Sharp MZ-80B Owner's Manual page 93
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- Owner's manual (138 pages) ,
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86
Modified Page Zero Addressing. The Z-80A has
a special
single byte CALL instruction to any of 8 locations in page
zero of memory. This instruction (which is referred to as a restart)
sets
the PC to an effective address in page zero. The
value of this instruction is that it allows a single byte to specify a complete 16-bit address where commonly
called
sub-
routines are located, thus saving memory space.
OP Code
one byte
b7
bo
Effective address is (b
5
b
4
b
3
000)
2
Relative Addressing. Relative addressing uses one byte of data following the OP
code
to specify a displacement from
the existing program to which a program jump can occur. This displacement is
a signed
two's
complement
number that
is added to the address of the OP
code
of the following instruction.
f-O_P_C_o_
d e
-----1
} Jump relative (one byte OP code)
Operand
8-bit two's complement displacement
added
to Address (A+2)
The value of relative addressing is that it allows jumps to nearby locations while only requiring two bytes of memory
space. For most programs, relative jumps are by far the most prevalent type of jump due to the proximity of
related
program segments.
Thus,
these instructions can significantly reduce memory
space
requirements. The signal displace-
ment can range between +127 and
-
128 from A+2. This allows for
a
total displacement of +129 to
-126 from
the
jump relative OP
code
address. Another major advantage is that it
allows
for relocatable
code.
Extended Addressing. Extended Addressing provides for two bytes (16 bits) of
address
to be included in the instruc-
tion
.
This data can be an
address
to which a program
can
jump or it can be an address where an
operand
is located.
f---O_P_C_o_d_e _
_
_ _ _ _
__ _
__ _ _
--1
}
one or two bytes
Low Order Address
or
Low order operand
High Order Address
or
High order operand
Extended addressing is required for
a
program to jump from any location in memory to any other location, or load
and
store data in any memory location.
When extended
addressing
is used to specify the source or destination address of an operand, the notation (nn) will
be used to indicate the content of memory at nn, where nn is the 16-bit address
specified
in the instruction
.
This means
that the two bytes
of
address nn
are
used as a pointer to a memory location. The use of the parentheses always means
that the value enclosed within them is used as a pointer to a memory
location.
For example
,
(1200) refers to the
contents of memory
at
location 1200.
Indexed
Addressing.
In this type of addressing, the byte of data following the OP
code
contains a displacement which
is added to one of the two index registers (the OP code specifies which index register is used) to form a pointer to mem-
ory. The contents of the index register
are
not altered by this operation.
OP Code
}
f------- -
------1
two byte OP code
OP Code
Displacement
Operand added to index register to form a pointer to memory
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