Valid HTML 4.01 Transitional Valid CSS Valid SVG 1.0

Me, myself & IT

Donald Knuth’s Algorithm D, its implementation in Hacker’s Delight, and elsewhere

Purpose
Introduction
Generic implementation in Hacker’s Delight
Specialised implementation in Hacker’s Delight
Flawed implementation in libdivide
Proper (and optimised) implementation …
… in ANSI C
… in Assembler

Purpose

Show some shortcomings of the multiple precision division implementations presented in Hacker’s Delight, and their effect elsewhere.

Introduction

In Volume 2: Seminumerical Algorithms, Chapter 4.3: Multiple-Precision Arithmetic of The Art of Computer Programming, Donald Ervin Knuth presents Algorithm D (Division of nonnegative integers):

Note: I added an introductory step D0 providing definitions and descriptions.

Algorithm D (Division of nonnegative integers).
D0. [Define]
Let U be the dividend (or numerator) of m+n digits, stored in an array of m+n+1 elements, one digit per element, with the most significant digit in element U[m+n−1] and the least significant digit in element U[0];
Let V be the divisor (or denominator) of n digits, stored in a second array of n elements, with n greater than 1, a non-zero most significant digit in element V[n−1] and the least significant digit in element V[0];
Let B be the base (or radix) of the digits (also limbs, places or words), typically a power of 2.
(The algorithm computes the quotient Q of m+1 digits as ⌊U ⁄ V⌋ (also U div V or U ÷ V), and the remainder R of n digits as U modulo V (also U mod V or U % V), using the following primitive operations:
addition or subtraction of two single-digit integers, giving a single-digit sum and a carry or a single-digit difference and a borrow;
multiplication of two single-digit integers, giving a double-digit product;
division of a double-digit integer by a single-digit integer, giving a single-digit quotient and a single-digit remainder.)
D1. [Normalize]
Set D to (B − 1) ÷ V[n−1];
Multiply all digits of U and V by D.
(On a binary computer, choose D to be a power of 2 instead of the value provided above; any value of D that results in V[n−1] not less than B ÷ 2 will suffice. If D is greater than 1, the eventual overflow digit of the dividend goes into element U[m+n].)
D2. [Initialize j]
Set the loop counter j to m.
D3. [Calculate Q′]
Set Q′ to (U[n+j] * B + U[n−1+j]) ÷ V[n−1];
Set R′ to (U[n+j] * B + U[n−1+j]) % V[n−1];
Test if Q′ equals B or Q′ * V[n−2] is greater than R′ * B + U[n−2+j];
If yes, then decrease Q′ by 1, increase R′ by V[n−1], and repeat this test while R is less than B.
D4. [Multiply and subtract]
Replace (U[n+j]U[n−1+j]…U[j]) by (U[n+j]U[n−1+j]…U[j]) − Q′ * (V[n−1]…V[1]V[0]).
(The digits (U[n+j]…U[j]) should be kept positive; if the result of this step is actually negative, (U[n+j]…U[j]) should be left as the true value plus Bn+1, namely as the B’s complement of the true value, and a borrow to the left should be remembered.)
D5. [Test remainder]
Set Q[j] to Q′;
If the result of step D4 was negative, i.e. the subtraction needed a borrow, then proceed with step D6; otherwise proceed with step D7.
D6. [Add back]
Decrease Q[j] by 1 and add (0V[n−1]…V[1]V[0]) to (U[n+j]U[n−1+j]…U[1+j]U[j]).
(A carry will occur to the left of U[n+j], and it should be ignored since it cancels with the borrow that occurred in step D4.)
D7. [Loop on j]
Decrease j by 1;
Test if j is not less than 0;
If yes, go back to step D3.
D8. [Unnormalize]
Now (Q[m]…Q[1]Q[0]) is the desired quotient, and the desired remainder may be obtained by dividing (U[n−1]…U[1]U[0]) by D.

Generic implementation in Hacker’s Delight

In his book Hacker’s Delight, Henry S. ("Hank") Warren presents the following generic implementation divmnu64.c of Donald Knuth’s Algorithm D in ANSI C:

Note: I removed some parts of the code which are not relevant here.

