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

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! 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???????? 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. 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 non-zero? Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.