118 lines
4.6 KiB
Plaintext
118 lines
4.6 KiB
Plaintext
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Disk Drives - what YOU are doing wrong!
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* Copyright 1988 Commodore-Amiga, Inc.
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* This information is provided "as is"; no warranties are made. All
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* use is at your own risk. No liability or responsibility is assumed.
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* Permission granted to reproduce, provided this notice remains.
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A distressing number of our third party hardware and software
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developers have been using the floppy disk drives in an incorrect
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manner. If you use the trackdisk.device you are safe. If you go
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directly to the hardware, or build hardware, then this article is
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for you.
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For Hardware types:
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1> The disk drive light should not flash on and off during
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access. Our drive activity light reflects the state of the
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motor. Typically the LED signal is driven by IN_USE (pin 4).
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2> For compatibility with future systems, we require that drives
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refuse to step past track zero. That is, if the head is already
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at zero, and an outward step is received, it should not move the
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head. The drive must still reset it's "DISKCHANGE" latch,
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however.
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3> The critical specifications for our 90mm (3.5") drives are:
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3ms track-to-track. 15ms settling time. >80% radial alignment
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using a Dysan Alignment disk. 500ms motor spinup. 800ms maximum
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power on delay.
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For Software types:
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This part is primarily directed at people who write custom
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boot-loaders for game software. Due to defective loaders, there
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are thousands of Amiga owners who can't load some games, and will
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NEVER BE ABLE TO BUY THEM.
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The ultimate source for information on drive timing comes from
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the manufacturer's specifications. This article simply
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highlights the most misused points.
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1> Don't make bad assumptions. For example; if you depend on the
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motor being on, turn it on before use. Don't assume that your
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boot code will be entered with the disk drive or system in any
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reliable state.
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2> **NEVER** use a loop like this for timing:
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move.w #$1000,D0
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busy_wait dbra d0,busy_wait
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This fails to produce accurate timing under a large number of
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circumstances. The speed of the above loop depends on what CPU
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is installed in the system, what video mode is selected, what type of memory
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the program is in, in what relation to vertical
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blank the code executes in, what the blitter is doing, what
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interrupts are enabled and more other factors than you want to
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think about.
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The 8520 chip provides fast, easy timing. See the companion
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article entitled "How to waste time".
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3> The STEP line must be used as a low-going pulse. The
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direction must be set up FIRST, with a separate write to the
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register. A typical use would be:
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or.b #%00000010,$bfd100 ;Set up direction
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and.b #%11111110,$bfd100 ;Pulse low
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nop ;Wait a bit
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nop ; " " "
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or.b #%00000001,$bfd100 ;Set it high again
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;-- now wait 3 miliseconds for the head to
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;-- get to the next track
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We specify that our drives must get to the next track within 3
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miliseconds. Some drives will step considerably faster, others
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will fail at or before 2.8 miliseconds. When the direction of
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step is changed, the settling time must also be added (a total
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minimum delay of 18 miliseconds).
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Note that the TRACK ZERO sensor will not be valid until the head
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actually reaches the track.
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4> When turning on the motor, wait for the READY signal to go low
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before reading or writing (steps are ok before then). Note that
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READY is only valid when the motor signal is ON.
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5> To determine if a disk is in the drive, look at the DISKCHANGE
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signal. If it is low,the disk has been removed (and possibly
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inserted again) since the last check. Step the head to reset the
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latch and examine the current state.
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6> Some code uses an extra track or two for storage or copy
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protection. We will not guarantee that our drives will have more
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than the normal 80 tracks. We will say that using one extra
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track is rather safe, two tracks is probably ok, and three tracks
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is a very bad idea.
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7> After a disk write DMA has finished, a delay of 1.2
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miliseconds is required before any other operations (drive
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select, step, head change, etc.). The type of disk drives we use
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have a gap between the erase head and the read-write head. The
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disk drive keeps the erase head enabled after the end of write gate to
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compensate for the gap. Failure wait out the delay may
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result in writing over innocent data on other tracks or sides.
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-Bryce Nesbitt
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