1-21. ELECTRONIC STATION (cont)
(3) Input/Output Priority.
(a) Input/output priority is established by CCA position within the card
cage or backplane.
CCA located in position A1 has highest priority while CCA in
position A13 has lowest priority.
Control store CCA occupies position A1 while I/0
interface CCAS begin at position A7.
HP-IB interface CCA at position A7 has highest
I/0 priority while buffer interface CCA A13 has the lowest priority.
(b) When an input/output device is ready to be serviced, it causes its I/0
interface CCA to request an interrupt so that the computer will interrupt the current
program and service the device.
With the I/0 interface CCA supplying its select code
to the address bus, an instruction in memory caused a program jump to the service
routine for that select code.
The instruction automatically saves the return address
for a later return to the CPU program.
(4) Input/Output Addressing.
An external device is connected by cable directly
to an I/0 interface located In the computer mainframe.
The I/0 interface, in turn,
plugs into one of the input/output slots in the backplane. Note that the select code
(I/0 address) of each interface is independent of its priority. The computer
communicates with a specific device on the basis of its select code, which is set by
switches on the I/0 interface CCA.
(5) Input/Output Processing.
Most I/O instructions are executed by an
input/output processor (IOP) chip on each I/0 interface CCA. When an I/0 interface
CCA determines it should execute the instruction fetched, it also decides if execution
will require interaction with the CPU.
(a) If the CPU is needed:
The interface requests an I/0 handshake with the CPU.
The CPU acknowledges the I/0 handshake request after DMA requests are
The interface that requested the handshake responds by driving a
command work onto the data bus at completion of the handshake.
(b) I/0 instructions are broken down into three groups:
Data transfer I/0 instructions
Status sensing I/0 instruction
Status altering instructions