Taken from the Ramsoft website - http://www.ramsoft.bbk.org/ - Broken Link!!!
Converted to HTML by Pete Robinson

                          RAMSOFT proudly presents:

                                THE COMPLETE
           -------------------------------------------------------
             D I S C i P L E / + D   T E C H N I C A L   I N F O
           -------------------------------------------------------

                          Revision 0.8 (11 Dec 1997)

                                SUMMARY:
                                ========

                     1 ..... DISCiPLE/+D general features
                     2 ..... Memory layout
                     3 ..... I/O ports
                     3.1 ... DISCiPLE port 7Bh (123 dec.)
                     4 ..... UFIA layout
                     5 ..... System calls (hook codes)
                     5.1 ... Internal system calls
                     6 ..... Disk layout
                     7 ..... Filesystem details
                     8 ..... File types table
                     9 ..... GDOS extended Basic commands
                     9.1 ... The snapshot button
                     9.2 ... GDOS/G+DOS and UNIDOS error messages
                    10 ..... GDOS/G+DOS and UNIDOS system variables
                    11 ..... Connectors pinouts
                    12 ..... VL1772 FDC programming info

DISCLAIMER
==========

This document may contain some errors. The authors do not assume any
responsability for losses and damages caused to your system by use of any
information reported here.

COMMON TERMS
============

  DRAM - The sector buffer.
   RPT - A system variable which points to a byte in the DRAM.
  UFIA - User File Information Area, a 24 byte structure which describes
         a file for system calls.
  DFCA - Disk File Channel Area.

1. DISCiPLE / +D features
=========================

     8 KB EPROM (for disk BIOS)
     8 KB RAM
     NMI button (snapshot)
     Parallel port (not bi-directional)
     Floppy disk port (controlled by VL1772 FDC)

     DISCiPLE-only features:

     Two ATARI joystick ports (Sinclair 1, Sinclair 2 / Kempston)
     Two network connectors (Interface 1 compatible, 3.5mm jack)
     Inhibit button (to lock out the interface)
     Through bus connector (to plug in other devices)

2. MEMORY LAYOUT
================

When the interface memory is paged in (see below), the first 16K of the
Z80 address space have this mapping:

  Address   DISCiPLE   +D
  ------------------------------
   0x0000   8K RAM     8K ROM
   0x2000   8K ROM     8K RAM

UNI-DOS memory mapping is the same as +D, even on DISCiPLE.
Read [3.1] to see how it is possible to swap ROM/RAM addresses on the
DISCiPLE.

3. DISCiPLE and PlusD I/O PORTS
===============================

NOTE: Joystick 1 is scanned both as Kempston and Sinclair 2 (keys 6,7,8,9,0)
      Joystick 2 is Sinclair 1 (keys 1,2,3,4,5)
      Network is a Interface 1 compatible net.
      +D lacks of joystick ports and network.

 Port (+D)  In              Out                       Notes
 ---------------------------------------------------------------------------
  1Bh (E3h) FDC status      FDC command               See also section 12
  5Bh (EBh) track register  track
  9Bh (F3h) sector regist.  sector register
  DBh (FBh) data register   data register

  1Fh (EFh) Joystick 1      control:                  Joy is DISCiPLE only
            b0 right        drive select
            b1 left         side select
            b2 down         single/double density
            b3 up           ROM bank select
            b4 fire         Inhibit switch control    Inhibit is DISCiPLE only
            b5  --          ext. select (?)
            b6 PRN BUSY     printer STROBE
            b7 network      network

  59h (N/A) wait when net=1     --                    DISCiPLE only

  7Bh (N/A) set boot        reset boot                DISCiPLE only, see [3.1]

  BBh (E7h) mem. page in    memory page out           (*)

  FBh (F7h)    --           printer data

  FEh (N/A) Joystick 2                                DISCiPLE only

(*) DISCiPLE and +D memories are also paged in whenever the Z80 fetches an
    instruction from the following addresses:

    0x0000, 0x0008, 0x0066, 0x028E.