/* This divides an n-word dividend by an m-word divisor, giving an
n-m+1-word quotient and m-word remainder. The bignums are in arrays of
words. Here a "word" is 32 bits. This routine is designed for a 64-bit
machine which has a 64/64 division instruction. */ 1
[…]
/* q[0], r[0], u[0], and v[0] contain the LEAST significant words.
(The sequence is in little-endian order).

This is a fairly precise implementation of Knuth's Algorithm D, for a
binary computer with base b = 2**32. The caller supplies:
   1. Space q for the quotient, m - n + 1 words (at least one).
   2. Space r for the remainder (optional), n words.
   3. The dividend u, m words, m >= 1.
   4. The divisor v, n words, n >= 2.
The most significant digit of the divisor, v[n-1], must be nonzero.  The
dividend u may have leading zeros; this just makes the algorithm take
longer and makes the quotient contain more leading zeros.  A value of
NULL may be given for the address of the remainder to signify that the
caller does not want the remainder.
   The program does not alter the input parameters u and v.
   The quotient and remainder returned may have leading zeros.  The
function itself returns a value of 0 for success and 1 for invalid
parameters (e.g., division by 0).
   For now, we must have m >= n.  Knuth's Algorithm D also requires
that the dividend be at least as long as the divisor.  (In his terms,
m >= 0 (unstated).  Therefore m+n >= n.) */

int divmnu(unsigned q[], unsigned r[],
     const unsigned u[], const unsigned v[],
     int m, int n) {

   const unsigned long long b = 4294967296LL; // Number base (2**32).
   unsigned *un, *vn;                         // Normalized form of u, v.
   unsigned long long qhat;                   // Estimated quotient digit.
   unsigned long long rhat;                   // A remainder.
   unsigned long long p;                      // Product of two digits.
   long long t, k;
   int s, i, j;

   if (m < n || n <= 1 || v[n-1] == 0)
      return 1;                         // Return if invalid param.
[…]
   /* Normalize by shifting v left just enough so that its high-order
   bit is on, and shift u left the same amount. We may have to append a
   high-order digit on the dividend; we do that unconditionally. */

   s = nlz(v[n-1]);             // 0 <= s <= 31.
   vn = (unsigned *)alloca(4*n);
   for (i = n - 1; i > 0; i--)
      vn[i] = (v[i] << s) | ((unsigned long long)v[i-1] >> (32-s));
   vn[0] = v[0] << s;

   un = (unsigned *)alloca(4*(m + 1));
   un[m] = (unsigned long long)u[m-1] >> (32-s);
   for (i = m - 1; i > 0; i--)
      un[i] = (u[i] << s) | ((unsigned long long)u[i-1] >> (32-s));
   un[0] = u[0] << s;

   for (j = m - n; j >= 0; j--) {       // Main loop.
      // Compute estimate qhat of q[j].
      qhat = (un[j+n]*b + un[j+n-1])/vn[n-1];
#ifdef OPTIMIZE
      rhat = (un[j+n]*b + un[j+n-1])%vn[n-1];
#else
2    rhat = (un[j+n]*b + un[j+n-1]) - qhat*vn[n-1];
#endif
again:
#ifdef OPTIMIZE
      if (qhat >= b || (unsigned)qhat*(unsigned long long)vn[n-2] > b*rhat + un[j+n-2])
#else
3    if (qhat >= b || qhat*vn[n-2] > b*rhat + un[j+n-2])
#endif
      { qhat = qhat - 1;
        rhat = rhat + vn[n-1];
        if (rhat < b) goto again;
      }

      // Multiply and subtract.
      k = 0;
      for (i = 0; i < n; i++) {
#ifdef OPTIMIZE
         p = (unsigned)qhat*(unsigned long long)vn[i];
#else
3       p = qhat*vn[i];
#endif
         t = un[i+j] - k - (p & 0xFFFFFFFFLL);
         un[i+j] = t;
         k = (p >> 32) - (t >> 32);
      }
      t = un[j+n] - k;
      un[j+n] = t;

