Amiga Developer CD 2.1/Amiga Developer CD v2.1.iso/Reference/Amiga_Mail_Vol1/Trackdisk/AmigaFloppy

Text File | 1999-10-27 | 5.4 KB | 134 lines

(c)  Copyright 1989-1999 Amiga, Inc.   All rights reserved.
The information contained herein is subject to change without notice, and
is provided "as is" without warranty of any kind, either expressed or implied.
The entire risk as to the use of this information is assumed by the user.



                          Amiga Floppy Drives

                           by Bryce Nesbitt


Some Amiga products that make direct use of the floppy disk drive hardware
do not work correctly.  As a general rule, using the trackdisk.device is
the best way to insure compatability.  But if you need to use the drive
hardware at a lower level, here are some pointers for correct usage.


Hardware

Hardware vendors that make floppy drives for the Amiga (or products
that interact with the floppy drives) should follow these guidelines:

1> The disk drive light should not flash on and off during
   access.  The Amiga drive activity light reflects the state of the
   motor.  Typically the LED signal is driven by IN_USE (pin 4).

2> Second drives for the A2000 must take MOTOR_ON from pin 6
   (normally DRIVE_SELECT_3).  Most drives have a jumper block
   that allows DRIVE_SELECT_3 to control the motor and, in turn, the
   activity light.  The current twisted ribbon cable used on the
   A2000 also connects the motor signal to IN_USE (pin 4).

   To convert an A2000 drive zero to drive one requires changing
   two jumpers.  The drive must respond to DRIVE_SELECT_1 (instead
   of DRIVE_SELECT_0) and it must take it's motor from DRIVE_SELECT_3
   (instead of MOTOR_ON).

3> For compatibility with future systems, disk drives MUST
   refuse to step past track zero.  That is, if the head is already
   at zero and an outward step is received, it should not move the
   head.  The drive must still reset it's DISKCHANGE latch, however.

4> The critical specifications for Amiga 90mm (3.5") drives are:
   3ms track-to-track, 15ms settling time,  >80% radial alignment
   using a Dysan Alignment disk, 500ms motor spinup and 800ms maximum
   power on delay.



Software

This section of the article is for those of you who write custom
boot-loaders for Amiga game software.  Because of defective loaders,
there are many Amiga owners unable to load certain games.  This will
lead some of these consumers to never again buy a program from that
product line.  Here are some tips to help you avoid problems with
custom boot-loaders.


1> Don't make bad assumptions.  For example, if you depend on the
   motor being on, turn it on before use.  Don't assume that your
   boot code will be entered with the disk drive or system in any
   known state.


2> Never use a busy-wait loop like this for timing:

        move.w    #$1000,D0
   busy_wait    dbra        d0,busy_wait

   This fails to produce accurate timing under a large number of
   circumstances.  The speed of the above loop depends on what CPU
   is installed in the system, what video mode is selected, what
   type of memory the program is in, the position of the vertical
   blank at run time, what the blitter is doing, what interrupts are
   enabled and many other factors.  Instead of a busy-wait loop, use
   the 8520 chip for timing as shown in the Amiga Mail article,
   "How to Waste Time".

3> The STEP line must be used as a low-going pulse.  The
   direction must be set up first, with a separate write to the
   register.  A typical use would be:

        or.b   #%00000010,$bfd100    ;Set up direction
        and.b  #%11111110,$bfd100    ;Pulse low
        nop                ;Wait a bit
        nop                ; "   "  "
        or.b   #%00000001,$bfd100    ;Set it high again
        ;-- now wait 3 milliseconds for the head to
        ;-- get to the next track

   We specify that our drives must get to the next track within 3
   milliseconds.  Some drives will step considerably faster.  Others
   will fail at or before 2.8 milliseconds.  When the direction of
   step is changed, the settling time must also be added - a total
   minimum delay of 18 milliseconds.

   Note that the TRACK ZERO sensor will not be valid until the head
   actually reaches the track.


4> When turning on the motor, wait for the READY signal to go low
   before reading or writing.  Steps are okay before then.  Note that
   READY is only valid when the motor signal is ON.


5> To determine if a disk is in the drive, look at the DISKCHANGE
   signal.  If it is low, the disk has been removed (and possibly
   inserted again) since the last check.  Step the head to reset the
   latch and examine the current state.


6> Some code uses an extra track or two for storage for copy
   protection.  We do not guarantee that our drives will have more
   than the normal 80 tracks.  We do guarantee however that using three
   extra tracks will fail.


7> After a disk write DMA has finished, a delay of 1.2 milliseconds
   is required before any other operations (drive select, step, head
   change, etc.).  The type of disk drives we use  have a gap between
   the erase head and the read-write head.  The disk drive keeps the
   erase head enabled after the end of write gate to compensate for
   the gap.  Failure to wait out the delay may result in writing over
   innocent data on other tracks or sides.


The ultimate source for information on drive timing comes from the
manufacturer's specifications.  This article simply highlights the most
misused points.  Try to use the trackdisk.device software to avoid
hardware dependencies but if you need more direct control of floppy
hardware, follow the guidlines given here.