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4.4.5
COMMAND SPECIFIC BITS
Byte 01, bits 01-04 specify options which depend upon the par-
ticular command.
4.4.6
RELATIVE ADDRESS BIT
The RELATIVE ADDRESS BIT
(Bit 0 of Byte 01) of the Group
1
commands is set to indicate that the block address portion of the
COB is a two's complement displacement.
This displacement is to
be added to the Block Address last accessed on the unit to form
the Block Address for the present command.
This feature is only
available when linking commands.
The feature requires that a
previous command in the linked group have accessed a block of
data on the device.
4.4.7
LOGICAL BLOCK ADDRESS
Group
0 commands contain 2l-bit starting block addresses while
Group 1 supports 32-bit block addressing.
4.4.8
NUMBER OF BLOCKS
A variable number of blocks may be transferred under a single
command.
Group 0 commands may transfer up to 256 blocks,
while
Group
I
commands may transfer up to 65,535 blocks.
A zero value
for a Group 0 command implies a 256 block transfer.
A
zero value
for a Group 1 command implies a zero length transfer.
4.4.9
CONTROL BYTE
The control byte is the last byte in a Class 00 or Class 01
command.
The command byte is defined as follows:
Bits 7-2
Bit 1
Bit 0
500504-00
Reserved;
must be zero
FLAG--This bit
indicates
that an
interrupt
is
requested for
this command
in a group of linked
commands.
Th is bi t may on ly be set for LINKED com-
mands.
LINK--This bit indicates the existence of a LINKED.
command which will be automatically executed upon
successful completion of the current command.
4-20
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