7.  Logical Interface 

7.1  General 

7.1.1  Bit Conventions

Bit names are shown in all upper case letters except where a lower case n 
precedes a bit name. This indicates that when nBIT=0 (bit is zero) the action 
is true and when nBIT=1 (bit is one) the action is false. If there is no 
preceding n, then when BIT=1 it is true, and when BIT=0 it is false. 

A bit can be set to one or cleared to zero and polarity influences whether it 
is to be interpreted as true or false:

          True      BIT=1       nBIT=0 
          False     BIT=0       nBIT=1 

7.1.2  Environment

The drives using this interface shall be programmed by the host computer to 
perform commands and return status to the host at command completion. When two 
drives are daisy chained on the interface, commands are written in parallel to 
both drives, and for all except the Execute Diagnostics command, only the 
selected drive executes the command. On an Execute Diagnostics command 
addressed to Drive 0, both drives shall execute the command, and Drive 1 shall 
post its status to Drive 0 via PDIAG-.

Drives are selected by the DRV bit in the Drive/Head Register (see 7.2.8), and 
by a jumper or switch on the drive designating it as either a Drive 0 or as 
Drive 1. When DRV=0, Drive 0 is selected. When DRV=1, Drive 1 is selected. 
When drives are daisy chained, one shall be set as Drive 0 and the other as 
Drive 1. When a single drive is attached to the interface it shall be set as 
Drive 0. 

Prior to the adoption of this standard, some drives may have provided jumpers 
to indicate Drive 0 with no Drive 1 present, or Drive 0 with Drive 1 present.

Throughout this document, drive selection always refers to the state of the 
DRV bit, and the position of the Drive 0/Drive 1 jumper or switch. 

7.2  I/O Register Descriptions 

Communication to or from the drive is through an I/O Register that routes the 
input or output data to or from registers (selected) by a code on signals from 
the host (CS1FX-, CS3FX-, DA2, DA1, DA0, DIOR- and DIOW-). 

The Command Block Registers are used for sending commands to the drive or 
posting status from the drive. 

The Control Block Registers are used for drive control and to post alternate 
status. 

Table 7-1 lists these registers and the addresses that select them. 

Logic conventions are:   A = signal asserted
                         N = signal negated
                         x = does not matter which it is

            TABLE 7-1:  I/O PORT FUNCTIONS/SELECTION ADDRESSES 
 +-------------------------------+-----------------------------------------+
 |         Addresses             |                 Functions               |
 |CS1FX-|CS3FX-| DA2 | DA1 | DA0 |    READ (DIOR-)     |   WRITE (DIOW-)   |
 +------+------+-----+-----+-----+---------------------+-------------------+
 |                                        Control Block Registers          |
 +------+------+-----+-----+-----+---------------------+-------------------+
 |  N   |  N   |  x  |  x  |  x  | Data Bus High Imped | Not used          |
 |  N   |  A   |  0  |  x  |  X  | Data Bus High Imped | Not used          |
 |  N   |  A   |  1  |  0  |  x  | Data Bus High Imped | Not used          |
 |  N   |  A   |  1  |  1  |  0  | Alternate Status    | Device Control    | 
 |  N   |  A   |  1  |  1  |  1  | Drive Address       | Not used          |
 +------+------+-----+-----+-----+---------------------+-------------------+
 |                                        Command Block Registers          |
 +------+------+-----+-----+-----+---------------------+-------------------+
 |  A   |  N   |  0  |  0  |  0  | Data                | Data              |
 |  A   |  N   |  0  |  0  |  1  | Error Register      | Features          |
 |  A   |  N   |  0  |  1  |  0  | Sector Count        | Sector Count      |
 |  A   |  N   |  0  |  1  |  1  | Sector Number       | Sector Number     | 
 |  A   |  N   |  1  |  0  |  0  | Cylinder Low        | Cylinder Low      |
 |  A   |  N   |  1  |  0  |  1  | Cylinder High       | Cylinder High     | 
 |  A   |  N   |  1  |  1  |  0  | Drive/Head          | Drive/Head        |
 |  A   |  N   |  1  |  1  |  1  | Status              | Command           |
 |  A   |  A   |  x  |  x  |  x  | Invalid Address     | Invalid Address   |
 +------+------+-----+-----+-----+---------------------+-------------------+

7.2.1  Alternate Status Register

This register contains the same information as the Status Register in the 
command block. The only difference being that reading this register does not 
imply interrupt acknowledge or clear a pending interrupt. 

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |  BSY  | DRDY  |  DWF  |  DSC  |  DRQ  | CORR  |  IDX  |  ERR  | 
     +-------+-------+-------+-------+-------+-------+-------+-------+

See 7.2.13 for definitions of the bits in this register. 

