To
reference a large range of locations in main memory a variety of addressing
technique are used. The direct, relative and register addressing modes are
discuss below:
Direct
addressing mode:
 |
| Direct addressing mode |
(i)
Address field contains address of
operand.
(ii)
Effective address (EA) = address
field (A)
(iii)
e. g ADD A
-Add contents of cell A to
accumulator
-Look in memory at address A for
operand.
(iv)
Single memory reference to access
data.
(v)
No additional calculations to work
out effective address.
(vi)
Limited address space.
Relative
addressing mode:
 |
| Relative addressing mode |
A =
contents of an address field in the instruction
R = contents
of an address field in the instruction that refers to a register
Relative
addressing, also called pc-relative addressing, the implicitly referenced
register is the program counter pc. That is, the next instruction address is
added to the address field to produce the EA. typically, the address field is
treated as a two’s complement number for this operation. Thus, the effective
address is a displacement relative to the address of the instruction.
Register
addressing mode:
 |
| Register addressing mode |
Register
Addressing is similar to direct addressing. The only difference is that the
address field refers to a register rather than a main memory address:
EA = R
The
advantages of register addressing are-
(i)
Only a small address field is
needed in the instruction.
(ii)
No time consuming memory references
are required. The memory access time for a register internal to the processor
is much less than that for a main memory address. The disadvantage of register addressing
is that the address space is very limited.
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