      q[j] = qhat;              // Store quotient digit.
      if (t < 0) {              // If we subtracted too
         q[j] = q[j] - 1;       // much, add back.
         k = 0;
         for (i = 0; i < n; i++) {
            t = (unsigned long long)un[i+j] + vn[i] + k;
            un[i+j] = t;
            k = t >> 32;
         }
         un[j+n] = un[j+n] + k;
      }
   } // End j.
   // If the caller wants the remainder, unnormalize
   // it and pass it back.
   if (r != NULL) {
      for (i = 0; i < n-1; i++)
         r[i] = (un[i] >> s) | ((unsigned long long)un[i+1] << (32-s));
      r[n-1] = un[n-1] >> s;
   }
   return 0;
}
[…]
This implementation exhibits the following misrepresentation and shortcomings:
  1. Contrary to the highlighted part of its initial comment, this routine does not need a 64-bit machine; it but needs support for 64-bit integers and elementary 64-bit arithmetic operations, which ANSI C compilers provide since the last millennium.
  2. Although a 64/64 division instruction is explicitly stated, the code doesn’t take full advantage of it: instead to use the % operator to compute get the remainder of the 64÷64-bit division (which comes for free on 64-bit machines, and almost for free with software implementations on 32-bit machines), it but performs an extraneous 64×64-bit multiplication — which is not free, especially on 32-bit machines!
  3. The previous argument also holds for the multiplications qhat*vn[n-2] and qhat*vn[i]: while an optimising compiler should detect that these are 32×32-bit multiplications returning a 64-bit product, I would not count on it — better give the compiler the proper hints!
Note: Knuth’s Algorithm D builds upon a 64÷32-bit division returning a 32-bit quotient and a 32-bit remainder, and a 32×32-bit multiplication returning a 64-bit product!

Specialised implementation in Hacker’s Delight

Hank also presents a simplified and specialised implementation divlu.c for unsigned 64÷32-bit division, based on a 32÷16-bit division returning a 32-bit quotient and a 16-bit remainder, i.e. suited for 16-bit processors:

Note: I removed some parts of the code which are not relevant here.

/* Long division, unsigned (64/32 ==> 32).
   This procedure performs unsigned "long division" i.e., division of a
64-bit unsigned dividend by a 32-bit unsigned divisor, producing a
32-bit quotient.  In the overflow cases (divide by 0, or quotient
exceeds 32 bits), it returns a remainder of 0xFFFFFFFF (an impossible
value).
   The dividend is u1 and u0, with u1 being the most significant word.
The divisor is parameter v. The value returned is the quotient.
   […] Several of the variables below could be
"short," but having them fullwords gives better code on gcc/Intel.
[…]
This is the version that's in the hacker book. */

unsigned divlu2(unsigned u1, unsigned u0, unsigned v,
                unsigned *r) {
   const unsigned b = 65536; // Number base (16 bits).
   unsigned un1, un0,        // Norm. dividend LSD's.
            vn1, vn0,        // Norm. divisor digits.
            q1, q0,          // Quotient digits.
            un32, un21, un10,// Dividend digit pairs.
            rhat;            // A remainder.
   int s;                    // Shift amount for norm.

   if (u1 >= v) {            // If overflow, set rem.
      if (r != NULL)         // to an impossible value,
         *r = 0xFFFFFFFF;    // and return the largest
      return 0xFFFFFFFF;}    // possible quotient.

   s = nlz(v);               // 0 <= s <= 31.
   v = v << s;               // Normalize divisor.
   vn1 = v >> 16;            // Break divisor up into
   vn0 = v & 0xFFFF;         // two 16-bit digits.

   un32 = (u1 << s) | (u0 >> 32 - s) & (-s >> 31);
   un10 = u0 << s;           // Shift dividend left.

   un1 = un10 >> 16;         // Break right half of
   un0 = un10 & 0xFFFF;      // dividend into two digits.

   q1 = un32/vn1;            // Compute the first
#ifdef OPTIMIZE
   rhat = un32%vn1;          // quotient digit, q1.
#else
   rhat = un32 - q1*vn1;     // quotient digit, q1.
#endif
again1:
   if (q1 >= b || q1*vn0 > b*rhat + un1) {
     q1 = q1 - 1;
     rhat = rhat + vn1;
     if (rhat < b) goto again1;}

   un21 = un32*b + un1 - q1*v;  // Multiply and subtract.

   q0 = un21/vn1;            // Compute the second
#ifdef OPTIMIZE
   rhat = un21%vn1;          // quotient digit, q1.
#else
   rhat = un21 - q0*vn1;     // quotient digit, q0.
#endif
again2:
   if (q0 >= b || q0*vn0 > b*rhat + un0) {
     q0 = q0 - 1;
     rhat = rhat + vn1;
     if (rhat < b) goto again2;}

   if (r != NULL)            // If remainder is wanted,
      *r = (un21*b + un0 - q0*v) >> s;     // return it.
   return q1*b + q0;
}
[…]
Especially notice the highlighted part of the initial comment!

