Scor­pio News

   

January–March 1987 – Volume 1. Issue 1.
Page 28 of 63

bytes indicated by the sector length byte. Finally, the two CRC bytes are written to the disk followed by one byte of ones (0FFH)

In the Gemini DDDS, QDSS and QDDS formats, there are ten sectors per track and consequently the pattern of ID blocks and data area will be repeated ten times on the track but with differing sector numbers.

Normally, the memory image of the track to be written during formatting would look something like the given below; the values given conform to the IBM System 34 format. When using the FD1797 FDC, the index mark is not required and so Gemini have left in out together with pre-index gap (gap 4) and the 0F6H bytes. In the example memory images given, the centre column contains the data sent to the FDC while the right hand column shows the data written to the disk surface.

IBM System 34 Format

Number
of bytes		Hex value
to FDC		byte sent
to disk

804E4E
120000
3F6C2
1FCFC; Index mark
504E4E

*120000
*3F5A1; Resets CRC generator
*1FEFE; ID Address mark
*1Track NoTrack No
*1Side NoSide No
*1Sector NoSector No
*10101; Sector Length (256 bytes)
*1F7CRC1
*CRC2
*224E4E
*120000
*3F5A1; Resets CRC generator
*1FBFB; Data Address mark
*256E5E5; Empty Data area
*1F7CRC1
*CRC2
*544E4E

**5984E4E

*Write this field 26 times
**Continue writing 4EH until next index pulse received
(Physical end of track).

By contrast, the track format for the Gemini QDDS format is given below:

Gemini QDDS Format

Number
of bytes		Hex value
to FDC		byte sent
to disk

324E4E

*120000
*3F5A1; Resets CRC generator
*1FEFE; ID Address mark
*1Track NoTrack No
*1Side NoSide No
Page 28 of 63