3.1 DISCiPLE PORT 7Bh (123 dec.) AND MEMORY ADDRESSES
======================================================

Port 7Bh is available only on the DISCiPLE and has a flip flop attached to it.
It can be used to swap the RAM/ROM addresses in this way:

Access  ROM      RAM      Purpose
----------------------------------
  IN    0x0000   0x2000   reset ff
 OUT    0x2000   0x0000   set ff

This feature is used by GDOS to know if it necessary to load the system
file from disk on boot or after two consecutive resets without any DOS
command between them; UNIDOS ignores this feature, so any swap attempt will
result in a system crash.

In GDOS there is a variable located in RAM at offset 0x1DE4 that is set to
0x44 ('D') after a BASIC syntax check (i.e. after a RST 08h with a code lower
than 1Bh) and after a bootstrap: this variable indicates that the DOS services
have been called almost once. Whenever the user resets the computer, the flip
flop attached to port 7Bh is reset, so the ROM will be placed at 0x0000. When
the first interrupt occurs, the keyboard scanning routine is called at 0x028E
and the DISCiPLE memory is automatically paged in. At offset 0x028E in the
DISCiPLE's ROM there's a routine that checks if the variable we said above
holds 0x44: if it's the case, then the same routine puts 00h in there to say
that DOS services haven't been called since last reset; otherwise the routine
sets the variable to 0x53 ('S') and copies the first 2335 (0x091F) bytes of
ROM in the RAM: in this case the system file has to be loaded again.
When all is finished, the memories will be swapped again (i.e. the flip flop
will be set) by OUTing to port 7Bh, the DISCiPLE paged out by OUTing to port
BBh and the keyscan routine is finally executed.

INning from port 7Bh has the same meaning of a system reset for the DOS, so
after reading 2 times from port 7Bh without typing a DOS command between them
the system file needs to be reloaded.

NOTE that since all this is based on the keyscan routine in the Spectrum's
ROM, nothing will happen by INning from port 7Bh if the call is not performed
(i.e. if interrupts are disabled in IM 1 or we're not in IM 1 or keyboard is
scanned in a custom way); however the last operation with port 7Bh must be an
OUT before the routine in the ROM is executed if you want to keep the system
safe by resetting once.

4. USER FILE INFORMATION AREA (UFIA)
====================================

   Offset Len   Meaning
   ---------------------------------------
        0   1   Drive number (1, 2 or '*' for current)
        1   1   Program number (in the directory)
        2   1   Stream number
        3   1   Device density type ('d'=DD, 'D'=SD)
        4   1   Directory description (see below)
        5  10   File name (padded with spaces)
        15  1   File type (see below)
        16  2   Length of file
        18  2   Start address
        20  2   Basic length
        22  2   Autostart line

5. SYSTEM CALLS (both DISCiPLE and +D)
======================================

To invoke system services you must use the IF1 protocol:

     RST 8
     DB #service

All the functions return an error code into register A.

IMPORTANT NOTES:
You cannot perform RST 8 calls from within a routine located into the
interface's RAM.
You must not call ROM address 0x028E (keyboard scanning routine) from
within an interrupt routine, since this would crash the Spectrum when a
DISCiPLE/+D is connected; some programs crash due to this fact
(eg SoundTracker v1.1) - you may try to correct the problem replacing the
CALL 0x028E with a RST 0x38.
A few programs (like some games converted to disk) do not use the RST 8
mechanism but make absolute CALLs instead - a very bad practice!