Flawed implementation in libdivide

Now take a look at the naïve and rather undelightawful adaption of this simplified and specialised variant provided in libdivide to implement an unsigned 128÷64-bit division:

Warning: don’t use this function; it demonstrates that its author copyist does not understand the original code and fails to read or ignores the comment, it exhibits poor performance, and it has a serious bug!

// libdivide.h
// Copyright 2010 - 2018 ridiculous_fish
[…]
// Code taken from Hacker's Delight:
// http://www.hackersdelight.org/HDcode/divlu.c.
// License permits inclusion here per:
// http://www.hackersdelight.org/permissions.htm

static uint64_t libdivide_128_div_64_to_64(uint64_t u1, uint64_t u0, uint64_t v, uint64_t *r) {
1  const uint64_t b = (1ULL << 32); // Number base (16 bits)
2  uint64_t un1, un0; // Norm. dividend LSD's
2  uint64_t vn1, vn0; // Norm. divisor digits
2  uint64_t q1, q0; // Quotient digits
3  uint64_t un64, un21, un10; // Dividend digit pairs
    uint64_t rhat; // A remainder
#ifdef OPTIMIZE
    uint64_t p; // A product
#endif
    int32_t s; // Shift amount for norm

    // If overflow, set rem. to an impossible value,
    // and return the largest possible quotient
    if (u1 >= v) {
        if (r != NULL)
            *r = (uint64_t) -1;
        return (uint64_t) -1;
    }

    // count leading zeros
    s = libdivide__count_leading_zeros64(v);
    if (s > 0) {
        // Normalize divisor
        v = v << s;
5      un64 = (u1 << s) | ((u0 >> (64 - s)) & (-s >> 31));
4      un10 = u0 << s; // Shift dividend left
    } else {
        // Avoid undefined behavior
6      un64 = u1 | u0;
4      un10 = u0;
    }

    // Break divisor up into two 32-bit digits
7  vn1 = v >> 32;
7  vn0 = v & 0xFFFFFFFF;

    // Break right half of dividend into two digits
7  un1 = un10 >> 32;
7  un0 = un10 & 0xFFFFFFFF;

    // Compute the first quotient digit, q1
    q1 = un64 / vn1;
#ifdef OPTIMIZE
    rhat = un64 % vn1;
    p = q1 * vn0;

    while (q1 >= b || p > b * rhat + un1) {
        q1 = q1 - 1;
        rhat = rhat + vn1;
        if (rhat >= b)
            break;
        p = p - vn0;
    }
#else
8  rhat = un64 - q1 * vn1;

9  while (q1 >= b || q1 * vn0 > b * rhat + un1) {
        q1 = q1 - 1;
        rhat = rhat + vn1;
        if (rhat >= b)
            break;
    }
#endif
     // Multiply and subtract
    un21 = un64 * b + un1 - q1 * v;

    // Compute the second quotient digit
    q0 = un21 / vn1;
#ifdef OPTIMIZE
    rhat = un21 % vn1;
    p = q0 * vn0;

    while (q0 >= b || p > b * rhat + un0) {
        q0 = q0 - 1;
        rhat = rhat + vn1;
        if (rhat >= b)
            break;
        p = p - vn0;
    }
#else
8  rhat = un21 - q0 * vn1;

