Why does the ADD command in Donald Knuths number one program written on MIX set the overflow to ON?

Question

Here is the program:

STZ 1
ENNX 1
STX 1(0:1)
SLAX 1
ENNA 1
INCX 1
ENT1 1
SRC 1
ADD 1
DEC1 -1
STZ 1
CMPA 1
MOVE -1,1(1)
NUM 1
CHAR 1
HLT 1

What I know so far:

STZ 1 sets the next instruction to NOP so the second instruction can be ignored
According to the answer - the ADD function should trigger the overflow.
The ADD function should add the contents of Memory 1 to Register A

Memory 1 is set to zero with the first command - the ADD function should simply add zero to Register A.

If Mem1 is set to zero how does this set the overflow toggle to on?

REF: The Art of Computer Programming Volume 1 Page 142 Question 18

Do you know for sure that the program is located starting at 0? The code shown does not have a `ORIG` directive. — Erik Eidt, Feb 16 '22 at 00:42
Aha the answer says these are the results unless the program begins in location 0. Many thanks. — Jonathan Harker, Feb 16 '22 at 07:51

score 2 · Accepted Answer · answered Feb 16 '22 at 16:16

The program appears to use location 1 as data, and so the code itself should be located somewhere past that, as a stab in the dark, say starting from 0x10. If the code is located starting at 0 (or 1) then it will overwrite itself, with undesired results.

Older computers, like PDP-8, would start the program counter at 0, but also use data down low like that (on the zero page), so you would see programs contain a first instruction at 0 being a JUMP to the actual code start, then data after that, then the actual code.

Why does the ADD command in Donald Knuths number one program written on MIX set the overflow to ON?

1 Answers1