This is the list of GDOS3 (G+DOS2) hook codes with input parameters:

  HXFER (33h) - transfer file description and header to the DFCA.
                IX = UFIA address
  OFSM  (34h) - open file sector map with the info in the DFCA.
                The RTP is set to the beginning of the DRAM.
  HOFLE (35h) - open a file.
                IX = UFIA
                Combines the previous two functions.
                Sets the last 9 bytes of UFIA with the file header.
  SBYT  (36h) - Save a byte to DRAM location pointed by RTP.
                A = byte to save.
                If the sector buffer is full, it is automatically saved
                to disk.
  HSVBK (37h) - Save a block of data.
                DE = start address of data.
                BC = number of bytes to save.
  CFSM  (38h) - Close file sector map.
                Flushes DRAM, closes file and updates the directory.
  PNTP  (39h) - Output a byte to the parallel port.
                A = byte to output.
  COPS  (3Ah) - Copy the screen to printer.
  HGFLE (3Bh) - Get a file from disk.
                IX = UFIA
                The first sector is loaded to DRAM and RPT is set to the
                first byte.
  LBYT  (3Ch) - Load the byte pointed by RPT.
                Returns A = byte read.
                If needed, another sector is read from the disk.
                RPT is updated consequently.
  HLDBK (3Dh) - Load a block of data.
                DE = start address (where the data will be put)
                BC = number of bytes to read
  WSAD  (3Eh) - Write the DRAM to a sector in the disk.
                D = track
                E = sector
                RPT is restored to the beginning of DRAM.
  RSAD  (3Fh) - Read a sector to DRAM.
                D = track
                E = sector
                Same as 3Eh.
  REST  (40h) - Reset drive and seek track 0.
                Drive number is specified into the UFIA.
  HERAZ (41h) - Erase the file on disk identified by UFIA.
                IX = UFIA address.

For the next ones I haven't exact information:

        (42h) - Large screen dump
        (43h) - Disk catalogue
        (44h) - Load sector
                IX = start address
        (45h) - Save sector
                IX = start address
        (46h) - Open and close streams (how?)
        (47h) - Page in

Now UNIDOS-only hook codes:

        (48h) - Load file (?)
        (49h) - Verify file (?)
        (4Ah) - Merge (?)
        (4Bh) - Save file (?)
        (4Ch) - Open file (?)
        (4Dh) - POINT (see UNIDOS)
        (4Eh) - Flush buffers to disk
        (4Fh) - Close file
        (50h) - Clear channels
        (51h) - Rename file (?)
        (52h) - Move stream
        (53h) - Select disk and directory

5.1 INTERNAL SYSTEM CALLS (both DISCiPLE and +D)
================================================

Here is the purpose of the RST commands when the DISCiPLE or the +D memories
are paged in.

RST 00h - Reset
RST 08h - Call system services; the required service code must follow (1)(2)
RST 10h - Call in Spectrum ROM; the routine address must follow (3)
RST 18h - GDOS, G+DOS : reserved (for syntax check)
          Uni-DOS : low-level system services; the required service code must
          follow
RST 20h - Print DOS error report : the error code must follow
RST 28h - Performs a RST 20h in the Spectrum ROM
RST 30h - Gets interpreter status : Z=0 if checking syntax, Z=1 if executing
RST 38h - Enables interrupts

(1) = ROM 1 must be paged in Spectrum 128K
(2) = The interface is automatically paged in by the hardware
(3) = Be sure that the right Spectrum ROM is paged in

6. DISK LAYOUT
==============

The disk has 80 tracks of 10 sectors (512 bytes high density, 256 bytes low
density) each.
The first four tracks of the disk (tracks 0-3 side 0) are reserved for the
system and contain the disk directory.
The directory consists of 80 consecutive file descriptors, each one taking
256 bytes; thus, the descriptor of file #48 resides in the first 256 bytes
of sector 4 track 2.
The directory has a fixed dimension and can only contain up to 80 files.
UNIDOS introduces subdirectories and allows to specify the maximum number
of file entries for each directory.

7. FILE DESCRIPTOR FORMAT
=========================

Now we will see the details about a single directory entry.
NOTE: All numbers are in decimal.

