Drive programming info by Cadaver, Ninja & DocBacardi

This rant summarizes some information about building IRQ-loaders with multi-
drive support. 1541,1571,1581,CMD FD,CMD HD all have a 6502-processor, so
very similar drivecode can be used in all of them.

0. What section of drivememory to use?


All drives have useable buffers from $0300 to $06ff

1. How to read sectors (by jobcodes)


The 'read' jobcode is the same for all drives, only the queue and
track/sector list differ. I can't remember how it worked on the 1581 and FD.
But at least for the HD it is special. It has 2 emulation mode job queues at
the 1541's and 1581's location, but both do not work with native partitions.
Only the 'native modus' queue works with all partition types:

job | track | sector | buffer
$20 | $2800 | $2801  | $0300
$21 | $2802 | $2803  | $0400
$22 | $2804 | $2805  | $0500
$23 ...

I think for 1581 and FD the jobs started at $02 and trk/sec at $0b/$0c, but I
have to check this first. (confirmed)


The 1581 and CMD FD/HD have an "execute job" routine in their ROM, which
is called in the following way, and will return job result in the
accumulator. Let's assume buffer 1 ($0400-$04ff) is being used.

(Common for all drives)

  ldx #$01                ;Buffer 1 job
  lda #$80                ;Job code: read sector

(For 1581)

  jsr $ff54               ;Execute job

(For CMD FD)

  jsr $ff54               ;Execute job
  lda $03                 ;Check returncode: because of firmware bug it
                          ;isn't returned in A!!! (Ninja)
(For CMD HD)

  jsr $ff4e ;Execute job

(For 1541)

1541/1571 need some additional operations, for example handling the
disk ID in case of disk change. Additionally I like to flash the led :)
This is what my 1541 job exec code in the loader is currently like,
it is called with $80 in the accumulator, and X isn't really needed:

  lda $1c00               ;Led on
  ora #$08
  sta $1c00
  cli                     ;Allow interrupts & execute command
  stx $01
  lda $01                 ;Poll for job completion &
  bmi wait                ;read returncode
  lda $16                  ;Handle disk ID change
  sta $12
  lda $17
  sta $13
  lda $1c00               ;Led off
  and #$f7
  sta $1c00

On all drives, if return value in accumulator is $02 or greater, an error of
some kind has occurred. $00 and $01 indicate sector was read OK. Also remember
to forbid interrupts after sector read so that your data transfer will work 

2. Where to find the directory?


1570 and 71 are like the 1541, the dir always starts at 18/1. All other
drives can have more than one directory, either in a 1581 subpartition or a
cmd subdirectory. But the dos keeps track of the directory start:

      | track | sector |
1541  | #$12  | #$01   |
1570  |       |        |
1571  |       |        |
1581  | $022B | #$03   |
FD    | $54   | $56    |
HD    | $2BA7 | $2BA9  |

3. How the serial port ($1800) works on different drives?


The bits are the same, only the location differs:

1541 : $1800
1570 : $1800
1571 : $1800
1581 : $4001
FD   : $4001
HD   : $8000

4. Where and how to detect the drive


I shamelessly ripped some code from DreamLoad to identify drive type, you
can see it at "drv_detect" label in the drivecode source. Basically this
involves searching for certain identifying values from the diskdrive ROM.

DreamLoad likes to do drive detection on the C64 side by peeking into drive
memory with "M-R" commands, but I do it directly on the diskdrive: the
detection routine is called when the drivecode first starts up, and it patches
the drivecode to use correct serial port address ($1800 access), zeropage
tempvariable, directory track/sector location, jobcode execution routine etc.

In my loader, the basic logic of the drivecode, and data transfer between
C64<->drive stays the same on all drives. DreamLoad does some more advanced
things, so it actually uploads different code to the diskdrive based on the
drive type.

5. Data transfer methods


Because 1581, CMD FD & CMD HD are 2MHz drives, you can't just rely on the
traditional 2-bit transfer routine (use both CLK & DATA lines for data transfer,
synchronize only at the beginning of a byte) unless you adjust the delays.
But there are 2 other methods you can use, that work with whatever drive speed.

1) Use 1-bit transfer. This is what the CovertBitops loader does. Disadvantage
   is speed.

2) Use ATN line to sync transfers, and CLK & DATA for the data. This is what
   DreamLoad uses. Downside is that only 1 drive can be connected on the serial
   bus while doing so.

6. Additional info

For further reference, AAY1541 & 1581 by The Dreams

and DreamLoad source code are recommended

as well as the CovertBitops Loadersystem this rant is originally based on