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MICROPROCESSING UNIT
The microprocessing unit (MPU) is the control center of
the printer. The MPU. controls the flow of information to
and from the controller. It is a microprogram-controlled
processor and performs internal machine functions such
as testing and comparing data, checking for· machine
errors, operating the printer, and storing and getting
data from read / write storage.
When a block of data is received, the M PU formats the
data into print lines using SNA character string (SCS)
control characters that are in the data. Formatting is
done only at the end of the lines (when the forms are
moving and the carrier is stopped). Two buffers are
used, ensuring that formatting is 1 line ahead of
printing.
The major parts of the MPU are read/write and
read-only storage, storage registers, work registers, and
an arithmetic and logic unit (ALU).
Microprocessing Unit
Read/Write Storage
Read / write storage is divided into 2 major parts: a data
buffer area and an I/O device control area. The data
buffer area is composed of two 256-byte receive
buffers and two 132-byte print buffers. The receive
buffers are used by the cable adapter for storing up to 1
complete block of input data from the controller. One
buffer holds data being formatted for printing while the
other receive buffer is accepting data from the
controller. One print line buffer is used for printing while
the other is being loaded with formatted data from a
receive buffer. The I/O device control area is divided
into parts for cable adapter control and MPU work
space.
Internal Processing Queues
Each printer has a work area large enough to hold up to
16 frames of commands or associated data. The
commands and data are processed from this queue on a
first-in and first-out basis.
1.-------------------,
I
Read-Only
I
I
Storage
I
I
Address
I
I
Register
~
I
Data
I
To Printer
Adapter
I
Register
I
I
I
Work
Address
I
I
I/O Control
I
Register
Register
I
2 Receive
I
Buffers
I
---- I
I
2 Print
I
Buffers
L ____________________ J
To Cable Adapter _ _ ---'
Theory,
Features
Theory
5256 MIM
4-15
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