HP 3000 III Series Manual page 168

Machine Instr uctions and Stack Opera"tions
LOAD/DE LETE
After Delete After Load
Initial Stack Instruction Instruction
0 - "
1
a---.
i
Q~
1
11 11 11
500 500 500
7 7 7
63
S--.
63 63
S---.
44
S---.
37
0 0
®
ADD/MUL TIPLV/SUBTRACT/NEGATE
After Add
After Multiply
After Subtract After Negate
Instruction Instruction Instruction
Instruction
0---+
1
O~
1
0----'
1 0--'
1
11 11 11 11
500 500
s---+
-200
S--.
200
7
S -----..
700
s---+
100
0 CD
0
®
Figure 4-15. Basic Arithmetric Stack Operations
To subtract (SUB), the top element is subtracted from the next-
to-top element. Thus the answer at (F) is the result of 500-700,
or -200. (As before, only the answer remains after computation
is performed.) Finally, at (G), negation is performed. This
simply re verses the's ign of the n umber of the TOS. In binary
form, a two's complement operation is performed.
Although the sequence (A) through (G) in figure 4-15 is a very
simple series of operations, it does illustrate the advantages of
the stack technique in computation. First, note that regardless
of how many elements of data there aTe or what memory cells they
occupy, the operand for each instruction is consistently the same
; the TOS. This permits implicit addressing; i.e., since the
operand is understood to be the TOS, it is not necessary
to~ive
an. operand address in the instruction word. Thus (except for
LOAD which must specify a relative address to load from), the
instruction can simply say "addu, pr "multiply", etc. The im-
mediate benefit of this is that it allows code compression. Two
instructions can be given in a single word. The sequence (D)
through (G) for example, can be given in two instr uction words.
Since this reduces the number of memory fetches, the speed of
computation is considerably increased. A second point to note is
that temporary storage of intermediate results is automatically
provided. For example, once the parameters 63 and 37 (C) have
been added, they are no longer required and are deleted. The
answer however, which is substituted on the TOS, is automatically
4-38