Since their inception, digital computers have contin-
uously become more efficient, expanding into new appli-
cations with each major technological improvement. The
advent of minicomputers enabled the inclusion of digital
computers as a permanent part of various process control
systems. Unfortunately, the size and cost of minicomputers
in "dedicated" applications has limited their use. Another
approach has been the use of custom built systems made up
of "random logic" (i .e., logic gates, flip-flops, counters, etc.).
However, the huge expense and development time involved
in the design and debugging of these systems has restricted
their use to large volume applications where the develop-
ment costs could be spread over a large number of machines.
Today, Intel offers the systems designer a new alter-
native ... the microcomputer. Utilizing the technologies and
experience gained in becoming the world's largest supplier
of LSI memory components, Intel has made the power of
the digital computer available at the integrated circuit level.
Using the n-channel silicon gate MOS process, Intel engi-
neers have implemented the fast (2/ls. cycle) and powerful
(72 basic instructions) 8080 microprocessor on a single LSI
chip. When this processor is combined with memory and
I/O circuits, the computer is complete. Intel offers a variety
of random-access memory (RAM), read-only memory (ROM)
and shift register circu~ts, that combine with the 8080 pro-
cessor to form the MCS-80 microcomputer system, a system
that can directly address and retrieve as many as 65,536
bytes stored in the memory devices.
The 8080 processor is packaged in a 40-pin dual in-line
package (DIP) that allows for remarkably easy interfacing.
The 8080 has a 16-bit address bus, a 8-bit bidirectional data
bus and fully decoded, TTL-compatible control outputs. In
addition to supporting up to 64K bytes of mixed RAM and
ROM memory, the 8080 can address up to 256 input ports
and 256 output ports; thus allowing for virtually unlimited
system expansion. The 8080 instruction set includes con-
ditional branching, decimal as well as binary arithmetic,
logical, register-to-register, stack control and memory refer-
ence instructions. In fact, the 8080 instruction set is power-
ful enough to rival the performance of many of the much
higher priced minicomputers, yet the 8080 is upward soft-
ware compatible with Intel's earlier 8008 microprocessor
(Le., programs written for the 8008 can be assembled and
executed on the 8080).
In addition to an extensive instruction set oriented to
problem solving, the 8080 has another significant feature-
SPEED. In contrast to random logic designs which tend to
work in parallel, the microcomputer works by sequentially
executing its program. As a result of this sequential execu-
tion, the number of tasks a microcomputer can undertake
in a given period of time is directly proportional to the
execution speed of the microcomputer. The speed of exe-
cution is the limiting factor of the realm of applications of
the microcomputer. The 8080, with instruction times as
short as 2 /lsec., is an order of magnitude faster than earlier
generations of microcomputers, and therefore has an ex-
panded field of potential applications.
The architecture of the 8080 also shows a significant
improvement over earlier microcomputer designs. The 8080
contains a 16-bit stack pointer that controls the addressing
of an external stack located in memory. The pointer can be
initialized via the proper instructions such that any portion
of external memory can be used as a last in/first out stack;
thus enabling almost unlimited subroutine nesting. The stack
pointer allows the contents of the program counter, the ac-
cumulator, the condition flags or any of the data registers to
be stored in or retrieved from the external stack. In addi-
tion, multi-level interrupt processing is possible using the
8080's stack control instructions. The status of the pro-
cessor can be "pushed" onto the stack when an interrupt is
accepted, then "popped" off the stack after the interrupt has
been serviced. This ability to save the contents of the pro-
cessor's registers is possible even if an interrupt service
routine, itself, is interrupted.
