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3
The central processing
unit,
which
consists
of control
and
arithmetic
sections,
performs
active
dynamically; it
serves as
the brain
of the
computer
and controls
it
overall
operation.
Its
operation, however, is made up
of
repetitions
of the following simple
operating
sequence:
1
.
A data item containing
an instruction
is read
from
storage.
2. The instruction is executed.
In other
words,
logically speaking it is
a
collection of data items in the storage unit
give
instructions that
cause
the
computer
to operate
in
a dynamic
manner.
This collection of data
items is
called a program. It is, therefore, necessary
to
prepare a program to indicate the steps of
a
job and store it in the
storage unit to
cause the
computer to
perform
the
job.
Inside the
computer, data and
control
signals
are
logically represented by binary numbers which are represented by
the digits
of 0
and 1
.
The number
of digits of a
binary number
(i.e., a sequence of Os and Is) is counted in terms of bits.
For
example, the 8-bit binary number
00110101
is
a data
item
which has a length of 8 bits (this
is
equivalent
to
53
in
decimal representation). Since bits are too small
to
be convenient
for
indicating the length of
data,
a
unit called the "byte" is used
to
indicate
a
data item of 8
bits. One
byte
can
represent
up
to
28
(=
2 56) different numbers.
The MZ-80A employs a Z80,
a
so-called 8-bit
microprocessor
(which process
one
byte
of
data
at
a time), as its
central
processing unit. Accordingly,
programs which
give
instructions and data
to be processed
are all
stored and trans¬
ferred in byte units. Byte locations in the
storage unit
are
designated by
a
2-byte pointer in
the central
processing
unit.
With this 2-byte pointer, the Z80 can address up to
216
(= 65536) locations. Since
2*°
(= 1024) represents 1
K
bytes,
the Z80 is
said
to
have an address space of 64
K
bytes. As mentioned above, the
MZ-80A
main storage unit
is made up
of 48 K
bytes,
or
3/4
of the Z80 RAM
(Random Access
Memory) address space.
RAM is a
type
of memory which
can
be
freely
read and written;
on the other
hand,
ROM
(Read Only
Memory)
can
only be
read.
The majority of special-purpose computers dedicated
to automatic
control
systems
and many personal
computers
have memories in which
1/3
to
1/2
or more of the memory
space
is composed
of ROM for
storage
of control or
system
programs (e.g., BASIC interpreter programs).
The use of RAM in the memory
configuration of the
MZ-80A
is based on
the premise that main memory should be freely available for a
variety
of
uses.
The
MZ-80A stores
all system programs
in external
files
from which they are loaded
into main
memory
by
a
monitor
program.
The
SA-5510 BASIC
interpreter,
one of the MZ-80A
system
programs, functions
to
translate BASIC
source
pro¬
grams
into
machine
code
for
execution.
The personal computer MZ-80A
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