9  while (q0 >= b || q0 * vn0 > b * rhat + un0) {
        q0 = q0 - 1;
        rhat = rhat + vn1;
        if (rhat >= b)
            break;
    }
#endif
    // If remainder is wanted, return it
    if (r != NULL)
        *r = (un21 * b + un0 - q0 * v) >> s;

    return q1 * b + q0;
}
[…]
This implementation has an obvious bug and multiple deficiencies (in order of appearance):
  1. The number base is 232, i.e. 32 bits, not 16 bits, as stated in the comment.
  2. The normalised (least significant) digits of dividend and divisor as well as the digits of the quotient fit in 32 bits, i.e. the variables un1, un0, vn1, vn0, q1 and q0 can and should of course be declared as uint32_t, not as uint64_t.
  3. The proper name for the variable with the two most significant digits of the dividend is un32, not un64.
  4. The variables un6432 and un10 are superfluous: the dividend u1 and u0 can and should of course be normalised in place, like the divisor v.
  5. The term & (-s >> 31) is superfluous: in the original code it is used to avoid the conditional expression if (s > 0) … else … and evaluates to either & 0 or & -1 (alias & 0xFFFFFFFF) there; here it always evaluates to & -1 (alias & 0xFFFFFFFFFFFFFFFF).
  6. The term | u0; is wrong and must be removed.
  7. The variables un1, un0, vn1 and vn0 are superfluous: they can and of course should be replaced by the expressions (uint32_t) (un10 >> 32), (uint32_t) un10, (uint32_t) (v >> 32) and (uint32_t) v respectively.
  8. The remainders should be computed as un6432 % vn1 and un21 % vn1, not as un6432 - q1 * vn1 and un21 - q0 * vn1 using expensive 64×64-bit multiplications: both hardware division instructions and software division routines typically return quotient and remainder together.
  9. The repeated (expensive) computation of the 64×64-bit products q1 * vn0 and q0 * vn0 inside the while … loops should be moved outside the loops.

Proper (and optimised) implementation …

… in ANSI C

This further simplified and optimised implementation of Algorithm D for unsigned 128÷64-bit division on 32-bit processors is based on a 64÷32-bit division returning a 64-bit quotient and a 32-bit remainder, trivially implemented per long (alias schoolbook) division using a 64÷32-bit division returning a 32-bit quotient and a 32-bit remainder:
// Copyleft © 2011-2019, Stefan Kanthak <‍stefan‍.‍kanthak‍@‍nexgo‍.‍de‍>

// Divide a 128-bit integer dividend, supplied as a pair of 64-bit
// integers in u0 and u1, by a 64-bit integer divisor, supplied in v;
// return the 64-bit quotient and optionally the 64-bit remainder in *r.

unsigned long long divllu(unsigned long long u0,
                          unsigned long long u1,
                          unsigned long long v,
                          unsigned long long *r) {
    unsigned long long qhat;        // A quotient.
    unsigned long long rhat;        // A remainder.
    unsigned long long uhat;        // A dividend digit pair.
    unsigned long      q0, q1;      // Quotient digits.
    unsigned long      s;           // Shift amount for norm.

    if (u1 >= v) {                  // If overflow, set rem.
        if (r != NULL)              // to an impossible value,
            *r = ~0ULL;             // and return the largest
        return ~0ULL;               // possible quotient.
    }

    s = __builtin_clzll(v);         // 0 <= s <= 63.
    if (s != 0) {
        v <<= s;                    // Normalize divisor.
        u1 <<= s;                   // Shift dividend left.
        u1 |= u0 >> (64 - s);
        u0 <<= s;
    }
                                    // Compute high quotient digit.
    qhat = u1 / (unsigned long) (v >> 32);
    rhat = u1 % (unsigned long) (v >> 32);

    while ((unsigned long) (qhat >> 32) != 0 ||
                                    // Both qhat and rhat are less 2**32 here!
           (unsigned long long) (unsigned long) qhat * (unsigned long) v >
           ((rhat << 32) | (unsigned long) (u0 >> 32))) {
        qhat -= 1;
        rhat += (unsigned long) (v >> 32);
        if ((unsigned long) (rhat >> 32) != 0) break;
    }

    q1 = (unsigned long) qhat;
                                    // Multiply and subtract.
    uhat = ((u1 << 32) | (unsigned long) (u0 >> 32)) - q1 * v;