General structure:

BYTE     MEANING
  0      File type (see FILE-TYPES table); 0= Erased or free entry
 1-10    Filename (padded with spaces)
11-12    Number of sectors occupated by the file (0-65535)
 13      Track number of the first sector of the file
 14      Sector number of the first sector of the file
15-209   Sector allocation bitmap. Each bit correspond to a disk sector.
         A bit is set if the corresponding sector belong to the file.
         Examples: byte 15, bit 0 corresponds to track 4, sector 1;
         byte 16, bit 3 means track 5, sector 2...
         IMPORTANT NOTE:
         The s.a.b. is used only during saving operations: the s.a.b. of
         all the 80 files are merged together (OR) so that the system
         knows which sectors are free (not allocated to any file).
         During loading a faster method is used: each sector contains
         only 510 bytes of data; the last two bytes contain the
         track number and the sector number of the next sector of the file,
         respectively. The last sector of the chain contains (0,0) as the
         last two bytes.

210-255  Depends on the file type.

BASIC (type 1)
---------------
210      Always 0
211-212  Length
213-214  Memory start address ( PROG when loading - usually 23755)
215-216  Length without variables
217-218  Autostart line
NOTE: These 9 bytes are also the first 9 bytes of the file.

NUMBER ARRAY (type 2)
---------------------
210      Always 1
211-212  Length
213-314  Memory start address.
215-216  Array name, probably ignored.
217-218  Not used
NOTE: These 9 bytes are also the first 9 bytes of the file.

STRING ARRAY (type 3)
---------------------
210      Always 2
211-218  Same as for type 2
NOTE: These 9 bytes are also the first 9 bytes of the file.

CODE FILE (type 4)
------------------
210      Always 3
211-212  Length
213-314  Start address
215-216  Not used
217-218  Autorun address

48K SNAPSHOT (type 5)
---------------------
210-218  Not used
219-255  Z80 registers

MDRV (type 6)
-------------
This is a microdrive cartridge image. Details omitted.

SCREEN$ (type 7)
----------------
Same as type 4 with Start=16384 and Length=6912

SPECIAL (type 8)
----------------
210-255  Any meaning assigned by the programmer.

128K SNAPSHOT (type 9)
----------------------
Same as 48K Snapshot. The first byte of the file is a copy of the page
register (port 0x7FFD), usually held in the system variable BANKM (23388).

OPENTYPE (type 10)
------------------
210     Number of 64K blocks in the file
211-212 Length of the last block
213-255 Not used

EXECUTE (type 11)
-----------------
210-255 Same as CODE file (type 4), but Length=510 and Start=0x1BD6 implicitly
        (0x3DB6 for +D). The sector is loaded into the interface RAM
        and executed (it should contain relocatable code!).

SUBDIRECTORY (type 12) - UNIDOS
-------------------------------
210-212 Same as Opentype (type 10). This file is always held on contiguous
        sectors. The last two bytes of a sector do not contain the address
        of the next sector. The structure is the same as the root directory,
        but the first entry contains the file header number of the parent
        directory. The last two bytes of the last sector contain 0xFFFF.

CREATE (type 13) - UNIDOS
-------------------------
210-255 Same as CODE file but the start address is ignored.

8. FILE-TYPES TABLE
===================

Code          Type
--------------------------
0             (ERASED)
1             BASIC
2             NUMBER ARRAY
3             STRING ARRAY
4             CODE
5             48K SNAPSHOT
6             MICRODRIVE
7             SCREEN$
8             SPECIAL
9             128K SNAPSHOT
10            OPENTYPE
11            EXECUTE
12            SUBDIRECTORY (UNIDOS)
13            CREATE (UNIDOS)

UNIDOS: Add 128 for hidden files and 64 for protected files.

