Indirect Branch Via Stack - HP 3000 III Series Manual

Machine Instructions and Stack Operations
The DB,
Q,
and S relative mode, however, are unconventional in
that they permit indirect branches through the data stack. (It
is both illegal and impossible to have a direct branch to the
stack: the coding of "01" for bits 5 and 6 encodes the Bee
instr uction. )
Figure 4-8 shows an example of the S- relative mode. Assume that
the instruction in location P specifies the S- relative mode,
with a displacement of 4, and indexing. This causes an indirect
branch to S-4 in the data stack. The content of S-4 is then ad-
ded to PB, thus point ing at loca tion "a" in the code segment.
Since indexing is specified, the value contained in the Index
Register is also added to the address being computed. Thus the
ultimate effective address for the branch (next
P)
is location
"a" displaced by the index value.
CODE
DATA
Displacement
4
DB
PB + (S-4)
+
PB + (S-41 +
x
a
S-41
S
t
=
P
a
P8
Next P
Figure 4-8. Indirect Branch Via Stack
Note particularly that the indirect address given in the stack is
relative to the program base, PB, not to P as is usually the
case. Also note that the displacement is relative to a location
in the stack (DB, Q, or S), and that indexing is applied after
the indirect addressing has been accomplished.
the DB, Q, and S modes depends on
For DB+, bits 8 through 15 provide a
For Q+, bits 9 through 15 provide a
For Q- and S-, bits 10 through 15 pro-
-63.
The displacement range for
which mode is se lected.
range of 0 through +255.
range of 0 through +127.
vide a range of 0 through
4-24