nominal speed of 300rpm, with some tolerance. <br> == Unconnected Pins == 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 FDC's Interrupt signal isn't used in the CPC. The FDC controller inside the CPC or in the [[Amstrad External Disk Drive|Amstrad DDI-1]] expansion doesn't have the US1 pin connected. This means it can only select floppy drives 0 and 1, and not drives 2 and 3. However, the FDC in the [[Vortex Disc Drives|Vortex disc interface]] does have the US1 pin connected, so it can manage up to 4 floppy drives. <br> == ABBA switch == The [[ABBA switch]] is a common hardware modification that allows users to swap between Drive A and Drive B. On Amstrad CPC, some games and other software were designed to run specifically from Drive A, creating a problem when the disk you needed was in Drive B. The ABBA switch resolved this issue by allowing you to "swap" the designation of the two drives. When the switch is activated, Drive A becomes Drive B, and vice versa, effectively swapping their roles. <br> == Ready / Disk Changed signal == This signal differs between floppy drives model: *For 3inch floppy drives (pin26), it is a "Ready" signal. The /RDY signal is active whenever a disk is installed and rotating in the drive. *For PC 3.5inch floppy drives (pin34), it is a "Disk Changed" signal. The /DSKCHG signal determines whether the same disk loaded during the previous disk access is still in the drive. The easiest solution to this issue is to force the Ready signal on the cable. However, the CPC will hang if you type the CAT command with no disk inserted (inserting a disk will unblock it). Alternatively, you can [[Modify PC floppy drives]] to create a Ready signal, or use Amiga floppy drives, which already have this signal. Gotek drives with FlashFloppy or HxC firmware can also be configured to simulate the Ready signal. Don't forget to use a proper floppy drive cable then, not the one commonly sold by CPC hobbyists with the Ready signal forced. <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.
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. The FDC controller can be used to control 80-tracks and/or double sided drives, though AMSDOS doesn't support such formats. AMSDOS supports a maximum of two disk drives only.
<br>
== Floppy Standard floppy disk format specification 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;"
| Diskette size || 356 KB || 338 KB || 308 KB
|}
Note: 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 796KB instead of 178KB for AMSDOS.
<br>
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
=== IBM format ===
Despite its name, the The IBM format is not only usable on PC 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 use switched to 9 sectors per track and the FAT12 filesystem instead. [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.
| 15 || Record Count (Number of 128 Byte Blocks) || Goes up to 128 (&80).
|-
| 16–31 || Cluster ID Block IDs where to find the file data || 1 byte is used per 1KB blocblock.
|}
Note: Each file is usually stored with an [[AMSDOS Header]] but can also be headerless.<br>
==== CAT'art ====
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.
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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 Track Format- 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:
* The Index Address Mark (IAM) signifies the beginning of a track.* The ID Address Mark (IDAM) marks the beginning of a sector's header.* The Data Address Mark DAM (DATA AMor DDAM) marks the beginning of the actual data in a sector.* There are 2 types of sectors based on their DATA AM value. Data Deleted data sectors are marked by an F8 byte instead of an FB byte.* In MFM encoding, while deleted sectors IDAM and DAM are marked always preceded by an F8 bytethree 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.
In MFM encoding, the IDAM and DATA AM 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.
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).
The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. The primary difference between 765 and 179x controllers is that the 765 only does standard track formats (preamble, marks and data fields), while the 179x will write anything you tell it in the write track (formatting a track) mode.
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=== GAPS Gaps Protection ===
GAPS Gaps was the most a common way to protect method of protecting floppy disks against copy 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.
=== Weak Sectors Protection ===
Another popular common copy-protection scheme was using (fully or partially) unmagnetized sectors. It involves leaving some portions of the track unmagnetized. 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, or do other protective measuresetc.
The trick is that the FDC cannot unmagnetize portions of a track sector or leave portions of a track 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).
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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 floppy disk controller (FDC) always knows where each bit begins and ends. Essentially, for 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
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=== 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.
In MFM, clock pulses are not always present. Instead, a clock pulse only occurs between two consecutive 0s. If a cell contains a 1 appears, it doesn’t need a clock pulse to mark it; instead, a transition is placed the signal will be recorded in the middle of the bit cell to represent the 1. Additionally, when If a cell contains a 0 follows , the signal will be recorded at the beginning of the cell, unless it was preceeded by a cell with a 1, no transition . If a signal is neededtransmitted for a 0 cell, further minimizing the number of magnetic changesit is used as clock signal.
This method greatly reduces clever encoding eliminates the need for separate clock signals for every bit, as they are implied by the number of transitions needed to represent data pattern. It allows the recording of double the density compared to FM, allowing MFM to store significantly more data in without increasing the same spacebit rate on the floppy material. Each bit cell has only half the size compared to FM.
[[File:MFM encoding scheme.png|600px]]
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 some fantasy floppy disks with up to 255 cylinders, and the FDC will handle them perfectly fine.
