The Best of Creative Computing Volume 2 (published 1977)
Microprocessors and Microcomputers - The State of The Art (Microcomputer vs. Minicomputer, Characteristics, Teledyne Systems TDY-52)
For an LSI machine to perform higher-level operations with
ease, microinstruction sequences corresponding to common
higher-level functions are stored in a separate read-only memory
(ROM) to be accessed, decoded, and executed on command.
These high-level sequences are called macroinstructions, the
medium in which system programmers usually code. Macroinstructions
in a microcomputer correspond to the basic
instructions of a minicomputer.
Microprogramming enables a systems designer to adapt
standard hardware to specific applications - perhaps the most
useful characteristic of a microcomputer, The designer can
construct macroinstructions that are best suited for the particular
functions to be performed, and incorporate them into the
microprocessor. For example, the instruction set of an existing
minicomputer can be completely or partially emulated to
minimize software development. Alternatively, a machine can
be built to perform functions peculiar to an application such as
word processing or data acquisition. This capability to adapt a
standard set of hardware modules to a variety of problems
combines the cost advantages of high-volume chip production
with the computing efficiency of tailored instruction sets.
MICROCOMPUTER vs MINICOMPUTER
Although stark and simplistic price comparisons are sometimes
misleading, it is not unfair to say that an LSI microprocessor
has a substantial cost advantage over a typical minicomputer
CPU. For example, a complete LSI CPU may be purchased for
as little as $300, compared
to $1000 to $2000 for a
minicomputer CPU.
The CPU power consumption
of an LSI microcomputer is 66
to 75 percent less than that of a
comparable minicomputer. For a
system containing but one CPU,
the difference would not be
significant considering the overall
system's power requirements.
However, in applications where
many CPUs are required, the
power difference would he
substantial.
An MOS/LSI microcomputer
operates at 50 to 33 percent of
the speed of commercially available minicomputers. Typical
memory-to-memory add times
for a moderately priced mini are
An example of a microcomputer is Teledyne Systems'
TDY-52, a programmable microcomputer contained within a
2" x 2" x O.2" package. Teledyne offers two different
configurations of the TDY-52: the TDY-52A, a package holding
a CPU with 8 registers, a 4K x 8-bit microinstruction ROM
control memory, 4K x 8-bit application program RAM
memory, a 2K bit scratchpad RAM, input multiplexer,
between 5 and Z0 microsec compared to 15 to 60 microsec for
a microcomputer. The speed of a microcomputer is derived
from the particular MOS process used in fabrication. As these
processes improve, so will the speed.
With integrated circuits, system reliability is largely a
function of the number of printed circuit (PC) board interconnections.
Since each LSI package replaces from 50 to 100
TTL packages. the interconnections required by microcomputers
are reduced and total system reliability is increased.
The LSI microcomputer can be built into a light and compact
configuration because of the higher number of gates per
package module and the simplicity of interconnection.
In summary, the LSI microcomputer offers better price-performance,
lower power consumption and heat dissipation,
higher reliability, and smaller physical size than a minicomputer.
The microcomputer further offers the flexibility of
microprogramming, which, in a given application, has many
advantages. Although execution speeds comparable to today's
minicomputer have not yet been achieved, several architectural
techniques have emerged which will eventually increase
microcomputer speeds.
CHARACTERISTICS
Microprocessor architecture is similar to that of a bus-oriented
minicomputer. Applications can generally be categorized by bit
width: four-bit microprocessors for calculators; eight-bit units
for microcontrollers; and sixteen-bit units for microcomputers.
The range of characteristics is broad:
Data Word Size
4 to 100 bits
Instruction Set
40 to 120
Instruction Format
8 to 24 bits
ROM
400 23-bit to 16K 8-bit
RAM
up to 65K 16-bit
General-Purpose Registers
1 to 16
Cycle Time to Fetch & Execute
An Instruction
0.54 to 62 microsec, with
5 to 10 microsec common
Stack Depth
2 to 32 levels
Interrupt Cepebility
None to full
Parallelism
mostly parallel
to serial/parallel
output buffer registers, priority interrupts, and oscillator;
and the TDY-52B, a general-purpose 16-bit microcomputer
with CPU and registers, priority interrupt, memory and I/O
address register, clock generator, timing and control, and
output buffers. Both configurations can also incorporate
additional ROM, RAM and ROM/RAM modules, contained
within another TDY-52 size package.
