Hi All, 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: 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 Reason for the question: I require the complete quotient ONLY when the remainder is ZERO (specifically when the dividend is not a prime number since (R2) is ALWAYS a prime number). Thus, if I can avoid performing the first division except when the remainder is zero and I require the high order 16 bits of the quotient to continue, I can optimize the code. Thus far I have tested the code on numbers up to 100,000,000 when I divide by 3 and the results are always correct. Can anyone confirm what I have found in practice? 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! For those who are interested, I am attempting to calculate the Mobius Function - mu(n). If you check on Google, mu(n) requires the number of factors of n. Thus while a sieve program can be used to remove the prime numbers, the rest of the numbers must be factored to obtain the number of factors. 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.