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== PC Floppy Drives Mass Storage via FDC ==
This chapter is relevant The Gotek drive (with a FlashFloppy or HxC firmware) has a special direct access mode that allows it to be used as many CPC users are using PC floppy drives on their CPCa 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]
=== 5This 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.25inch PC diskette format ===
The IBM PC supports two standard diskette formats on a 5This special Gotek direct access mode is supported in [[SymbOS]].25 inch drive[https:* The first is double sided, double density, 40 cylinders, which yields a capacity of 360KB per diskette//www. This is the original IBM PC formatcpcwiki. The double density drive rotates at 300 RPM.* The second format was introduced eu/forum/applications/symbos-with the PC-AT. It is double sided, quad density, 80 cylinders, with a total capacity of 1.2 MB. The high density drive rotates at '''360 RPM''', so only 15 sectors can be written on a track instead of 18.hxc-direct-access-fat32-support/ Source]
=== 3.5inch PC diskette format ===<br>
The IBM PS/2 introduced two standard 3.5 inch diskette formats:== FDC Block Diagram == * The first FDC chip is a double sidedquite different from the other CPC chips. It contains its own internal CPU, double density, 80 cylinders, format yielding a capacity of 720KB. The double density drive rotates at 300 RPMROM and RAM.* The other is double sided, quad density, 80 cylinders, with a total capacity of 1.44 MB. The high density drive rotates at 300 RPM[[File:WDC37C65 block diagram.png|700px]]
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=== Internal details of the chip ===
* [https://scarybeastsecurity.blogspot.com/2020/11/reverse-engineering-forgotten-1970s.html The decapped photos Internally this is a microcoded part with a primative controller of Intel 8271, NEC D765 and Intel 8272 chips] prove that NEC D765 and Intel 8272 chips are identicalNEC’s own design.
* From [https://hackaday.com/2012/08/13/taking-a-look-at-decapped-ics/#comment-734991 a comment in the hackaday website]: "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]
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== = Intel 8272 FDC Block Diagram ===
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. [Filehttps:WDC37C65 block diagram//classiccmp.png|700pxorg/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.
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== Generic System Diagram ==
The Amstrad CPC and Amstrad Plus do not have a DMA controller associated with the FDC. The INT pin of And the FDC is not connected either. The clock runs at 4MHz instead of 8MHz.
[[File:FDC765 - System Diagram.png|800px]]
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=== 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 FDC's Interrupt signal isn't used in the CPC.
The FDC controller inside the CPC or in the [[Amstrad External Disk Drive|Amstrad DDI-1]] expansion doesn't have the US1 pin connected. This means it can only select floppy drives 0 and 1, and not drives 2 and 3. And AMSDOS supports a maximum of two disk drives only.
However, the FDC in the [[Vortex Disc Drives|Vortex disc interface]] does have the US1 pin connected, and so it can manage up to 4 floppy drives.
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=== ABBA switch ===
The [[ABBA switch]] is a common hardware modification that allows users to swap between Drive A and Drive B.
On Amstrad CPC, some games and other software were designed to run specifically from Drive A, creating a problem when the disk you needed was in Drive B.
The ABBA switch resolved this issue by allowing you to "swap" the designation of the two drives. When the switch is activated, Drive A becomes Drive B, and vice versa, effectively swapping their roles.
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== PC to CPC floppy connector ==
[[File:Cpc6128floppytopcfloppy.gif]]
<br>
=== Ready / Disk Changed signal ===
This signal differs between floppy drives model:
*For 3inch floppy drives, it is a "Ready" signal (pin 26). The /RDY signal turns on when a disk is inserted and spinning in the drive, usually after detecting 2 or 3 index pulses, depending on the drive model. It turns off immediately if the disk is removed or the motor stops.
*For PC 3.5inch floppy drives, it is a "Disk Changed" signal (pin 34). The /DSKCHG signal determines whether the same disk loaded during the previous disk access is still in the drive.
The easiest solution to this issue is to force the Ready signal on the cable. However, the CPC will hang if you type the CAT command with no disk inserted (inserting a disk will unblock it).
Alternatively, you can [[Modify PC floppy drives]] to create a Ready signal, or use Amiga floppy drives, which already have this signal.
Gotek drives with FlashFloppy or HxC firmware can also be configured to simulate the Ready signal. Don't forget to use a proper floppy drive cable then, not the one commonly sold by CPC hobbyists with the Ready signal forced.
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=== PC Floppy Drive types ===
The IBM PC supports 3 standard diskette formats on a 5.25 inch drive:
* The earliest IBM PCs used single-sided, double density, 40 cylinders, 300 RPM, floppy drives which yield a capacity of 160 KB per side. And MS-DOS 1.0 only supported single-sided floppy drives.
* Double-sided floppy drives were introduced in the IBM PC in 1982 and are supported in MS-DOS 1.1. They originally had a capacity of 320 KB per diskette. MS-DOS 2.0 extended the capacity to 360 KB with a new disk format (9 sectors per track instead of 8). [https://minuszerodegrees.net/5150/early/5150_early.htm Source]
* A new floppy drive was introduced with the PC/AT in 1984. It is double sided, quad density, 80 cylinders, with a total capacity of 1.2 MB. The high density drive rotates at '''360 RPM''', so only 15 sectors can be written on a track instead of 18.
