Commodore Amiga A500 Technical Reference Manual page 190

This four bit field can be used to specify options as indicated below:
Initialize Unit 1
(Opcode CC)
Change Command
Block
(Class 0, Opcode F)
Bit 7
=
0
5 bit correction span (default value)
=
1 1 1 bit correction span
Bit 6
=
0
Retries
&
ECC enabled (default value)
=
1 Retries
&
ECC disabled
Bit 5
=
0
Not Used
Bit
4
=
0
Not Used
Step Rate
Step Rate
l 4
=
11.1 usec
Step Rate l 5
=
3 0 usec
All
Others
=
3 msec
Possible Error Codes
No error, invalid command.
DMA
error.
Action
This command with initialize or set drive parameters of unit 1 only.
This allows for the HDC to support two different drive types at the
same time. The action of this command is identical to the action of
the 'Set Drive Parameter' command noted above except that it will
effect only unit 1. For command details see section 6.2.9.
Action
The Change Command Block is used to move the location of the
command block from the default on power up to a new location.
Bytes 6 and
7
of the command block are used as indirect address
pointers for the beginning of a
7
byte block of memory organized as
follows:
Table 5-1
0.
Change Command Block Address
D7 D6 D5 D4 D3
D2
D1
DO
0 0
A23 High Order
DMA
Byte A1 6
A1 5
Mid Order
DMA
Bfle
A08
A07 Low Order
DMA
Byte
0