NCR PC4I, Owner'S Manual

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HISTORICAL DATA
The execution of the program instructions currently in the computer
is also performed in the CPU. The machine-language instructions are
loaded into an instruction register and decoded into either specially-
wired circuits or unique microcoding to accomplish the steps of that
specific instruction.
The CPU contains:
1. Control circuitry, including a next instruction address register.
2. Greater, less than, and equal indicators to reflect the result of the
latest comparison instruction executed.
3. Arithmetic overflow and zero result indicators.
4. In many cases, one or more sets of “address registers” (storage
areas each large enough to store a memory address, such as the
base address for a block of coding, a block of data, or a table of
information; a return point for a branch out-of-line; or an
interrupt routine address).
Data to be processed arithmetically or logically are normally
retrieved from internal memory into the CPU under control of the
CPU. Data resulting from CPU operations are stored back into
internal memory.
Memory
The term memory encompasses the various hardware devices used by
the computer to “remember” or store data and instruction sets it
requires to process data. Two types of memory are used: internal and
external.
Internal Memory
Internal memory is the storage area within the computer to which
CPU has direct access. The “memory size” of the computer is
normally stated as the number of bytes of internal memory, such as
128KB, 640KB, where K (Greek kilo, or thousand) represents 2 to the
10th power or 1024 bytes.
Two primary types of internal memory are random access memory
(RAM), sometimes called “User memory” in smaller personal
computers, and read only memory (ROM).
RAM is used as temporary memory. Only the programs and data with
which the computer is currently working are in memory at any
instant. The operating system programs and the applications
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