Addressing in Detail
Each instruction in machine code is represented by a series of
binary digits. There are two parts to each instruction, an operation (the o=
pcode)
and the data. The data is what the operation is being applied to, there are=
a
number of different ways in which this data can be represented, and this is
known as addressing. Usually the address of the data is given rather than j=
ust
the data.
Direct
Addressing
This is where the address part of the instruction holds the
address of where the data is. For example if an instruction was 00110111 and
the first 3 bits were the instruction e.g. ADD and the last 5 bits were the
address, then the computer would know to add whatever is in the data locati=
on 10111
to the accumulator. Most bytes are bigger than 8 bits, more like 32 or 64 b=
its.
However it does not give access to enough memory, and needs supplementing w=
ith
other methods.
Immediate
Addressing
This is when the value in the instruction is not an address at=
all
but the actual data. This is very simple, although not often used because t=
he
program parameters cannot be changed. This means that the data being operat=
ed
on can’t be adjusted and only uses constants.
Indirect
Addressing
This is where the real address is stored in the memory so the
value in the address part of the instruction is pointing to the address of =
the
data. This method is useful because the amount of space in a location is mu=
ch bigger
than the space in the address part of the instruction. Therefore we can sto=
re
larger addresses and use more memory.
Relative
Addressing
This is direct addressing that does not commence from the star=
t of
the address of the memory. It begins from a fixed point, and all addresses =
are
relative to that point.
Indexed
Addressing
This Is where the address part of t=
he
instruction is added to the value held in a special register. The special
register use is the Index Register (IR). The result of this is then the
required address.
&nb=
sp;
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