external status words and data words returned (input)
from the channel. The starting address of the chain (and
other data) is provided by the load control memory
command. The chains starting address is stored in the
channels chain address pointer portion of I/O control
memory. The contents of the I/O control memory are
used by the IOC to control all channel operations
including execution of chaining instructions.
Input/output operations are initiated by the CPU.
Computers with an IOC will begin I/O control functions
only after an initiate I/O or equivalent instruction is
executed by the CPU. I/O operations under the control
of the computer program control the external
equipment. Computer instructions inform the external
equipment which type of operations to perform with
function codes. Computer instructions also specify
memory areas for input and output information.
Input/output operations do not accept data from
external equipments or send information to them unless
memory areas for the data have been specified by the
computer programs. Whenever an external equipment
is ready to send or receive data, a request signal is sent
to the computer.
How the I/O section notifies or
interrupts the control section that an external equipment
is ready to send or receive information/data depends on
the type of computer.
Some constants in all I/O
operations include the following:
When the transfer will begin,
How many words or bytes will be transferred,
Word or byte size,
When each individual word or byte is actually
When the transfer will terminate.
I/O operations require circuitry that must take
usually a maintenance panel or some equivalent. You
can use this option for troubleshooting purposes.
Consult the operators section of your computers
technical manual. As far as the operating modes for I/O
operations, these options are usually established at the
factory. Again, they usually apply to computers that
have an IOC. Some of the operating modes for I/O
operations include the following:
l Single-channel The single-channel operating
mode allows external equipments to communicate with
the computer via one input/output channel.
l Dual-channel The dual-channel operating
mode is used by computers with smaller word sizes, say
16 bits, to communicate with external devices using a
larger word size (30 or 32 bits). In a dual/channel mode,
the data lines for two channels are combined under
control of the lower order channel. A pair of
sequentially numbered channels (0 and 1,2 and 3, and
so forth) is used for dual-channel operations. The even
numbered channel provides the control signals and
lower half or lower order data bits. The odd numbered
channel provides the upper half or upper order data bits
only. The exchange of information over the dual
channel is controlled by the even numbered channels
interface signals. Dual channels may use the computer
peripheral or intercomputer channel signals.
@ Externally specified address (ESA) The
externally specified address mode provides the external
devices with a means of specifying an absolute memory
location for storage (write) or retrieval (read) of
information on a word-by-word basis.
l Externally specified index (ESI) The
externally specified index mode is identical to regular
transfers (input, output, external interrupt, and external
function) except that the IOC requires the external
device to specify an index address in main memory.
l Intercomputer channel (IC) The intercomputer
action in a specific sequence of events to communicate
with the external equipment. In I/O operations, we
examine operating modes, I/O circuits, and I/O
Similar to the CPU, some computers have the
capability to select operating modes. These options are
usually found with computers that have an IOC. They
can be found on the computers controlling device,
channel mode permits communication between two
CPUs. In this mode, each computer appears as an
external device to the other. During operations, the
computer that is outputting the data is defined as the
sending computer. The computer that is receiving the
data from the sending computer is defined as the
In chapter 4, we discussed the circuits used by
computers. We also discussed some of the same circuit
types in the CPU and memory sections. I/O is no