Sample track data for a track formatted to logical track 9, logical head
0, logical sectors 0 to 9, each 256 (2^(7+1)) bytes long.

INDEX PULSE
000 : FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF : gap1   16x&FF

010 : 00 00 00 00 00 00                               : sync    6x&00
016 : FE                                              : ID mark   &FE
017 : 09 00 00 01                                     : trk, hd, sec, sz
01B : F7 FF                                           : CRC
01D : FF FF FF FF FF FF FF FF FF FF                   : gap2   10x&FF
027 : 00 00 00 00 00 00                               : sync    6x&00
02D : FB                                              : Data mark &FB
02E : E5 E5 E5 E5 E5 E5 .. .. .. E5 E5 E5 E5 E5 E5 E5 : Data  256x&xx
12E : F7 FF                                           : CRC
130 : FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF    : gap3   15x&FF

13F : 00 00 00 00 00 00                               : sync
145 : FE                                              : ID mark   &FE
146 : 09 00 01 01                                     : trk, hd, sec, sz
14A : F7 FF                                           : CRC
14C : FF FF FF FF FF FF FF FF FF FF                   : gap2   10x&FF
156 : 00 00 00 00 00 00                               : sync    6x&00
15C : FB                                              : Data mark &FB
15D : E5 E5 E5 E5 E5 E5 .. .. .. E5 E5 E5 E5 E5 E5 E5 : Data  256x&xx
25D : F7 FF                                           : CRC
25F : FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF    : gap3   15x&FF

26E : 00 00 00 00 00 00                               : sync    6x&00
etc...

AB7 : 00 00 00 00 00 00                               : sync
ABD : FE                                              : ID mark   &FE
ABE : 09 00 09 01                                     : trk, hd, sec, sz
AC2 : F7 FF                                           : CRC
AC4 : FF FF FF FF FF FF FF FF FF FF                   : gap2   10x&FF
ACE : 00 00 00 00 00 00                               : sync    6x&00
AD4 : FB                                              : Data mark &FB
AD5 : E5 E5 E5 E5 E5 E5 .. .. .. E5 E5 E5 E5 E5 E5 E5 : Data  256x&xx
BD5 : F7 FF                                           : CRC
BD7 : FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF    : gap3   15x&FF

BE6 : FF FF FF FF FF FF .. .. .. FF FF FF FF FF FF FF : gap4   30x&FF
C04 : FF FF FF FF FF FF .. .. .. FF FF FF FF FF FF FF : gap5   49x&FF
C35 :                                                 : end of track data

3125 bytes

gap5 is written until the index pulse comes back round, and so mustn't be
longer than will fit in the track. By the time gap5 has been written, gap1
is likely to have passed under the read/write head, and so will have been
partially overwritten. gap5 is often set to 0 to ensure that the track
data does not overrun itself.

In the raw disk image file, the track data is prefixed by the physical
track number, the physical side and the track density.  Three bytes are
then thrown away from the end of the gap5 data. gaps are always written as
&FFs, so these three bytes are reconstructed as &FFs.

Some documentation describes gap4 as following the final gap3, so in this
example gap4=30.  Some document describes gap4 as including the final
gap3, so in this example gap4=45.  Usually, only gap1, gap3 and gap5 are
adjustable. I prefer to regard gap4 as following the final gap3, so then
each sector has the same logical structure.

As can be seen, if the track was formatted as a single sector, you could
fit 3k on it, as opposed to 2.5k by formatting as 10 sectors of 256 bytes.
Unfortunately, 3k is not one of the size units that a sector can be
formatted to.