Seagate STT8000A - Manual (Page 60)

Chapter 5 ATAPI Interface
Page 52 STT8000A Product Manual
Read command
Bit
Byte
7 6 5 4 3 2 1 0
0 Operation code (08h)
1 Reserved 1 (Fixed)
2 (MSB)
3 Transfer Length
4 (LSB)
5 Reserved
The Transfer Length specifies the number of fixed 512 byte blocks to be transferred.
A transfer length of 0 indicates that no data is to be transferred but will initiate a
read-ahead.
The DSC bit is reset (0) after this command is accepted and is set (1) when at least
52 blocks in the buffer are available for the next READ command. Issuing a READ
command when the DSC bit is reset (0) will keep the ATA bus busy (BSY = 1) while
the drive is reading the required data into the buffer. This delay could be more than
a minute if exhaustive retries are required to read the data.