Editing Addressing mode (section)

==={{anchor|REGISTER PLUS IMMIEDIATE CONSTANT}}Base plus offset, and variations===
This is sometimes referred to as 'base plus displacement' or 'truncated'.
    +------+-----+-----+----------------+
    | load | reg | base|     offset     |  reg := RAM[base + offset]
    +------+-----+-----+----------------+
 
    (Effective address = offset + contents of specified base register)

If the offset is zero, this becomes an example of ''register indirect'' addressing; the effective address is just the value in the base register.

On many RISC machines, register&nbsp;0 is fixed at the value zero. If register&nbsp;0 is used as the base register, this becomes an example of ''absolute addressing''. However, only a small portion of memory can be accessed.

The offset is often small in relation to the size of current computer memories. However, the principle of [[locality of reference]] applies: over a short time span, most of the data items a program wants to access are fairly close to each other.

This addressing mode is closely related to the indexed absolute addressing mode.

''Example 1'':
Within a subroutine a programmer will mainly be interested in the parameters and the local variables, for which one base register (the [[frame pointer]]) usually suffices. If this routine is a class method in an object-oriented language, then a second base register is needed which points at the attributes for the current object ('''this''' or '''self''' in some high level languages).

''Example 2'':
If the base register contains the address of a [[composite type]] (a record or structure), the offset can be usually used to select a field from that record.