Commodore Amiga A500 Technical Reference Manual page 180

Logical Sector Address
A logical sector address is a 21 bit unsigned integer that specifies a
unique physical sector. The one-to-one correspondence between the
set of logical sector addresses and the set of physical sectors is com-
puted by the HDC from the Cylinder (C), Head (H), and Sector (S)
address, as well as the drive parameters, heads per drive (HD) and
Sectors per track (ST):
C, H and S can be derived from L, HD, and ST as follows:
S
=
L Modulo ST
H
=
(
(L-S)IST) Modulo HD
C
=
( (
(L-S)IST)-H)IHD
This field specifies a sector or the first sector for the Read and Write
Drive commands. When only a track specification is required, the sec-
tor number implied by the Logical Sector Address is ignored. Hence
each format type command begins operation at the beginning of the
track containing the specified sector. The HDC will report an invalid
command, if the logical address specified is out of range.
Block Count
The sector count is a parameter for each data transfer command. It
specifies the number of logical sectors t o be transferred during any
disk READ or WRITE operations. The sector count is an unsigned,
non-zero integer. All zeros in the sector count field specify a count of
256.
For a format command, the number of sectors t o be formatted per
track is specified by this byte. The interleave factor need not be ex-
plicitly furnished by the host, since it is implicitly contained in the in-
terleave table furnished by the host.
Control Field
The control field is reserved for future use.
DNIA Memory Address
The next three bytes, bytes 6,7, and 8, make up the 2 3 bit address
which points t o the block of 51
2
byte t o be transfer via DMA. This
block of memory contains data bytes or specifies an address value as
required by the command t o be executed. Since the
R I ' -
bit is part
of the LD2 memory address counter, address bits A1 -A23 are shifted
right 1 bit by the HDC before being stored for command execution.