7.2.2  Command Register 

This register contains the command code being sent to the drive. Command 
execution begins immediately after this register is written. The executable 
commands, the command codes, and the necessary parameters for each command are 
listed in Table 9-1.

7.2.3  Cylinder High Register

This register contains the high order bits of the starting cylinder address 
for any disk access. At the end of the command, this register is updated to 
reflect the current cylinder number. The most significant bits of the cylinder 
address shall be loaded into the cylinder high Register. 

..rm102
  NOTE: Prior to the introduction of this standard, only the lower 2 bits of 
        this register were valid, limiting cylinder address to 10 bits i.e. 
        1,024 cylinders.

7.2.4  Cylinder Low Register

This register contains the low order 8 bits of the starting cylinder address 
for any disk access. At the end of the command, this register is updated to 
reflect the current cylinder number. 

7.2.5  Data Register

This 16-bit register is used to transfer data blocks between the device data 
buffer and the host. It is also the register through which sector information 
is transferred on a Format command. Data transfers may be either PIO or DMA. 

7.2.6  Device Control Register

The bits in this register are as follows: 

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   x   |   x   |   x   |   x   |   1   | SRST  |  nIEN |   0   | 
     +-------+-------+-------+-------+-------+-------+-------+-------+

 - SRST is the host software reset bit. The drive is held reset when this bit 
   is set. If two disk drives are daisy chained on the interface, this bit 
   resets both simultaneously. Drive 1 is not required to execute the DASP- 
   handshake procedure.
 - nIEN is the enable bit for the drive interrupt to the host. When nIEN=0, 
   and the drive is selected, INTRQ shall be enabled through a tri-state 
   buffer. When nIEN=1, or the drive is not selected, the INTRQ signal shall 
   be in a high impedance state. 

7.2.7  Drive Address Register

This register contains the inverted drive select and head select addresses of 
the currently selected drive. The bits in this register are as follows: 

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |  HiZ  |  nWTG |  nHS3 |  nHS2 |  nHS1 |  nHS0 |  nDS1 |  nDS0 | 
     +-------+-------+-------+-------+-------+-------+-------+-------+

 - HiZ shall always be in a high impedance state. 
 - nWTG is the Write Gate bit. When writing to the disk drive is in progress, 
   nWTG=0.
 - nHS3 through nHS0 are the one's complement of the binary coded address of 
   the currently selected head. For example, if nHS3 through nHS0 are 1100b, 
   respectively, head 3 is selected. nHS3 is the most significant bit. 
 - nDS1 is the drive select bit for drive 1. When drive 1 is selected and 
   active, nDS1=0. 
 - nDS0 is the drive select bit for drive 0. When drive 0 is selected and 
   active, nDS0=0.

..rm102
  NOTE: Care should be used when interpreting these bits, as they do not 
        always represent the expected status of drive operations at the 
        instant the status was put into this register. This is because of the 
        use of cacheing, translate mode and the Drive 0/Drive 1 concept with 
        each drive having its own embedded controller. 

7.2.8  Drive/Head Register

This register contains the drive and head numbers. The contents of this 
register define the number of heads minus 1, when executing an Initialize 
Drive Parameters command. 

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   1   |   0   |   1   |  DRV  |  HS3  |  HS2  |  HS1  |  HS0  | 
     +-------+-------+-------+-------+-------+-------+-------+-------+

 - DRV is the binary encoded drive select number. When DRV=0, Drive 0 is 
   selected. When DRV=1, Drive 1 is selected.
 - HS3 through HS0 contain the binary coded address of the head to be selected 
   e.g. if HS3 through HS0 are 0011b, respectively, head 3 will be selected. 
   HS3 is the most significant bit. At command completion, this register is 
   updated to reflect the currently selected head. 

7.2.9  Error Register 

This register contains status from the last command executed by the drive or a 
Diagnostic Code. 

At the completion of any command except Execute Drive Diagnostic, the contents 
of this register are valid when ERR=1 in the Status Register. 

Following a power on, a reset, or completion of an Execute Drive Diagnostic 
command, this register contains a Diagnostic Code (see Table 9-2). 

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     | BBK   |  UNC  |   0   |  IDNF |   0   | ABRT  | TK0NF |  AMNF |
     +-------+-------+-------+-------+-------+-------+-------+-------+

 - BBK (Bad Block Detected) indicates a bad block mark was detected in the 
   requested sector's ID field. 
 - UNC (Uncorrectable Data Error) indicates an uncorrectable data error has 
   been encountered. 
 - IDNF (ID Not Found) indicates the requested sector's ID field could not be 
   found. 
 - ABRT (Aborted Command) indicates the requested command has been aborted due 
   to a drive status error (Not Ready, Write Fault, etc.) or because the 
   command code is invalid. 
 - TK0NF (Track 0 Not Found) indicates track 0 has not been found during a 
   Recalibrate command. 
 - AMNF (Address Mark Not Found) indicates the data address mark has not been 
   found after finding the correct ID field. 
 - Unused bits are cleared to zero. 