9. GDOS EXTENDED BASIC SYNTAX
=============================

GDOS extends the BASIC to provide support for disk operations.
When you switch the Spectrum on, you must initialize the system; to do so,
insert a disk containing the operating system file ("SYS*" on DISCiPLE and
"+SYS*" for +D) into the first drive and then enter "RUN".
The DOS also looks for the first file called "auto*" and runs it if found.
NOTE: File names are case insensitive.

To show the disk contents, enter:

        CAT 1 (for drive 1)     or
        CAT * (current drive)

To LOAD/SAVE a file:

        LOAD d;"filename" [CODE|SCREEN$|DATA|etc]
        LOAD p
        SAVE d;"program" [CODE|LINE|SCREEN$|etc]

        where  is a drive number (1-2)
               is a file number (1-80)

The file names are not case sensitive and you can use wildcards ('*' and
'?' characters work exactly like you'd expect).
You must specify 'S' to load a 48K snapshot and 'K' for a 128K snapshot.
Case of letters 'S' and 'K' is important.

Examples:

 LOAD d1;"screen" SCREEN$
 LOAD d*;"pippo"             loads program pippo from the current drive
 LOAD d1;"snap128"K          loads the 128K snapshot 'snap128'
 SAVE d2;"rom" CODE 0,16384

Note: "d1" with lowcase 'd' refers to DS DD disks (80 tracks double sided);
in ROM version 3 the only difference seems to be that if you use 'D' a CAT
command is also performed after the operation.
Early versions of the OS used "D1" (capital 'D') for single sided disks.
This applies always when you have to specify the 'd' field.
Please use only DS/DD disks.

Each file can be referred both though its name and its directory number, so
if file "screen" is listed as number 7 you may also enter:

 LOAD p7

Note that if you use the abbreviated notation, each file will be loaded
accordingly to its type (i.e. you can a CODE file will be loaded into memory
at its start address).

Of course MERGE and VERIFY are also available with a similar syntax.

To erase a file from the disk, enter:

 ERASE d1;"file2del"
 ERASE d1;"*"                dangerous!

To format a disk, use:

 FORMAT d1

GDOS also extends streams, so that you can redirect a stream to a file and
vice versa. We will not analyze the extended syntax of OPEN, CLOSE and
MOVE here.

DISCiPLE recognizes the Microdrive syntax, so you can enter commands like:

 LOAD *"m";1;"pippo"

which will load the BASIC program pippo from drive 1. All your microdrive
programs should run over GDOS without modifications. Remember that PlusD
does not support the IF1 syntax.
Besides, DISCiPLE implements the IF1 command set for network operations
(with some enhancements).

UNI-DOS specific syntax coming soon.

9.1 THE SNAPSHOT BUTTON
=======================

When you press the magic button, an NMI is generated and control passes to
address 0x0066 (102 dec.) of the DISCiPLE/+D memory. The consequent behaviour
of the system depends on the particular System version loaded. Under the
standard systems (eg. systems 3a/3b/3d), you have to hold down CAPS SHIFT
while pressing the button and then colored stripes appear in the border and
five keys are active:

  1 = Dump screen to printer
  2 = Big screen dump (A4)
  3 = Save current screen to disk
  4 = Save a 48K snapshot
  5 = Save a 128K snapshot.

Snapshots are saved to disk with names like "Snap1A" and subsequent indexes
depending on their position in the directory. When saving a 128K snap, the
system stops after creating the file and waits for the user to specify if the
screen has changed since the beginning of the operation; the user must respond
pressing either 'y' or 'n'. This happens because unlike Multiface the DISCiPLE
and the +D haven't got any flip flop to store the status of port 0x7FFD bit 3
which tells which videoram is displayed. Before waiting for the user
intervention, the system pages the first videoram (0x4000 page 5), so if the
image displayed changes then it means that the second videoram was previously
paged. Well, that's it!

After that, control is passed back to the interrupted program.

NOTE: The snapshot routine corrupts the stack with six bytes (PC,AF and R+F).
This may cause some programs which use the stack in a particular way to
crash if the magic button is pressed at certain times (eg. Batman the Movie).

