PDP-11 Integer Divide Instruction "Div", To: General Discussion: On-Topic and Off-Topic Post

Jerome H. Fine jhfinedp3k at compsys.to
Tue Feb 14 07:28:54 CST 2006

 >Johnny Billquist wrote:

> >"Jerome H. Fine" wrote:
> > I have noticed what may be an interesting result when I use the
> > PDP-11 Integer Divide Instruction "Div".  Since I have noticed
> > at least one individual who worked on the microcode for the
> > PDP-11, perhaps there is an explicit "Yes / No" answer to my
> > question:
> Since this actually have nothing to do with the microcode, and 
> actually is nothing specific with the PDP-11 DIV instruction, just 
> about anyone should be able to answer definitely. 

Jerome Fine replies:

I suggest that you might not be aware of the exact implementation
of the PDP-11 integer "Div" instruction when an overflow occurs.

Please see below!

> > If I divide 196612 by 3 - i.e. "Div (R2),R0" where R0 = 3, R1 = 4, 
> (R2) = 3
> > the result (in addition to the condition bits) is R0 = 1, R1 = 1 
> which is
> > exactly correct if the quotient is regarded as a 32 bit result with 
> R0 being
> > the low order 16 bits of that result and the high order 32 bits are
> somewhere
> > else - probably inaccessible as far as programming is concerned, but 
> easily
> > obtained by:
> >   Mov R1-(SP)    ; Save low order 16 bits of dividend
> >   Mov R0,R1       ;  Divide high order 16 bits
> >   Clr    R0            ;      of dividend
> >   Div    (R2),R0   ;      by the divisor
> >   Mov  R0,R3      ; Save high order 16 bits of quotient
> >   Mov  R1,R0      ; Divide the remainder
> >   Mov  (SP)+,R1 ;      of the dividend
> >   Div    (R2),R0   ;      by the divisor
> > i.e. R3 now contains the high order 16 bits of the 32 bit quotient
> > with R0 holding the low order 16 bits of the 32 bit quotient
> What you have implemented here, as well as described, is the exact way 
> you should have been taught how to do division on paper in elementary 
> school.
> Yes, that algorithm is valid, and can be extended to arbitrary sizes, 
> as long as you remember the full method.

I agree that the above code is the "correct" method to ensure
a valid result.  BUT, that is NOT what I am attempting to determine.

Specifically, I have found that the following code also works:
   Mov  R0,R3
   Div    (R2),R0  ; First Divide Instruction
   Tst     R1
   Bne    Somewhere - since the quotient is not of interest when a 
non-zero remainder
    Mov  R0,-(SP)
    Mov  R3,R1
    Clr     R0
    Div     (R2),R0  ; Second Divide Instruction
    Mov   (SP)+,R1
At this point, R0 / R1 now contains 32 bit quotient IF the first
"Div" instruction places the low order 16 bits of the 32 bit
quotient into R0.  I have found this result in practice and since
there is a VERY  HIGH probability that the remainder is NOT
zero, the above code is MUCH faster.

Again, the specific question is IF the quotient of the "Div" instruction
is the low order 16 bits of a 32 bit quotient all of the time or just
when the high order 16 bits are all zero????????

> > Can anyone confirm what I have found in practice?
> Certainly. It's basic math, the way it's taught in elementary school.
> That was atleast the first way I was taught how do do divides on big 
> numbers on paper.

I learned that also, but the observation is not relevant
to my question.

I realize that the DEC manual description of the "Div" instruction
does not address the situation when the quotient exceeds 65535
(decimal) or 16 bits, but again, perhaps someone who knows
the microcode might have an answer.

> > Even better would be a method of retrieving the high
> > order 16 bits of the quotient in a manner which takes
> > fewer instructions and without a second divide instruction!
> I doubt you'll find it.

I  AGREE!!  It would have been "nice" though if
DEC knew where the value was and made that high
order 16 bits available via the next instruction
if the user needed it.  That information would
also have exactly defined whether or not the low
order 16 bits of the quotient and the remainder
were correct all of the time.  Any comments on these
TWO observations?

I realize that the instruction set is long past being
subject to change in DEC hardware, but that does not
mean that an emulator could not manage to make a few
small but vital improvements.  And certainly, at least
in SIMH, it is possible to examine the code to determine
the answer to my original question.  Does anyone have
the code for the "Div" emulation in SIMH and what does
happen when the high order 16 bits of the quotient are

Sincerely yours,

Jerome Fine
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