Floppy Disk Controller: Difference between revisions

in/out of the FIFO buffer -- or wait for an IRQ6 to do the same thing. When the transfer is complete, read the "result" bytes to see

if there were any errors. See below for more detail.

=== There are 3 "Modes" ===

can just ignore the other 2 modes. Or you can try to handle all three, by only using registers and commands that are identical

in all 3 modes. The mode does not affect the driver code much, except for detecting "Disk Change".

== Registers ==

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Most important is RQM, which is set when it is OK (or necessary!) to read/write data from/to the FIFO port.

DIO and BUSY should be checked to verify proper command termination (the end of "result phase" and beginning of "command phase").

can always be set to zero safely. So, even though they have different bit definitions, you always set them with identical values, and

one or the other of DSR and CCR can be ignored in any modern system.

The bottom 2 bits specify the data transfer rate to/from the drive. You want both bits set to zero for a 1.44MB floppy drive.

Note: a reset procedure does not affect this register. However, if you have drives of different types on the bus which

# Issue your command byte plus some parameter bytes (the "command phase").

# Exchange data with the drive / "seek" the drive heads (the "execution phase").

# Read any "result bytes" produced by the command (the "result phase").

# The commands "Recalibrate", "Seek", and "Seek Relative" require an additional "Sense Interrupt" command to be sent.

# Wait for the resulting IRQ6.

# Send a Sense Interrupt command (required).

# Do a Drive Select procedure for the next drive to be accessed.

# Send the correct Datarate setting to CCR. Usually this is a 0 (1.44MB floppy drive).

# If the newly selected drive is a different type than the previously selected drive (or changing from PIO to DMA mode), send a new Specify command.

# Set the "drive select" bits (and the other bitflags) in DOR properly, including possibly turning on the Motor bit for the drive (if it will be accessed soon).

# Send your chosen command byte to the FIFO port (port 0x3F5).

# In a loop: loop on reading MSR until RQM = 1, then send the next parameter byte for the command to the FIFO port.

# begin a loop:

# Either poll MSR until RQM = 1, or wait for an IRQ6, using some waiting method.

# In an inner loop: transfer a byte in or out of the FIFO port via a system buffer, then read MSR. Repeat while RQM = 1 and NDMA = 1 ((Value & 0xa0) == 0xa0).

# if NDMA = 1, loop back to the beginning of the outer loop, unless your data buffer ran out (detect underflow/overflow).

## In a loop: read the next result byte from the FIFO, loop on reading MSR until RQM = 1, verify CMD BSY = 1 and DIO = 1 ((Value & 0x50) == 0x50).

# After reading all the expected result bytes: check them for error conditions, verify that RQM = 1, CMD BSY = 0, and DIO = 0. '''If not''' retry the

* Third parameter byte = precomp_val = 0

* No result bytes.

==== Version ====

* Version command = 0x10

* No parameter bytes.

* First result byte = 0x90

* '''or''' Unlock command = 0x14

* No parameter bytes.

* First result byte = lock bit << 4

* Second parameter byte = HLT_value << 1 | NDMA

* No result bytes.

===== SRT, HLT and HUT =====

bitflag in the controller. It is required in four circumstances that produce interrupts.

# After doing a Controller Reset procedure (you may need to send 4 of them, then).

# After the completion of a Seek command (or Relative Seek).

# After the completion of a Recalibrate command.

the DOR and you are using polling instead.

* Sense Interrupt command = 0x8

* First result byte = [[#st0|st0]]

* Second result byte = controller's idea of the current cylinder

The command will only try a maximum of 80 head assembly steps. So, theoretically, it may take several Recalibrates to move the head back

* Recalibrate command = 0x7

* First parameter byte = drive number = 0 to 3.

* No result bytes.

It is possible to poll the "disk active" bits in the MSR to find out when the head movement is finished.

A Sense Interrupt command is required after this command completes, to clear it from being BUSY. (Multiple Sense Interrupts,

if you ran multiple simultaneous Recalibrates.)

* Second parameter byte = requested cylinder number

* No result bytes.

It is possible to poll the "disk active" bits in the MSR to find out when the head movement is finished.

* First parameter byte = (Drive 3 enable << 5) | (Drive 2 enable << 4) | (Drive 1 enable << 3) | (Drive 0 enable << 2)

* No result bytes.

You also need to set CCR/DSR for the 1M datarate (value = 3) to access a 2.88M drive.