7.2.10  Features Register

This register is command specific and may be used to enable and disable 
features of the interface e.g. by the Set Features Command to enable and 
disable cacheing. 

This register may be ignored by some drives. 

Some hosts, based on definitions prior to the completion of this standard, set 
values in this register to designate a recommended Write Precompensation 
Cylinder value. 

7.2.11  Sector Count Register

This register contains the number of sectors of data requested to be 
transferred on a read or write operation between the host and the drive. If 
the value in this register is zero, a count of 256 sectors is specified. 

If this register is zero at command completion, the command was successful. If 
not successfully completed, the register contains the number of sectors which 
need to be transferred in order to complete the request.

The contents of this register may be defined otherwise on some commands e.g. 
Initialize Drive Parameters, Format Track or Write Same commands. 

7.2.12  Sector Number Register

This register contains the starting sector number for any disk data access for 
the subsequent command. The sector number may be from 1 to the maximum number 
of sectors per track. 

See the command descriptions for contents of the register at command 
completion (whether successful or unsuccessful).

..Typically, when a read or write command completes successfully, the value 
..in the register is the last sector number accessed modulo the number of 
..sectors per track. Typically, if a command does not complete successfully, 
..the register contains the sector number at which an error occurred on a 
..multiple sector transfer. 
..
7.2.13  Status Register

This register contains the drive status. The contents of this register are 
updated at the completion of each command. When BSY is cleared, the other bits 
in this register shall be valid within 400 nsec. If BSY=1, no other bits in 
this register are valid. If the host reads this register when an interrupt is 
pending, it is considered to be the interrupt acknowledge. Any pending 
interrupt is cleared whenever this register is read. 

..rm102
  NOTE: If Drive 1 is not detected as being present, Drive 0 clears the Drive 
        1 Status Register to 00h (indicating that the drive is Not Ready).

         7       6       5       4       3       2       1       0 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |  BSY  | DRDY  |  DWF  |  DSC  |  DRQ  | CORR  |  IDX  |  ERR  | 
     +-------+-------+-------+-------+-------+-------+-------+-------+

 - BSY (Busy) is set whenever the drive has access to the Command Block 
   Registers. The host should not access the Command Block Register when 
   BSY=1. When BSY=1, a read of any Command Block Register shall return the 
   contents of the Status Register. This bit is set by the drive (which may be 
   able to respond at times when the media cannot be accessed) under the 
   following circumstances: 
   a) within 400 nsec after the negation of RESET- or after SRST has been set 
      in the Device Control Register. Following acceptance of a reset it is 
      recommended that BSY be set for no longer than 30 seconds by Drive 1 and 
      no longer than 31 seconds by Drive 0.
   b) within 400 nsec of a host write of the Command Register with a Read, 
      Read Long, Read Buffer, Seek, Recalibrate, Initialize Drive Parameters, 
      Read Verify, Identify Drive, or Execute Drive Diagnostic command. 
   c) within 5 usecs following transfer of 512 bytes of data during execution 
      of a Write, Format Track, or Write Buffer command, or 512 bytes of data 
      and the appropriate number of ECC bytes during the execution of a Write 
      Long command. 
 - DRDY (Drive Ready) indicates that the drive is capable of responding to a 
   command. When there is an error, this bit is not changed until the Status 
   Register is read by the host, at which time the bit again indicates the 
   current readiness of the drive. This bit shall be cleared at power on and 
   remain cleared until the drive is ready to accept a command. 
 - DWF (Drive Write Fault) indicates the current write fault status. When an 
   error occurs, this bit shall not be changed until the Status Register is 
   read by the host, at which time the bit again indicates the current write 
   fault status. 
 - DSC (Drive Seek Complete) indicates that the drive heads are settled over a 
   track. When an error occurs, this bit shall not be changed until the Status 
   Register is read by the host, at which time the bit again indicates the 
   current Seek Complete status. 
 - DRQ (Data Request) indicates that the drive is ready to transfer a word or 
   byte of data between the host and the drive.
 - CORR (Corrected Data) indicates that a correctable data error was 
   encountered and the data has been corrected. This condition does not 
   terminate a data transfer. 
 - IDX (Index) is set once per disk revolution. 
 - ERR (Error) indicates that an error occurred during execution of the 
   previous command. The bits in the Error Register have additional 
   information regarding the cause of the error. 

