87
An example
of
an indexed instruction would be to
lo
ad the contents
of
the memory location (Index Register+
Dis-
placement) into the accumulator. The displacement
is
a signed two's complement number. Indexed addressi
ng
greatly
simplifies programs using tables
of
data since the index register can point
to
the start
of
any table. Two index registers
are provided since very often operations require two or more tables. Indexed addressing also allows for relocatable
code.
The two index registers in the
Z-80A are referred
to
as
IX and IY.
To
indicate indexed addressing the notation:
(IX+
d) or (IY + d)
is
used. Here d
is
the displacement specified after the
OP
code. The parentheses indicate
that
this value
is
used
as
a
pointer to external memory.
Register Addressing. Many
of
the Z-80A OP codes contain bits
of
information
that
specify which CPU register
is
to
be
used for
an
operation. An example
of
register addressing would be
to
load the data
in
register B into register
C.
Implied Addressing. Implied addressing refers to operations where the
OP
code automatically implies one or more CPU
registers
as
containing the operands. An example
is
the set
of
arithmetic operations where the accumulator
is
always
implied to be the destination
of
the results.
Register Indirect Addressing. This type
of
addressing specifies a 16-bit CPU register pair (such
as
HL) to
be
used
as
a
pointer to any location in memory. This type
of
instruction
is
very powerful and it
is
used in a wide range
of
applica-
tions.
I
OP
Code I } one or two bytes
An examp
le
of
this type
of
instruction would be to load the accumulator with the data in the memory location
pointed to by the HL register contents. Indexed addressing
is
actually a form
of
register indirect addressing except
that
a displacement
is
added with indexed addressing. Register indirect addressing allows for very powerful
but
simple to
implem
ent
memory accesses. The block move and search commands in the Z-80A are extensions
of
this type
of
address-
ing where automatic register incrementing, decrementing and comparing has been added. The notation for indicating
register indirect addressing
is
to
put
parentheses around the name
of
the register
that
is
to be used
as
the pointer.
For
example, the symbol
(HL)
specifies
that
the contents
of
the HL register are to be used
as
a pointer to a memory location. Often register indirect
addressing
is
used to specify 16-bit operands. In this case , the register contents point to the lower order portion
of
the
operand while the register contents are automatically incremented
to
obtain the upper portion
of
the operand.
Bit Addressing. The
Z-80A contains a large number
of
bit set, reset and test instructions. These instructions allow any
memory location or
CPU register to be specified for a bit operation through one
of
three previous addressing modes
(register, register indirect and indexed) while three bits
in
the OP code specify which
of
the eight bits
is
to be manipul-
ated.
ADDRESSING MODE COMBINATIONS
Many instructions include more than one operand (such
as
arithmetic instructions or loads). In these cases, two
types
of
addressing may be employed.
For
example, load can use immediate addressing to specify the source and
register indirect
or
indexed addressing
to
specify the destination.
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