9.2 GDOS and UNIDOS ERROR MESSAGES
==================================

Here's a list of the error codes for both systems available for the RST 20h
service.

-------------------------------------------------------------
Code |        GDOS                |        G+DOS
-------------------------------------------------------------
 00  |  Nonsense in GDOS          |  Nonsense in G+DOS
 01  |  Nonsense in GNOS          |  Nonsense in GNOS
 02  |  Statement end error       |  Statement END error
 03  |  BREAK requested           |  BREAK requested
 04  |  SECTOR error              |  SECTOR error
 05  |  FORMAT data lost          |  FORMAT data lost
 06  |  NO DISC in drive          |  CHECK DISC in drive
 07  |  No "SYSTEM" file          |  NO "+SYS  " file
 08  |  Invalid FILE NAME         |  Invalid FILE NAME
 09  |  Invalid STATION           |  Invalid STATION
 10  |  Invalid DEVICE            |  Invalid DEVICE
 11  |  VARIABLE not found        |  VARIABLE not found
 12  |  VERIFY failed             |  VERIFY failed
 13  |  Wrong FILE type           |  Wrong FILE type
 14  |  MERGE error               |  MERGE error
 15  |  CODE error                |  CODE error
 16  |  PUPIL set                 |  PUPIL set
 17  |  Invalid CODE              |  Invalid CODE
 18  |  Reading a WRITE file      |  Reading a WRITE file
 19  |  Writing a READ file       |  Writing a READ file
 20  |  O.K. GDOS 3               |  O.K. G+DOS
 21  |  Network OFF               |  Network OFF
 22  |  Wrong DRIVE               |  Wrong DRIVE
 23  |  Disc write PROTECTED      |  Disc write PROTECTED
 24  |  Not enough SPACE on disc  |  Not enough SPACE on disc
 25  |  Directory FULL            |  Directory FULL
 26  |  File NOT FOUND            |  File NOT FOUND
 27  |  END of file               |  END of file
 28  |  File NAME used            |  File NAME used
 29  |  Not a MASTER station      |  NO G+DOS loaded
 30  |  STREAM used               |  STREAM used
 31  |  CHANNEL used              |  CHANNEL used
-------------------------------------------------------------

----------------------------------
 Code |        UNIDOS
----------------------------------
 128  |  Nonsense in Uni-Dos
 129  |  O.K Uni-Dos
 130  |  Break requested
 131  |  Corrupt sector
 132  |  Sector missing
 133  |  Check disc in drive
 134  |  DOS file not found
 135  |  Invalid filename
 136  |  Invalid sector number
 137  |  Invalid device/channel
 138  |  Wrong stream type
 139  |  Verification failed
 140  |  Wrong file type
 141  |  CODE parameter error
 142  |  Directory not found
 143  |  File has zero length
 144  |  Reading a write file
 145  |  Writing a read file
 146  |  POINT outside file
 147  |  Channel out of order
 148  |  Illegal drive number
 149  |  Disc write protected
 150  |  Not enough disc space
 151  |  Directory full
 152  |  File not found
 153  |  End of file
 154  |  Filename already used
 155  |  File still open
 156  |  File in use
 157  |  Channel already open
 158  |  Protected file
 159  |  Unavailable RST 8

10. GDOS and UNIDOS SYSTEM VARIABLES
====================================

System variables are modified with the POKE command in the following form:

            POKE @var, value

      where var   is a variable number and
            value is the new variable value.

