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When several data channels require main storage cycles
at the same time, the sequence of transmission is handled
automatically according to priorities previously established
by the user.
The following DP I/O devices are under data channel
control:
IBM 1442 Card Read Punch
IBM 1443 Printer
IBM 1810 Disk Storage
IBM 2311 Disk Storage Drive (via 2841 Storage Control
and selector channel)
IBM 2401 Magnetic Tape Unit
Direct Program Control
Direct program control transfers individual words of data
between processor-controller main storage and the I/O
device at a rate controlled by the program and maximum
speed of the I/O device. The processor-controller program
is interrupted for each word transfer and must branch to a
routine which services the I/O device. Therefore, the logical
condition of the processor-controller is changed.
The following DP I/O devices are under direct program
control:
IBM 1053 Printer
IBM 1054 Paper Tape Reader
IBM 1055 Paper Tape Punch
IBM 1627 Plotter
IBM 1816 Printer-Keyboard
Input/Output Interrupts
Interrupt is described as an automatic branch in the normal
program sequence based upon an external condition.
Input/output interrupts are initiated by such conditions
as:
1.
Termination of an I/O operation by certain error
conditions.
2.
Completion of the programmed instruction.
3.
Operator intervention at the I/O devices.
4.
I/O unit operating under direct program control
requires service.
Input/output interrupts enable the processor-controller to
provide appropriate programmed responses to conditions that
occur in I/O devices.
Input/output interrupts are assigned a priority sequence
similar to the data channels. This priority sequence arrange-
ment allows most efficient use of all I/O devices.
Introduction
3
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