HP 3000 III Series Manual page 178

Machine Instructions and Stack Operations
followed each time by a stack marker. Thus, when the final LOAD
Q-4 occurs in preparation for the zero test, the stack appears as
shown at (N).
4-25. FIRST EXIT. The check at address 115 now finds that the
parameter is zero. The checked copy of the parameter is deleted
from the stack (P), figure 4-21 and the branch at address 116
transfers control to address 117 (rather than 121). As mentioned
earlier in paragraph 4-20, an assumed value of 1 is necessary as
an initial previous product in order to begin the mUltiplication
loops. This is accomplished by a LDI 1 instruction (address 117),
which puts a 1 on the top of the stack (Q). Then an uncondition-
al branch at address 120 transfers control to address 126, where
the "1" on the top of the stack is stored into the location re-
served for this recursion's answer as shown at (R). The next
instruction is the EXIT 1 instruction at address 127. This caus-
es Q to move back six locations (Del ta Q
=
6) and S five loca-
tions (EXIT 1 deletes one of the two parameters), as shown at
(S). The return address for the P Register is the MPYM Q-4 in-
struction at address 125. This causes the parameter at Q-4 (1)
to be multiplied by the 1 an the top of the stack, leaving the
answer as the new TOS element. Since lxl
=
1 there is no appar-
ent change from
(5)
to (T) but, in fact, a multiplication has
occurred.
4-26. FIRST RECURSIVE EXIT. The answer of the first mUltiplica-
tion is now stored in the location reserved for it (Q-5) as shown
at (U), figure 4-21 by the STOR Q-S instruction at address 126.
The next instruction, at 127, is again the EXIT 1 instruction
which moves back the stack as shown at (V) and returns the P Reg-
ister to the MPYM Q-4 instruction at address 125. The parameter
for multiplication (at Q-4) is now 2 and, therefore, the multip-
lication result at (W) is 2. Again, this is stored back in the
location reserved for it (Q-5) as shown at (X).
4-27. SUCCESSIVE EXITS. After saving the result, the next EXIT 1
is encountered again, causing the Sand Q stack pointers to move
back to the next marker, leaving the answer 2 on the TOS. The
return for the P Register is again 125, the MPYM
Q-4 instruction
mUltiplies 2x3, and the following STOR Q-5 puts the answer 6 into
the reserved location as shown at (Y), figure 4-21. Similarly,
the last recursive exit causes the value 6 to be left on the TOS
when the last return to address 125 is made. Then the final mul-
tiplication mUltiplies 6x4 and the last STOR Q-5 instruction puts
the answer 24 into the location originally reserved for the end
result FACTORIAL. The last EXIT instruction finds the return for
the Q Register (Delta Q) pointing back to the origin of an earl-
ier procedure and therefore, is not shown in the stack diagram at
(Z). However, since one parameter is saved, the final answer re-
mains on the TOS as shown. Meanwhile, the P Register returns to
the next instruction in the outer block which is the STOR DB 15
instructi.on at address 133. This saves the answer in the global
area and a final PCAL returns control to the system.
4-48