ZiLOG Z80-CPU Technical Manual page 28

An example of an indexed instruction would be to load the contents of the memory location
(Index Register
+
Displacement) into the accumulator. The displacement is a signed two's complement
number. Indexed addressing 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-80 are referred to as IX and IY. To indicate indexed addressing the
notation:
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-80 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 applications.
I
OP Code
I}
one or two bytes
An example 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 implement memory accesses. The block move and search commands
in the Z-80 are extensions of this type of addressing 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-80 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 manipulated.
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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