Addressing Modes - Control Data 3300 Reference Manual

Special index considerations apply to BDP instructions where an index register
flag is pre sent. A flag defines which index register is used for indexing:
EXAMPLE:
If s = 00413, Bs = 2, and (B2
=
00364, then S = s
+
[BsJ or "the
modified address'S' equals the unmodified address's' added to the
contents of the index register as defined by Bs".
Thus:
S
=
s
+ [
BsJ
S 00413
+
(B2)
S
=
00413
+
00364
S
=
00777
Some BDP instructions, i. e. , PAK, CVBD, DTA, etc. utilize both word and
character addresses in their formats. Although the first two bits preceding the
address are unused and not part of the word address, the lower 15 bits of this
word are added to the contents of the specified index register. The lower two
bits of the specified index register must be set to "l's" to allow for an end-
around carry during the index addition.
EXAMPLE:
161514
Addressing Modes
020100
n
~
V
[Bn] are added to
the first 15 bits
, ,
,
I
Three modes of addressing are used in the computer: No Address, Direct Ad-
dress, and Indirect Address.
No Address
This mode is used when an operand 'y' or a shift count 'k' is placed directly in-
to 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 ad-
dress '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 Bb, M
=
m
+
(BD).
5-7
Rev. B