HP 3000 III Series Manual page 67

System/CPU Overview
2-48. ADDRESSING CONVENTIONS
2-49. Memory Addressing
Throughout this manual the terms "displacement", "effective ad-
dress ", "relative address ", and "base" are used in connection
with memory addressing. These terms are defined as follows:
Displacement is a positive number given in the instruction
word pointing to a location plus or minus that number of lo-
cations from a given reference cell which is also given in the
instr uction word.
Effective address is always an absolute address. It mayor
may not be the location indicated by the displacement number.
The effective address is the final computed address after
displacement calculation, indirect addressing (if any), and
indexing (if any) have all been resolved.
Relative address is the address obtained by subtracting the
base fran the effective address.
Base is either the program base (PB) address or the data base
(DB) address.
The canputer system uses relative addressing almost exclusively.
Addressing can be relative to the location pointed to by the
P Register, DB Register, Q :R:gister, or S-pointer. As shown in
figure 2-14, memory address instructions (paragraph 2-44) use
bits 6 through 15 for "mode and displacement" and addressing can
be + or relative to P or Q, but only + relative to DB and -
relative to S. The relative addressing displacement ranges for
the various modes are also shown in figure 2-14. (It should be
noted that these ranges apply only to direct, unindexed addres-
sing. Indirect addressing and indexing are discussed separately
in paragraphs 2-50 and 2-53.) The variety of displacement ranges
is due to the particular coding required to specify a given mode.
For example, only two bits (6 and 7) are required to specify the
P+, P-, and DB+ relative modes. This leaves bits 8 through 15
available for
displaceme~t
which, therefore, can be any value
from 0 through 255. For Q+ relative mode, bits 9 through 15 give
a displacement range fram 0 through 127. For Q- and S-relative
modes, bits 10 through 15 give a displacement range from 0
through 63. In order to provide the most efficient usage of
bits, the mode codes are assigned according to the respective
needs of each displacement range.
Referring to figure 2-14, note that the DB+, Q-, Q+, and S-ranges
can overlap. Also, DB+, Q+, and S- can actually address words
currently held in the TOS registers. P+ and P- addressing modes
are typically used for branches and referencing of literals. The
DB+ mode is used for referencing global variables and pointers
(i .e., indirect addresses). The Q+ and Q- modes are useful for
local variable storage and passing of procedure parameters
respectively. The S- mode is typically used for accessing param-
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