HP 3000 III Series Manual page 176

Machine Instructions and Stack Operations
4-21. MAIN PROGRAM CALL. As in previous examples,the main program
has already reserved global space for the final answer (Y) before
the procedure is callede When the call is given, the ZERO,NOP
instruction at address 130 reserves space for the procedure re-
sult, FACTORIAL. (Compare stacks (a), figure 4-20 and (Z), figure
4-21. This is the first stack addition due to calling the pro-
cedure. Next, the actual parameter 4 is loaded on (B), and then
the PCAL instruction is issued. This causes the first stack
marker to be loaded (C). This marker differs from the ones that
follow in that is contains return information to the ovter block
which called the present procedure (i.e., the return P word is a
P-relative address for return to the caller in the code segment
and Delta
Q
points back to the
Q
value that the caller was using
earlier in the stack). Now, Sand Q are both pointing at the last
word of the first marker for this procedure.
4-22. TEST FOR ZERO. At addresses 114 and 115 (stack (D) and (E),
figure 4-20), the procedure parameter is first tested for zero.
This is done by copying it onto the top of the stack (LOAD Q-4)
and a CMFI 0 instruction. This instruction sets the condition
code according to comparison results and deletes the tested word
(E). Sin ce the fir s t te s t i s no n- z e r
0 (
i . e., 4), the bra nch
instruction at line 116 transfers control to address 121 (i .e.,
P+ 4). This test and branch will be repeated in each of the fol-
lowing recursion loops until the parameter has become zero.
4-23. FIRST RECURSIVE CALL. The branch to address 121 causes the
procedure to call itself. As usual, the first action of the call
is to load the procedure parameters onto the stack. The para-
meters in this case are the variable FACTORIAL and a decremented
form of the original passed parameter. Thus the ZERO, NOP in-
struction reserves a location for FACTORIAL (F), figure 4-20
strictly for use by this recursion (i.e., distinct from the final
FACTORIAL location reserved at (A). Then (G and H), the new par-
ameter, is obta ined by' cop ying the preced ing val ue to the top of
the stack (LOAD Q-4) and decrementing with a SUBI 1 instruction.
Afte r load ing parameters for the new call, another PCAL instr uc-
tion is issued. This causes a new stack marker (I), figure 4-20
and, via the Segment Transfer Table, control is transferred back
to the starting point of the procedure at address 114. The new
stack marker gives as its return P value the address immediately
following the PCAL which is 125. (This will be important to re-
member when the exit sequence is discussed.) Also, the Delta Q
value is 6 since the previous Delta
Q was six locations back.
4-24. SUCCESSIVE RECURSIONS. Next, all the previously des.cr ibed
steps are repeated, beginning with paragraph 4-22. Since the
parameter is 3 on the second recursion, the branch to address 121
again occurs. The first actions, again, are to reserve a loca-
tion for this recursion's answer
(J),
figure 4-20 and to load a
decremented parameter value of 2 (K) and (L). After this, the
procedure call back to the beginning is made again which results
in another stack marker (M) that is identical to the one gen-
erated on the first recursion. The third and fourth recursions
repeat the entire process again, loading parameters of 1 and 0
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