channel facilities are free, START
ently selects the
Channel Address Word
Successful execution of START
to fetch a channel address word (
main-storage location 72. The
location in main storage where the channel program
Figure 18 shows the format for the
specify the location of the first channel command
o 34 7 8 31
Figure 18. Channel Address Word Format
Channel Command Word
The byte location specified by the
eight bytes of information that the channel fetches
from main storage. These 64 bits of information are
called a channel command word
that directs channel operations. If more than one
ccw is to be used, each ccw points to the next ccw
to be fetched, except for the last ccw in the chain,
which identifies itself as the last in the chain. Figure
19 shows the format for ccw's.
Transfer In Channel
The read command causes a read operation from the
storage to be used.
Bits 8-31 specify the
Bits 32-36 are
Bit 32 causes the address portion of the next
Bit 33 causes the command code and data address in the next
Bit 34 causes a
Bit 35 suppresses the transfer of information to main storage.
Bit 36 causes an interruption as
Bits 37-39 must contain zeros.
Bits 48-63 specify the number of bytes in the operation a
Figure 19. Channel Command Word Format
The write command causes a write operation on the
age to be written.
The read-backward command causes a read operation
in which the external document is moved in a back
ward direction. Bytes read backward are placed in
descending main storage locations.
The control command contains information used to
control the selected
tion is caned an order. Orders are peculiar to the par
tions as rewinding a tape unit, searching for a par
ticular track in disk storage, or line skipping on a
printer. The relationship of
mands, and orders is shown in Figure 20.
storage location to which status information is trans-