As we have seen, all symbolic
addresses are based on variants of the concept of
base address (stored in a base register) and an offset.
Note that the offset, encoded
as a 12–bit unsigned integer, is always non–negative.
The possible offset values
range from 0 through 4095 inclusive.
We now introduce a way to
reference a storage position relative to the symbolic
address of another label. This allows direct reference to unlabeled storage.
The form of a relative address
where N is the byte offset of
the desired storage relative to the symbolic address associated with LABEL.
Again, note the lack of spaces
in the relative address. This is
Consider the two data
F1 DC F‘0’
A four–byte full-word.
F2 DC F‘2’
Another full-word at address F1 + 4
Consider the following two
instructions. They are identical.
L R6, F2
L R6, F1+4
Addressing: A More Common Use
The most common use of relative
addressing is to access an unlabeled section
of a multi–byte storage area associated with a symbolic address.
Consider the following very
common declaration for card data. It
sets aside a storage
of 80 bytes to receive the 80 characters associated with standard card input.
While only the first byte (at
offset 0 from CARDIN) is directly named, we may use
relative addressing to access any byte directly. Consider this figure.
The second byte of input it is
at address CARDIN+1, the third at CARDIN+2, etc.
Remember that the byte at address
is the character in column (N + 1)
of the card. Punched cards do not have a column 0.