The IBM PS/2 introduced two standard 3.5 inch diskette formats:
* The first is a double sided, double density, 80 cylinders, format yielding a capacity of 720KB. The double density drive rotates at 300 RPM.
* The other is double sided, quad density, 80 cylinders, with a total capacity of 1.44 MB. The high density drive rotates at 300 RPM.
Fun fact: The original IBM PC didn’t have a hard drive, even as an option. And MS-DOS 1.x didn't have any support for hard drives either. Hard drives only became a thing with the introduction of the IBM PC/XT and MS-DOS 2.0 in 1983. But the floppy drive was not the only option to load programs on the original IBM PC. You could also use a tape deck, just like on Amstrad CPC. However, the tape port disappeared with the PC/XT. [https://en.wikipedia.org/wiki/IBM_cassette_tape Source]
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== FDD Block Diagram ==
[[File:Floppy Disk Drive - Block Diagram.png|700px]]
<br>
== Chip Variants ==
=== IC Models used in CPC ===
More than one manufacturer made 765 compatible ICs. These are the ones known to be used in the CPC by looking at pictures of CPC mainboards. All should operate almost identically.
* NEC D765AC [https://www.cpcwiki.eu/imgs/3/3c/CPC664_Z70205_MC0005B_PCB_Top.jpg Source]
* NEC D765AC-2 [https://www.cpcwiki.eu/imgs/4/45/CPC6128_PCB_Top_%28Z70210_MC0009A%29.jpg Source]
* UMC UM8272A [[Media:Amstrad cpc 6128 azerty (f) placa2.jpg|Source]]
* Zilog Z765APS [https://www.cpcwiki.eu/imgs/f/fb/CPC6128_Z70290_MC0020C_PCB_Top.jpg Source]
* [[Zilog]] Z0765A08PSC [https://www.cpcwiki.eu/imgs/6/67/CPC6128_PCB_Top_%28Z70290_MC0020F%29.jpg Source]
The following data seperators are used:
* FDC9216 [https://www.cpcwiki.eu/imgs/4/45/CPC6128_PCB_Top_%28Z70210_MC0009A%29.jpg Source]
* SED9420C [https://www.cpcwiki.eu/imgs/3/3c/CPC6128_PCB_Top_%28Z70290_MC0020A%29.jpg Source]
The CPC464, CPC472, 464 Plus and GX4000 are not equipped with a FDC chip.
All the floppy disk drive models used by Amstrad are referenced here: [https://www.cpcwiki.eu/index.php/Amstrad_FDD_part Amstrad FDD part]
=== Other Variants ===
NEC has developed various successors to the original uPD765, such as the uPD72065 (used in the [[Sharp X68000]]), uPD72067, and uPD72069.
Intel has also produced successors of the 8272, such as the 82072, 82077 and 82078.
The КР1810ВГ72А is a Soviet clone of the Intel i8272. It is used in the [[Aleste 520EX]] clone of the Amstrad CPC computer.
The [https://map.grauw.nl/resources/disk/toshiba_tc8566af.pdf Toshiba TC8566AF] is also a variant of the FDC765.
=== Competitors ===
The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. Its main differences are:
* the 765 has 3 scan commands, while the 179x has none.
* the 765 has 5 status registers, while the 179x only has one.
* the 765 has multi-byte "command phase", while commands are issued to the 179x by writing a single byte to the Command Register, after having set parameters in its dedicated registers.
* the 765 has multi-byte "result phase", while the 179x has none. After a command completes, all the status information is contained in the Status Register as a single byte.
* the 765 has drive select pins to directly select one of four floppy drives, while the 179x has none. Instead, external logic must be used to handle multiple drives.
* the 765 only does standard track formats (preamble, marks and data fields), while the 179x will write anything you tell it in the write track (formatting a track) mode.
Some other computers adopted 3inch floppy disks like Amstrad, but used different FDC chips:
* [[Oric-1/Atmos|Oric]] uses the WD1773 (Jasmin) or WD1793 (Microdisc) FDC chip
* [[Tatung Einstein]] uses the WD1770 FDC chip
Third-party 3inch floppy disk drives (like the Amdek Amdisk) were also available for many systems: Apple II, Atari 8-bit, Tandy CoCo, BBC Micro, etc..
<br>
* [[Media:UPD765_App_Note_Mar79.pdf| NEC uPD765 Datasheet preliminary (1979)]] [[Media:D765 NEC.pdf]] [[Media:UPD765-NEC.pdf]] [[Media:Z765A datasheet.pdf]] - uPD765 disc controller
* [[Media:Intel 8272A Datasheet preliminary (1982).pdf]] [[Media:Datasheet.hk_d8272a_2873060.pdf|Intel 8272A Datasheet (1986)]] [[Media:Floppy Disk Controller(FDC) UM8272A UM8272A-4.pdf]] - Licensed clone of the uPD765
* [[Media:WD37C65 datasheet.pdf]] - NEC 765 controller clone with onboard data seperator and AT/EISA compatibility
* [[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)
*[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]]