Floppy Disk Controller: Difference between revisions

=== How Many Drives? ===

It is probably wisest to always get the drive count and types from [[CMOS]], register 0x10.

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

If you try to turn off the IRQs in Bochs (to use pure polling PIO mode), Bochs will panic. It may also not be able to handle

non-DMA data transfers properly, even if IRQs are turned on. It is currently mainly designed to model the floppy using DMA.

in all 3 modes.

There are only a few commands/conditions that produce interrupts: a reset (in polling mode only), Seek, Recalibrate, or one of the

read/write/verify/format commands. Several commands do not produce any result bytes, either. If you want to verify that a silent

access STATUS_REGISTER_B on FDC 1, you would use FDC1_BASE_ADDRESS + STATUS_REGISTER_B = 0x370 + 1 = 0x371.

As said above, the most common controller chip has 3 modes, and many bitflags in the registers are different (or opposite!) depending on

the mode. However, all of the important registers and bitflags remain the same between modes.

They are the following:

FIFO: The FIFO register may not have a 16byte buffer in all modes, but this is a minor difference that does not really affect its operation.

TDR: The Tape Drive Register is identical in all modes, but it is useless (you will never find functional equipment that requires it).

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the hardware "untoggles" reset mode automatically after the proper delay.

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

The bottom 2 bits of DSR match CCR, and setting one of them sets the other. The upper 6 bits on both DSR and CCR default to 0, and

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

Note: There is also a 300Kbps (value = 1), and a 250Kbps setting (value = 2) but they are only for utterly obsolete drives/media.

This bit (bit 7, value = 0x80) is fairly useful. It gets set if the floppy door was opened/closed. Sadly, almost all the emulator

programs set and clear this bit completely inappropriately (especially after a reset).

Note2: You must turn on the drive motor bit before you access the DIR register for a selected drive (you do not have to wait

Basically, you want to keep a flag for whether there is media in each drive. If Disk Change is set and there was media,

the OS should get a signal that the previous media was ejected.

this means you should check the value of Disk Change just prior to every Seek command that you do, because otherwise you

will lose any Disk Change information. This is also true for implied seeks, and relative seeks.

== Programming Details ==

# Turn on the drive's motor and select the drive, using an "outb" command to the DOR IO port.

# Issue your command byte plus some parameter bytes (the "command phase") to the FIFO IO port.

# Exchange data with the drive / "seek" the drive heads (the "execution phase"), on the FIFO IO port.

The reason to leave the motor on is that your driver may not know if there is a queue of sector reads or writes

that are going to be executed next. If there are going to be more drive accesses immediately, they won't need to wait

The suggested delays when turning the motor on are:

==== Reinitialization ====

So, when your OS is initializing:

# Send a Version command to the controller.

# Verify that the result byte is 0x90 -- if it is not, it might be a good idea to abort and not support the floppy subsystem. Almost all of the code based on this article will work, even on the oldest chipsets -- but there are a few commands that will not.

# If you don't want to bother having to send another Configure command after every Reset procedure, then:

## send a Lock command.

# Do a Controller Reset procedure.

==== Controller Reset ====

## Bit 2 (value = 4) in the DOR: Save the current/"original" value of the DOR, write a 0 to the DOR, wait 4 microseconds, then write the original value (bit 2 is always set) back to the DOR.

## '''or''' Bit 7 (value = 0x80) in the DSR: Precalculate a good value for the DSR (generally 0), and OR it with 0x80. Write that value to the DSR.

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

Note2: Emulators will often set the Disk Change flag to "true" after a reset, '''but this does not happen on real hardware''' -- it is

a shared bug in all the emulators that do it.

==== Drive Selection ====

command fails -- this may be the reason why.

# Read DIR. If the "Disk Change" bitflag is set to "true", then the floppy drive door was opened, so the OS needs to test if a new disk is in the drive.

# Verify that RQM = 1 and DIO = 0 ((Value & 0xc0) == 0x80) -- if not, reset the controller and start all over.

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

# Either Execution or Result Phase begins when all parameter bytes have been sent, depending on whether you are in PIO mode, and the command has an Execution phase. If using DMA, or the command does not perform read/write/head movement operations, skip to the Result Phase.

# (In PIO Mode Execution phase) read MSR, verify NDMA = 1 ((Value & 0x20) == 0x20) -- if it's not set, the command has no Execution phase, so skip to Result phase.

==== Configure ====

This command initializes controller-specific values: the data buffer "threshold" value, implied seek enable, FIFO disable, polling enable.

If you enable implied seeks, then you don't have to send Seek commands (or Sense Interrupt commands for the Seek commands).

transfer!).

after every reset. Always turn it off, if you send a Configure command.

* Configure command = 0x13

* First parameter byte = 0

* Third parameter byte = precomp_val = 0

* No result bytes.

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

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

# After the completion of a Recalibrate command.

These are the only times when you should send a Sense Interrupt. You should still send them even if you have IRQs turned off in

* Sense Interrupt command = 0x8

Note: if you try to read the result bytes without waiting for RQM to set, then you are likely to always get an incorrect result value

of 0. This is also likely to get your driver out of sync with the FDC for input/output. The correct value of st0 after a reset should

==== Recalibrate ====