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 Devices:
 trackdisk.device
  CMD_CLEAR
  CMD_READ
  CMD_UPDATE
  CMD_WRITE
  TD_ADDCHANGEINT
  TD_CHANGENUM
  TD_CHANGESTATE
  TD_EJECT
  TD_FORMAT
  TD_GETDRIVETYPE
  TD_GETGEOMETRY
  TD_GETNUMTRACKS
  TD_MOTOR
  TD_PROTSTATUS
  TD_RAWREAD
  TD_RAWWRITE
  TD_REMCHANGEINT
  TD_SEEK
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GuruMeditation
Docs » Autodocs » trackdisk.device » TD_RAWREAD

NAME

	TD_RAWREAD/ETD_RAWREAD -- read raw data from the disk.

FUNCTION

	These commands read a track of raw data from disk and deposits it in
the provided buffer. The data is taken straight from the disk with
no processing done on it. It will appear exactly as the bits come out
off the disk, hopefully in some legal MFM format.
This interface is intended for sophisticated programmers only.
Amiga reserves the right to make enhancements to the disk
format in the future. We will provide compatibility via the
CMD_READ and ETD_READ commands, anyone using TD_RAWREAD is
bypassing this upwards compatibility, and may thus stop working.
IO REQUEST INPUT
io_Device preset by the call to OpenDevice()
io_Unit preset by the call to OpenDevice()
io_Command TD_RAWREAD or ETD_RAWREAD.
io_Flags if the IOTDB_INDEXSYNC bit is set then the driver
will make a best effort attempt to start reading
from the index mark. Note that there will be at
least some delay, and perhaps a great deal of delay
(for example if interrupts have been disabled).
io_Length Length of buffer in bytes, with a maximum of 32768
bytes.
io_Data Pointer to CHIP memory buffer where raw track data is
to be deposited.
io_Offset The number of the track to read in.
iotd_Count (ETD_RAWREAD only) maximum allowable change counter
value.
IO REQUEST RESULT
io_Error - 0 for success, or an error code as defined in
<devices/trackdisk.h>

NOTES

	The track buffer provided MUST be in CHIP memory
There is a delay between the index pulse and the start of bits
coming in from the drive (e.g. dma started). This delay
is in the range of 135-200 microseconds. This delay breaks
down as follows: 55 microsecs is software interrupt overhead
(this is the time from interrupt to the write of the DSKLEN
register). 66 microsecs is one horizontal line delay (remember
that disk IO is synchronized with agnus' display fetches).
The last variable (0-65 microsecs) is an additional scan line
since DSKLEN is poked anywhere in the horizontal line. This leaves
15 microsecs unaccounted for... Sigh.
In short, You will almost never get bits within the first 135
microseconds of the index pulse, and may not get it until 200
microseconds. At 4 microsecs/bit, this works out to be between
4 and 7 bytes of user data of delay.

BUGS

	This command does not work reliably under versions of Kickstart
earlier than V36, especially on systems with 1 floppy drive.

SEE ALSO

TD_RAWWRITE()

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