Date   : Sat, 12 Jun 1999 17:57:43 +0100
From   : Tom <T.W.Seddon@...>
Subject: Re: BBC 8271 commands

Michael Foot wrote:
> FE80 read status register, write command
> FE81 read result register, write parameter
> FE84 read/write data register

These are correct, according to The Advanced Disk User Guide at least.
But there is also the reset register accessible at &FE82 (write-only).
This is roughly what the ADUG has to say: (ignoring the bits about Tube
compatibility)

Reset

The reset command differs from all others. It has a two-byte opcode, &01
followed by &00, which are sent in succession to the Reset Register
rather than the Command Register.

The Reset command is exactly equivalent to the RESET input pin on the
8271. Note that the 8271 needs time to process the first byte (&01)
before it receives the second (&00). A minimum of 10 clock cycles (NOP x
5) is required.

The reset command causes the following to happen:

* all output signals to the disk drive are forced low
* any command in progress is halted
* the status register is zeroised [sic]
* the 8271 goes idle
* the head is unloaded

Apropos of little I remembered the other day that there was a book
giving a commented disassembly of the OS 1.20 ROM. Does anybody have any
details of this book, or is my mind playing tricks again? It sounds like
a really useful book, and would also be a handy source for the OS ROM
:-)

-- 
--Tom
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