Chapter
1.
Assembly
Language
and
Processors
OVERVIEW
OF
8080/8085
HARDWARE
To the programmer, the
computer
comprises the following parts:
•
Memory
•
The program cou nter
•
Work registers
•
Condition flags
•
The stack and stack pointer
•
Input/output
ports
•
The instruction set
Of the
components
listed above, memory
is
not part
of
the processor,
but
is
of
interest to the programmer.
Memory
Since the program required to drive a microprocessor resides
in
memory.
all
microprocessor applications require
some memory. There
Me
two general types
of
memory: read only memory (ROM) and random access memory
(RAM).
ROM
As
the name implies, the processor can only read
instructiom
and data from
ROM;
it
cannot
alter the
contents
of
ROM.
By
contrast, the processor can both read from and write to RAM. Instructions and unchanging data
are permanently fixed into
ROM
and remain intact whether or not power
is
applied to the system. For this
reason,
ROM
is
typically used for program
<.torage
in
single-purpose microprocessor applications. With
ROM
you
can be certain
that
the program
is
ready for execution when power
is
applied to the system. With
RAM
a program
must
be
loaded into memory each time power
is
applied to the processor. Notice, however,
that
storing programs
in
RAM
allows a multi·purpose system since different programs can be loaded to serve different needs.
Two
special types
of
ROM
- PROM (Programmable Read Only Memory) and EPROM (Eraseable Programmable
Read
Only Memory)
..
are frequently used during program development. These memories are useful during
program development since they can
be
altered by a special PROM programmer.
In
high-volume commercial
applications, these special memories are usu.llly replaced by less expensive
ROM's.
RAM
Even if your program resides entirely
in
ROM, your application
is
likely to require some random access memory.
Any time your program
attempts
to write any data to memory,
that
memory must
be
RAM. Also, if your pro-
gram uses the stack, you need RAM. If your program modifies any
of
its own instructions (this procedure
is
discou raged), those instructions must reside
in
RAM.
The mix
of
ROM
and
RAM
in
an applicaticn
is
important
to both the system designer and the programmer.
Normally, the programmer must know the physical addresses
of
the
RAM
in
the system
so
that
data variables
1·5
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