Addressing Modes - Control Data 3100 Reference Manual

ADDRESSING MODES
Three modes of addressing are used in the computer: No Address, Direct Address, and
Indirect Address.
No Address
This mode is used when an operand 'y' or a shift count 'k' is placed directly into the lower
portion of an instruction word. Symbols 'a' and 'b' are not used as addressing mode and
index designators with any of the no address instructions.
Direct Address
The direct addressing mode is used in any instruction in which an operand address 'm'
is stored in the lower portion of the initial instruction word. This mode is specified by
making 'a' equal to O. In many instructions. address 'm' may be modified (indexed) by
adding to it the contents of register B
b
,
M=m
+
(B
b
).
Indirect Address
It is possible to use indirect addressing only with instructions that require an execution
address 'm'. For applicable instructions, indirect addressing is specified by making 'a'
equal to 1. Several levels (or steps) of indirect addressing may be used to reach the exe-
cution address; however, execution time is delayed in direct proportion to the number
of steps. The search for a final execution address continues until 'a' equals O. It is im-
portant to note that direct or indirect addressing and address modification are two dis-
tinct and independent steps. In any particular instruction, one may be specified without
the other. Figure 7-3 shows the indirect addressing routine for a 3100 Computer.
Original instruction
possibly containing
'a' and/or 'b'
No
Go to address M.
Acquire new
terms a, b, & m.
Add the
(B
b
)
to m.
No
Execute instruction
using address M.
Figure 7-3. Indexing and Indirect Addressing Routine Flow Chart
NOTE
Unless it is otherwise stated, indirect addressing follows the above routine through-
out the list of instructions.
7-4