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4.0 INSTRUCTION SET
The Z-80A CPU can execute 158 different instruction types including all 78
of
the 8080A CPU. The instructions
can be broken down into the following major groups:
• Load and Exchange
• Block Transfer and Search
• Arithmetic and Logical
• Rotate and Shift
• Bit Manipulation (set, reset, test)
•
Jump,
Call and Return
•
Input/Output
• Basic CPU Control
4.1
INTRODUCTION
TO INSTRUCTION TYPES
The load instructions move data internally between CPU
register~
or between CPU registers and external memory.
All
of
these instructions must specify a source location from which the data
is
to
be moved and a destination location.
The source location
is
not
altered
by
a load instruction. Examples
of
load group instructions include moves between
any
of
the general purpose registers such as move the data to Register B from Register C. This group also includes load
immediate
to
any CPU register or to any
to
Register B from Register C. This group also includes load immediate
to
any
CPU
register or to any external memory location. Other types
of
load instructions allow transfer between CPU registers
and memory locations. The exchange instructions can trade the contents
of
two registers.
A unique set
of
block transfer instructions
is
provided in
the
Z-80A. With a single instruction a block
of
memory
of
any
size
can be moved
to
any other location in memory. This set
of
block moves
is
extremely valuable when large
strings
of
data must be processed. The Z-80A block search instructions are also valuable for this type
of
processing.
With a single instruction, a block
of
external memory
of
any desired length can be searched for any 8-bit character.
Once the character
is
found
or
the end
of
the block
is
reached, the instruction automatically terminates. Both the block
transfer and the block search instructions can be interrupted during their execution so
as
to
not
occupy the CPU for
long periods
of
time.
The arithmetic and logical instructions operate on data stored
in
the accumulator and other general purpose CPU
registers or external memory locations. The results
of
the operations are placed in the accumulator and the appropriate
flags
are set according to the result
of
the operation. An example
of
an arithmetic operation
is
adding the accumulator
to
the
contents
of
an external memory location. The results
of
the addition are placed in the accumulator. This group
also includes 16-bit addition and subtraction between 16-bit
CPU registers.
The rotate and shift group allows any register or any memory location to
be
rotated right or left with or without
carry either arithmetic or logical. Also , a digit
in
the accumulator can be rotated right or left with two digits in any
memory location.
The bit manipulation instructions allow any bit in the accumulator, any general purpose register or any external
memory location
to
be set, reset or tested with a single instruction.
For
example, the most significant
bit
of
register H
can be reset. This group
is
especially useful
in
control applications and for controlling software flags in general purpose
programming.
The jump, call and return instructions are used to transfer between various locations in the user's program. This
group uses several different techniques for obtaining the new program counter address from specific external memory
locations. A unique type
of
call
is
the restart instruction. This instruction actually contains the new address as a
part
of
the 8-bit OP code. This
is
possible since only 8 separate addresses located
in
page zero
of
the external memory may be
specified. Program jumps may also
be
achieved
by
loading register HL, IX or IY directly into the PC, thus allowing the
jump address
to
be a complex function
of
the routine being executed.
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