Disk Controller

The disk controller consists of a ROM that adds disk commands to Extended Color BASIC, a disk controller chip, and a little glue to make it all work.

The disk controller chip is a Western Digital 1793 (or 1773?), and has four registers which can be accessed at addresses $FF48 through $FF4B inclusive. There is also one control register at $FF40 that is not part of the controller chip, that is used to enable the drive motors, select lines, and so on. Here is a map:

In general, you make the disk controller execute a command by writing a command byte into the Command register, and see the results by reading the Status register. During the execution of a read command, you have to load data bytes from the data register, where they appear as they come from the disk; during a write, you have to put bytes into the data register from where they will be written onto the disk. The speed of reads and writes is constant; if your program does not read or write bytes into the Data register quickly enough, the chance is gone, and the command will not complete successfully.

Control Register

This is not part of the controller chip, but is part of the "glue" that makes it all fit together.

You can force the drive motor to turn off by writing a zero into this. You can also force the motor on by writing non-zero there, but the documentation doesn't explain which bits do what. Don't panic; more details are on the way.

This register is write-only; you can't tell what value was last written there just by reading it. So Disk BASIC probably keeps a copy in somewhere of each value that it writes here. This means that if your programs write here, it might confuse BASIC, because the last thing it put here is no longer true. Be careful about that.

Since this register is not part of the disk controller chip, there are other things to be careful of as well. For instance, the disk controller's Track register normally contains the track number at which the disk drive head is positioned. But if you have more than one disk drive, you can switch which one you are using by writing some value into this Control register. The disk controller does not know that it is now talking to a new disk drive, and the head position of the new disk drive may be different from the old one. So the controller might think it is at track 10, where the old drive was. But maybe the new drive's head is over track 23. As soon as the controller tries to execute any read or write command, it will notice that the data coming from the disk drive is claiming track 23, instead of the 10 it expected. So the controller will return an error instead of executing the command.

more to come...

Track and Sector registers

The Track and Sector registers just hold track and sector numbers. The Track register normally reflects the current position of the head; you don't normally write into it. To get to a track you use the Seek command and put the desired track number in the data register. When that command has completed successfully, the track number will be in the Track register. In contrast, the Sector register is used to tell the controller which sector you want; you write into it.

You can, of course, write into the Track register. But if it doesn't match the position of the head, the controller will produce an error if you try to read or write a sector - it notices that it is not at the track it expected.

Still, there is a good reason to write into the Track register if you have more than one disk drive. Whenever you select a new disk drive by writing into the Control register, you may want to update the controller's Track register with the position of the new drive.

Data Register

more to come...

Command/Status Register

Writing into this register gives a command to the disk controller chip. Reading from it tells you the status of the command's execution. In effect, this is two registers sharing the same address; the Command register is write-only, and the Status register is read-only.

Command bits

There are four types of disk commands.
Type I - Restore, Seek, Step, Step In, and Step Out.
Type II - Read Sector and Write Sector.
Type III - Read Track, Write Track, and Read Address.
Type IV - Force Interrupt.

more to come...

Status bits

Bits in the error code are defined as follows. (This info comes from the 1793 data sheet.) The 1793 data sheet is more terse than the 1771 data sheet which was mistakenly refernced before, so I've filled in some missing descriptions in italics, like this; some from the 1771 data sheet and others that just seem obvious. Take those additions with a grain of salt, although I *think* they are right.

Note that some of the bits have different meanings based on which type of command caused them to be set. The status word is defined only for commands of type I, II, and III. The status of a type IV command, Force Interrupt, depends on the command that was interrupted.

Signals are named from the perspective of the 1793 disk controller chip, so "input" means input to that chip from either the computer or the disk drive, and "output" means an output from the 1793 to one of those. Singal names preceded by an asterisk "*" indicate that the signal is active-low, or inverted, so that "0" means true and "1" means false.

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24-Jun-99
yakowenk@csx.unxc.edu
(remove all "x"s to get a valid address)