Scor­pio News


January–March 1987, Volume 1, Issue 1

Page 29 of 63

sa Sector No Sector No rt v2 02"; Sector Length (512 bytes) rs “7 crea : Ree + 22 oa aE * 2 00 oo 7 3 F: AL Resets CRC generator soa A 5 + 2 BS 5 1 7 onc + cxcz + 30 ae aE “512 4 «

+ Write this field 20 times + Continue writing #E unt{l next index pulse received (Physical end of track)

‘As can be seen from the examples, there are gaps containing zero and/or SEH bytes beeween the 1D blacks and da ‘Defore ane after the index mark (if present) and at the end of the trac gape are there co allow the FDC to Synchronize with the disk to read the data and ID blocks.

The gaps before the index mark and at the end of the track are known The gap after the sadex mark and before the first ID block is known Gap Peeparaces, ‘TD block from Its ascoctated data area while Gap 3

Separates the data area from the next ID block on the disk.

The values of the bytes used to create the gaps are different for single density and double deneity mades as are the number of bytes uted. Below is an extract from the manufacturers data sheet for the PD1797 showing the byte values and byte counts for the gaps. The values shown for the byte count# are minimum except where shown.

Single Density Double Density

Gap 1 16 bytes HF 32 bytes 4E cap 2 11 bytes FF 22 bytes 4E ‘ 6 bytes 00 12 bytes 07

3 bytes AL

ap 3 10 bytes FF 26 bytes ue ” 4 bytes 00 8 bytes 00

3 bytes AL

Gap & 16 bytes FF 16 bytes 4E

+ Byte counts must be exact t+ Byte counts are minimum except exactly 3 bytes of Al must be written,

The physical data is held on the disk surface as # serial data stream. In double density mode a 250s pulse is cont to the disk drive for each flux transition This would imply that a pulse is sent to the drive each time the data changed from Otol or 1 to0. Inaddition to the data, a clock 1a recorded on the disk surface This clock {s picked off when reading the disk to synchronize the date being read. The presence of a clock on the disk ig ured to good effect by the FDI797 ‘when sending control bytes to che disk.

sn in the example track for byte values of OFSH and OF6H ge Yalues OALH and OC2H on the dlck surface. However, #0 that these bytes can be ‘Meeinguished from data bytes of the same values, clock pulses ace delibecately

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 29 of 63