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Prime Proprietary
Principles of Operation
4.2.11.1.1 Output/Input The A Register (OTA/INA)
OTA and INA PIO commands are quite similar.
The OTA moves a data word
from the A Register to a controller.
The INA moves a data word froma
controller to the A Register.
The A Register is a high-speed register
located in the central processor's high-speed register files,
For
example,
assume the program that the CPU is executing contains a
command to output a
character
from
memory
to
the
teletype.
The
program
would first move the character from memory to. the A Register.
Then, the CPU would send the OTA instruction to the controller on
the
BPA.
The
OTA's
device
address
would be the device address of the
teletype controller.
The
function
code
would
be
the
code
that
instructs
the
controller to output the data word to the device.
The
CPU sends the character in the A Register to
the
controller
on_
the
BPD.
The
CPU
sends
the BPCPIO control signal on the BPC.
All the
controllers on the bus examine the device address of
the
instruction
to
see
if
it
is
theirs.
The
addresses
controller
accepts the
information from the I/O Bus and instructs the teletype to
print
the
character.
When
an
OTA
or an INA is issued to a controller, the controller may
not be ready to accept the command.
To
allow
for
such
conditions,
there is a control line (BPCREDY, controller ready) that tells the CPU
whether
the
controller
is
ready
to
accept
the
command.
If the
controller is not ready, the CPU executes the next instruction in
the
program,
without
transferring the data.
If the controller is ready,
the CPU outputs the data and skips the next instruction
in
sequence.
It
is common programming practice for the instruction following these
commands to be a JUMP back.
If
the
controller
is
not
ready,
the
program re-executes the PIO command.
If the controller can accept the
data, the program is allowed to progress by skipping the JUMP back.
Once
the
instruction
is
decoded
and
accepted
by
the
teletype
controller, the character
is
sent
through
the
controller
to
the
device.
If the operation is an INA, a data word is input to the CPU.
The controller obtains the word from the teletype device and places it
on the BPD.
The CPU retrieves it and stores it
in
the
A
Register.
The
CPU
then
sends
a strobe (BPCSTRB) to the controller indicating
that the data has been received.
OTA and INA can be used to pass a variety of information
between
the
CPU and the controllers depending on how the controller is designed to
operate for the instruction being processed.
When an OTA or an INA to
device
address
'20 (Control Panel) is executed, the device is always
ready, so the instruction never skips.
4.2.11.1.2 Skip On Condition Set (SKS)
The SKS instruction examines the controller's ready state,
just
like
OTA/INA instructions.
If the line is true, the program skips the next
instruction,
If
the
line
is
false, the program executes the next
instruction.
4 - 27
070-C
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