TD_RAWREAD/ETD_RAWREAD - read raw data from the disk.

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. Commodore-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_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).

If the IOTDB_WORDSYNC bit is set it will use the WORDSYNC ability to sync on $4489. Sorry, you cannot

specify the sync pattern. You may set both INDEXSYNC and WORDSYNC, in which case it will do index sync first, then start a wordsync read.

io_Length

Length of buffer in bytes, with a maximum of 32766 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_Error

0 for success, or an error code as defined in <devices/trackdisk.h>

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.

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

TD_RAWWRITE