Channel Servicing; Servicing A Data Transfer - RCA 70/46 Reference Manual

CHANNEL
SERVICING
(Cont'd)
Servicing a Data Transfer
Block
1
Block 2
Block
3
Input/Output Operation
nique insures that the interference to selector channel servicing caused by
the multiplexor channel is not greater than that of an additional selector
channel.
• Once an input/output operation has been initiated, it proceeds under
control of the appropriate channel and registers in scratch-pad memory
and non-addressable main memory (multiplexor devices). When an input/
output operation has been initiated and the input/output device is ready
to send or receive a data byte, it asks the processor for a service request.
When the processor honors this service request, servicing of a data trans-
fer occurs.
Because servicing a data transfer requires that the channel utilize
main memory, scratch-pad memory and non-addressable main memory
(multiplexor devices), normal mode processing is held off until the servic-
ing has been completed. Servicing of a data transfer is time-shared with
normal mode processing. Between service requests, processing is resumed,
or another channel is permitted to service its device (s ) .
If a burst mode operation has been initiated to the multiplexor chan-
nel, the channd operates similar to a selector and only one device is serv-
iced. Service requests by devices other than the one operating in burst
mode are ignored until the multiplexor channel is operating in the multi-
plexor mode. This occurs at the conclusion of the burst operation when
the last data byte has been serviced (prior to interrupt).
Servicing of a data transfer causes the following events to occur (see
figure 9).
• If the service request comes from a device control electronics con-
nected to the mUltiplexor channel which is operating in the multiplex mode,
the processor gets the device address and fetches the appropriate sub-
channel registers in non-addressable main memory. These registers are
placed in processor utility registers in scratch-pad memory. (They are not
sent to the multiplexor channel registers in scratch-pad memory.) If the
service request comes from a device control electronics connected to the
mUltiplexor channel which is operating in the burst mode or from a device
connected to a selector channel, the appropriate channel registers in
scratch-pad memory are used to service the data transfer.
• A test is made
to.
see if the Program Controlled Interrupt (PCI) flag
is set. If it is, the channel interrupt bit is set in the Interrupt Flag register
and an interrupt occurs, if permitted. The PCI flag is reset and the
program control interrupt bit is set in the channel status byte.
• A test is made to see if the device control electronics requesting service
has indicated an end condition. An end condition is indicated when one of
the following occurs:
1. The processor reaches a byte count lapse. If this occurs, the processor
tells the device control electronics
to
indicate an end condition on
the next data service request.
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