Actually, the '@' operator is interpreted as an offset into the interface's
RAM. The base addresses are the following:

 0x2000 UNIDOS and G+DOS (+D)
 0x0298 GDOS (DISCiPLE)

List of the system variables:

  0  Flash border during disk operations. Set to 0 to leave the border
     unaltered. This byte is ANDed with the sector number currently accessed
     and then sent to port 0xFE.
  1  Drive 1 capacity = number of tracks + 128 if double sided.
  2  Drive 2 capacity, same as above.
  3  Drive stepping rate. Set to 1 for the minimum (1ms).
     WARNING: Poking 0 here may lock your disk drive. It can be unlocked
     reinstalling the OS from tape.
  4  GDOS: disable Centronics printer port (0=enabled)
     UNIDOS: Enable BREAK key if set.
  5  Printer line length in number of characters (default 80)
  6  Printer control flag. If set the codes sent to the printer are not
     filtered (binary output). Necesssary to send control codes to the
     printer.
  7  Printer line spacing expressed in n/72 of an inch. It is sent to the
     printer before avery CR (default GDOS=12, UNIDOS=8).
  8  Number of line feeds after CR (default: GDOS=1 UNIDOS=0).
  9  Left margin for printing. This is the number of spaces inserted before
     the first character of a line (default=0).
 10  Printer flag.
     GDOS: if set then the printer driver generates the graphic representation
     of 'œ' and ',' (default=1).
     UNIDOS: printer flag (default=0x80).
 11  GDOS: network station number (default=1).
     UNIDOS: Centronics enable, same as GDOS variable #4 (default=1).
 12  UNIDOS: printer column number (default=1).
 13  UNIDOS: CLS# screen color.
 14  Extended syntax address (2 bytes). This address is called on error which
     are not related to hook codes and DOS syntax. Can be used to add extra
     commands. Ignored if 0.
 16  UNIDOS: Interrupt address (2 bytes).
 18  UNIDOS: Printer initialization codes (8 bytes). They are sent to the
     printer after a NEW or before pressing the 'P' key during a snapshot.
 26  UNIDOS: Set character pitch (8 bytes).
 34  UNIDOS: Set n/72 line space (8 bytes).
 42  UNIDOS: Set UDG bit graphics density (8 bytes).
 50  UNIDOS: Second initialise codes (8 bytes).
 58  UNIDOS: Codes for 'œ' (8 bytes).
 66  UNIDOS: Codes for '#' (8 bytes).
 74  UNIDOS: Codes for (C) (8 bytes).
 82  UNIDOS: Save SCREEN$ 2 parameters (7 bytes).
 89  UNIDOS: set dump graphics (8 bytes).
 97  UNIDOS: address of extra error messages (default=0x1C68).
 99  UNIDOS: error code
100  UNIDOS: address of LPRINT routine (default=0x34AA).
102  UNIDOS: DOS error return address (default=0x0000).
104  UNIDOS: snapshot workspace (20 bytes).
124  UNIDOS: called on reset (default=0x0000).
126  UNIDOS: called on boot (default=0x21A4).
7667 UNIDOS: set this to 0 to reset DOS.

11. CONNECTORS PINOUTS
======================

The following pinouts are viewed from the back of the interface.

            DISC CONNECTOR
  ------------------------------------   1-33 Ground (0V)   22 Write data
 | 33                             3 1 |     8 Index         24 Write gate
 |  o o o o o o o o o o o o o o o o o |    10 Disk1 select  26 Track00
 |                                    |    12 Disk2 select  28 Write protect
 |  o o o o o o o o o o o o o o o o o |    16 Motor on      30 Read data
 | 34                             4 2 |    18 Step dir.     32 Side select
  ------------------------------------     20 Step pulse

        PRINTER CONNECTOR
   ----------------------------      1 Strobe        13 D5
  | 25                     3 1 |     3 D0            15 D6
  |  o o o o o o o o o o o o o |     5 D1            17 D7
  |                            |     7 D2            21 Input busy
  |  o o o o o o o o o o o o o |     9 D3          2-22 Ground (0V)
  | 26                     4 2 |    11 D4
   ----------------------------

The parallel pinout allows direct connection to a Centronics connector
simply with a flat cable.

