Lesson 6: Instruction Set
From "AMERICAN LABORATORY"
by Jonathan A. Titus, David G. Larsen and Peter R. Rony
The MOV and MVI 8080 Instructions
IN THE October 1976 column, we
indicated that seven generalpurpose
registers (13, C, D, E, H,
L, and the accumulator) exist within
an 8080 microprocessor chip and
that cach register operates on eight
bits at a time. These registers are
used for many varied purposes, e.g.,
the storage of an 8-bit constant, an 8bit
timing byte, a 16-bit pointer address,
01' an intermediate result from
an arithmetic or logical operation.
Let us cxamine thc data transfer
instructions MOV D,S in the 8080 instruction
set to sec how the generalpurposc
registers can be used. We
will assume that some data arc
initially present within each register.
There arc sixty-three different MOV
instruclions. each of which speci ries
both the SOtlfCC register S of thc data
and the destination register D to
which thc data are moved. For example,
to move data from register E to
registcr B, you would use the instruction
MClV B, E, which is the Intel
mnclllonic notation for the operation
of moving data to rcgister B from
registcr E. Unfortunately, an 8080bascd
microcomputer has no way to
understand or interpret a mnemonic
instl'llction Stich as MOV Il, n. What is
required is the binary representation
for MOV n, E: 01000011 2• Th~se
eight bits, which comprise the instruction
code for MOV n, E, can be
manipulated digitally; i.e., they can
bc storcd in a semiconductor
mcmory device, tmnsmitted over a
data bUfi, storcd in an instruction
register within the 8080 chip, and
dccoded by an i llstruction decoder
into a scries of actions that the 8080
performs internally.
Read More: The MOV and MVI 8080 Instructions.pdf [165KB]
The 8080 Logical Instructions
THE CONCEPT of the one important use for multibit logical instructions, such as AND, OR,
Exclusive-OR, and COMPLEMENT, has been discussed. I In this column the twenty-eight logical instructions in
the 8080A instruction set are summarized. It is very important to note that in the case of each logical instruction,
the result is stored in the accumulator. The previous contents of the accumulator is one of the
logical variables in the two-variable logical operation, or in the case of the complement instruction, the only
logical variable.
Read More: The 8008 Logical Instructions.pdf [49.7KB]
How does a microcomputer make a decision?
ONE of the most important
programming characteristics
of any digital computer, including
a microcomputer, is the
ability to make a decision. For a
typical microcomputer, we can define
a decision as the process of
determining further action based
upon the logic state of a flag. Aflag
is a single flip-flop that can be
either set or cleared in response to
operations occurring within the microcomputer
system. A change of
state 0 f the flag is usually an
indication either that a particular
operation has been completed or
that a certain condition exists as a
result of a microcomputer operation.
Flags can be located either
internal or external to the microprocessor
chip; the ones that we
shall discuss in this column are the
internal flags, which are set or
cleared in response to specific types
of microprocessor instructions such
as arithmetic and logical instructions.
The flags that are located within
the microprocessor chip are typically
associated with the arithmetic-
logic unit (AL U], a region within
the chip where all arithmetic and
logical operations are performed. In
the 8080 microprocessor chip, for
example, five flags indicate the following
conditions:
Read More: How Does A Microcomputer Make A Decision.pdf [37.7KB]