A Guide to Array Implementation 

Prepare index registers x and y for runtime.

For declaration var w, a[10]; record w as variable and
a as array together with the size 10 or 11 according to
your definition. For assignment statement such as

   a[i+j]:=w+a[j-k];

the value of the left index i+j is stored in register y
and that of the right index j-k in register x. Those can be
processed by expression. To load the value of a[j-k], use
the new machine instruction ldx.


   stx 0,0    store the top of stack into x
   sty 0,0    store the top of stack into y
   ldx l,a    load s[base(l)+a+x] into the top of stack
   ssy l,a    store the top of stack into s[base(l)+a+y]

Example  $ for <=

var i, y, z[10];
begin
  i:=1;
    while i$9 do begin
      z[i]:=i;
      i:=i+1;
   end;
  y:=0; i:=0;
  while i$9 do begin
      y:=y+z[i];
    i:=i+1;
  end;
  write(y);
  write(z[5]);
end.

 0 jmp 0 1 
 1 inc 0 15 
 2 lit 0 1 
 3 sto 0 3       i:=1;
 4 lod 0 3       load i
 5 lit 0 9       load literal 9
 6 opr 0 11      i<=9?
 7 jpc 0 17      jump to 17 if false
 8 lod 0 3       load i
 9 sty 0 0       store stack-top to index register y
10 lod 0 3       load i
11 ssy 0 5       z[i]:=i;
12 lod 0 3 
13 lit 0 1 
14 opr 0 2 
15 sto 0 3       i:=i+1;
16 jmp 0 4       go back to while
17 lit 0 0 
18 sto 0 4       y:=0;
19 lit 0 0 
20 sto 0 3       i:=0;
21 lod 0 3 
22 lit 0 9 
23 opr 0 11      i<=9?
24 jpc 0 36      jump to 36 if false
25 lod 0 4       load y
26 lod 0 3       load i
27 stx 0 0       store stack-top to index register x
28 ldx 0 5       load z[i]
29 opr 0 2       y+z[i]
30 sto 0 4       y:=y+z[i];
31 lod 0 3 
32 lit 0 1 
33 opr 0 2 
34 sto 0 3       i:=i+1;
35 jmp 0 21      go back to while
36 lod 0 4 
37 wrt 0 0 
38 lit 0 5      
39 stx 0 0       index register x is set to 5
40 ldx 0 5       load z[5]
41 wrt 0 0       write z[5], which is stack-top
42 opr 0 0 

start PL/0
45
5
 END PL/0

start PL/0
0  
1  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
2  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 
3  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 
4  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 
5  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 9 
6  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 
7  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 
8  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 
9  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 
10  0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 
11  0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 
12  0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 
13  0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 
14  0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 2 
15  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 
16  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 
4  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 2 
5  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 2 9 
6  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 1 
7  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 
8  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 2 
9  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 
10  0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 2 
11  0 0 0 2 0 0 1 2 0 0 0 0 0 0 0 
12  0 0 0 2 0 0 1 2 0 0 0 0 0 0 0 2 
13  0 0 0 2 0 0 1 2 0 0 0 0 0 0 0 2 1 
14  0 0 0 2 0 0 1 2 0 0 0 0 0 0 0 3 
15  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 
16  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 
4  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 3 
5  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 3 9 
6  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 1 
7  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 
8  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 3 
9  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 
10  0 0 0 3 0 0 1 2 0 0 0 0 0 0 0 3 
11  0 0 0 3 0 0 1 2 3 0 0 0 0 0 0 
12  0 0 0 3 0 0 1 2 3 0 0 0 0 0 0 3 
13  0 0 0 3 0 0 1 2 3 0 0 0 0 0 0 3 1 
14  0 0 0 3 0 0 1 2 3 0 0 0 0 0 0 4 
15  0 0 0 4 0 0 1 2 3 0 0 0 0 0 0 
16  0 0 0 4 0 0 1 2 3 0 0 0 0 0 0 
...
45
5
END PL/0