12. VL1772 PROGRAMMING INFO
===========================

Here is the low-level technical information about the VL1772 floppy
disk controller. Some devices equipped with this component are MGT DISCiPLE,
MGT PLUS D, MGT SAM COUPE and ATARI ST.
See section 3 for the port addresses of the FDC registers in the DISCiPLE
and the +D.

----------------------
-COMMAND REGISTER (W)-
----------------------

Commands are divided into four classes. The lower 4 bits of the command byte
have a different meaning depending on the command class; remember to OR them
with the command codes given below.

Type 1 commands:
----------------

  b0-b1 = Stepping rate
          00 =  6 ms
          01 = 12 ms
          10 = 20 ms
          11 = 30 ms
     b2 = Verify track
     b3 = Load/unload head at beginning

Command name    Code  Comments
----------------------------------------------------------------------------
RESTORE         0x00  Restore disk head to track 0
SEEK            0x10  Seek a track (send the track number to the DATA reg.)
STEP_NUPD       0x20  Step using current dir without updating track register
STEP_UPD        0x30  Step drive using current direction
STEP_IN_NUPD    0x40  Increase track without updating track register
STEP_IN_UPD     0x50  Increase track
STEP_OUT_NUPD   0x60  Decrease track without updating track register
STEP_OUT_UPD    0x70  Decrease track

Type 2 commands:
----------------

     b0 = f8 (deleted dam) / fb (dam) if set in READ commands
     b1 = Enable side compare
     b2 = 15 ms delay
     b3 = Compare for side 1/0

Command name    Code  Comments
----------------------------------------------------------------------------
READ_1SECTOR    0x80  Read one sector
READ_MSECTOR    0x90  Read multiple sectors
WRITE_1SECTOR   0xA0  Write one sector
WRITE_MSECTOR   0xB0  Write multiple sectors

Type 3 commands:
----------------

  b0-b1 = 0
     b2 = 15 ms delay
     b3 = 0

Command name    Code  Notes
----------------------------------------------------------------------------
READ_ADDRESS    0xC0  Read address
READ_TRACK      0xE0  Read a whole track
WRITE_TRACK     0xF0  Write a whole track

Type 4 commands:
----------------

     b0 = Not ready to read transition
     b1 = Ready to not read transition
     b2 = Index pulse
     b3 = Immediate interrupt, requires reset
  b0-b3 = 0000 -> Terminate with no interrupt

Command name    Code  Notes
----------------------------------------------------------------------------
FORCE_INTERPT   0xD0  Force interrupt

---------------------
-STATUS REGISTER (R)-
---------------------

Some bits assume a different meaning depending on the last command issued.

After a Type 1 command:

Bit   Meaning             Comments
----------------------------------------------------------------------------
 0    BUSY                Wait BUSY=0 for a new command
 1    INDEX PULSE         Index pulse
 2    TRACK00             Signals head on track 00
 3    CRC ERROR           Sector corrupted
 4    SEEK ERROR          Seek error
 5    HEAD LOADED         Head loaded
 6    WRITE PROTECT       Disk is write protected
 7    MOTOR ON            Motor is on or drive not ready

After a Type 2/3 command:

Bit   Meaning             Comments
----------------------------------------------------------------------------
 0    BUSY                Wait BUSY=0 for a new command
 1    DRQ                 Need to send or read data from DATA register
 2    LOST DATA           Error (eg you did not respect I/O timings)
 3    CRC ERROR           Sector corrupted
 4    RECORD NOT FOUND    Non-existent track/sector or no more data to read
 5    REC.TYP/WR.FAULT    Read: record type; Write: write fault
 6    WRITE PROTECT       Disk is write protected
 7    MOTOR ON            Motor is on or drive not ready

---------------------
-TRACK REGISTER (RW)-
---------------------

Contains the current track number.

----------------------
-SECTOR REGISTER (RW)-
----------------------

Current sector number for read/write operations.

--------------------
-DATA REGISTER (RW)-
--------------------

Here you may read and write the data to the controller.