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SERVICE CYCLES
Four service cycles take place during data transfers between
memory and the disk. The lOP and the controller continu-
ally interact during these service cycles - reporting status,
errors, and the successful conclusion of operations. The
descriptions presented below do not include information on
I/O instructions (or on Overflow and Carry indicators, Con-
dition Codes, or the Device Status Byte) since the relevant
I/O instruction has already been executed when these cy-
cles occur.
The following service cycles occur:
•
Order Out relates to an order, e. g., Seek or Write,
sent to disk.
•
Data Out relates to the writing of data on the disk.
•
Data In relates to the reading of data from the disk.
•
Order In relates to the Operational Status Byte.
ORDER OUT
The Order Out service cycle specifies the operation to oc-
cur next. An Order Out cycle, for example, always occurs
after a successful SIO instruction and is always initiated by
the controller. In effect, the controller signals the lOP
for an order, and the lOP fetches the order and sends it to
the controller for execution. During this cycle, the lOP
a Iso stores the byte count, flags, and memory address in
its internal registers.
DATA OUT
The Data Out service cycle transfers data from memory to
the disk. The lOP accesses the memory location pointed
to by the address field of the lOCO, checks the flags, ad-
justs the byte count, sets operational flags or error indi-
cators, and transfers the data to the controller. The con-
troller in turn directs the disk drive to write the data on
the disk. The lOP terminates this cycle with or without a
Terminal Order, based on the channel condition.
DATA IN
The Data In service cycle transfers data from the disk to
memory. The lOP accepts the data from the controller,
checks the flogs, adjusts the byte count, sets any requi red
operational flags or error indicators, and stores the data
beginning at the memory location pointed to by the address
field of the lOCO. The lOP terminates this cycle with or
without a Terminal Order, based on the channel condition.
ORDER IN
The Order In service cycle transfers the status from the con-
troller to the lOP. The lOP always terminates this cycle
with a Terminal Order. The following flogs will appear in
the Operational Status Byte:
Transmission Error.
Incorrect Length.
Channel End.
Unusua I End.
TERMINAL ORDER
Byte count error.
The controller encountered a
norma I channel end.
The controller encountered an
unusual end condition.
For the Order Out or Order In service cycles, the lOP al-
ways sends a Terminal Order (one byte) to the controller
before terminating the service cycle. For the Data Out or
Data In service cycles, the lOP sends a Terminal Order
only when an lOP halt is detected or a zero byte count ex-
ists; otherwise, the lOP signals the controller to disconnect
without a Terminal Order.
The bits in the Terminal Order byte have the following
significance:
Bit 0
Interrupt. A value of 1 triggers an interrupt
signal.
Bit 1
Count Done. This bit is set to 1 if the byte
count is reduced to zero and the data cha i n-
i ng flog is set to 1.
Bit 2
Command chain. This bit islset to 1 if the
!
command chaining flag is set to 1.
I
i
Bit 3
lOP holt. This bit is set to 1 when the lOP
halts as the result of an error.
Bit 4
Address error. This bit is set to 1 if the lOP
sensed a parity error in the device address
during the service cycle.
Bits 5-7
Reserved.
DISK ORGANIZA liON
I
The upper portion of Figure 7 illustrates the organization of
tracks (concentric circles) and sectors (sector marks) on a
disk, while the lower portion illustrates the sector format.
Each di sk has 408 tracks, 16 sectors on each surface, and
204 cylinders.
The track, sector, and cylinder addresses
given below are identical for all disks, irrespective of the
total number in a system.
TRACKS
The track addresses begin at the periphery of the disk and
continue in ascending order t'oward the center.
The
Disk Organization
9
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