                                    // Compute low quotient digit.
    qhat = uhat / (unsigned long) (v >> 32);
    rhat = uhat % (unsigned long) (v >> 32);

    while ((unsigned long) (qhat >> 32) != 0 ||
                                    // Both qhat and rhat are less 2**32 here!
           (unsigned long long) (unsigned long) qhat * (unsigned long) v >
           ((rhat << 32) | (unsigned long) u0)) {
        qhat -= 1;
        rhat += (unsigned long) (v >> 32);
        if ((unsigned long) (rhat >> 32) != 0) break;
    }

    q0 = (unsigned long) qhat;

    if (r != NULL)                  // If remainder is wanted, return it.
        *r = (((uhat << 32) | (unsigned long) u0) - q0 * v) >> s;

    return ((unsigned long long) q1 << 32) | q0;
}

… in Assembler

Finally the implementation of this optimised simplified variant in assembly language:

Note: this code runs on 33 year old Intel® i386 processors, and of course on current compatible processors too, about twice as fast as the ANSI C version.

; Copyright © 2011-2019, Stefan Kanthak <‍stefan‍.‍kanthak‍@‍nexgo‍.‍de‍>

	.386
	.model	flat, C
	.code

divllu	proc	public

	push	ebx
	push	ebp
	push	edi
	push	esi
[…]
	mov	esi, [esp+40]
	mov	edi, [esp+36]	; esi:edi = divisor

NORMALIZE:
	mov	ecx, 31
	bsr	ebx, esi	; ebx = index of leading '1' bit in high dword of divisor
	jz	@F		; high dword of divisor (and high dword of dividend) = 0?

	mov	edx, [esp+32]
	mov	eax, [esp+28]	; edx:eax = high qword of dividend

	sub	ecx, ebx	; ecx = number of leading '0' bits in (high dword of) divisor
	jz	READY		; number of leading '0' bits = 0?

	shld	esi, edi, cl
	shl	edi, cl		; esi:edi = divisor'
	mov	[esp+40], esi
	mov	[esp+36], edi	; save normalised divisor'

	mov	ebx, [esp+24]
	mov	ebp, [esp+20]	; ebx:ebp = low qword of dividend
	shld	edx, eax, cl
	shld	eax, ebx, cl
	shld	ebx, ebp, cl
	shl	ebp, cl		; edx:eax:ebx:ebp = dividend'
	mov	[esp+32], edx
	mov	[esp+28], eax
	mov	[esp+24], ebx
	mov	[esp+20], ebp	; save normalised dividend'
	jmp	READY

@@:
	mov	edx, [esp+28]
	mov	eax, [esp+24]
	mov	ebx, [esp+20]	; edx:eax:ebx = lowest dwords of dividend
				;             = highest dwords of dividend'
	mov	esi, edi	; esi = low dword of divisor
				;     = high dword of divisor'
	bsr	edi, edi	; edi = index of leading '1' bit in low dword of divisor
				;     = index of leading '1' bit in high dword of divisor'
	sub	ecx, edi	; ecx = number of leading '0' bits in high dword of divisor'
	lea	ecx, [ecx+32]	; ecx = number of leading '0' bits in divisor
	jz	@F		; number of leading '0' bits = 32?

	shl	esi, cl		; esi:0 = divisor'
	shld	edx, eax, cl
	shld	eax, ebx, cl
	shl	ebx, cl		; edx:eax:ebx:0 = dividend'

@@:
	xor	edi, edi	; edi = 0
				;     = low dword of divisor'
				;     = low dword of dividend'
	mov	[esp+40], esi
	mov	[esp+36], edi	; save normalised divisor'
	mov	[esp+32], edx
	mov	[esp+28], eax
	mov	[esp+24], ebx
	mov	[esp+20], esi	; save normalised dividend'

READY:
	push	ecx		; save number of leading '0' bits in divisor

DIVISION1:
	cmp	esi, edx
	jna	OVERFLOW1	; overflow with normal division?

				; "normal" division
NORMAL1:
	div	esi		; eax = (low dword of) quotient',
				; edx = (low dword of) remainder'
	mov	ebp, edx	; ebp = (low dword of) remainder'
	mov	ecx, eax
	xor	ebx, ebx	; ebx:ecx = quotient'
	jmp	CHECK1
				; "long" division
OVERFLOW1:
	mov	ecx, eax
	mov	eax, edx
	xor	edx, edx
	div	esi		; eax = high dword of quotient',
				; edx = high dword of remainder'
	mov	ebx, eax	; ebx = high dword of quotient'
	mov	eax, ecx
	div	esi		; eax = low dword of quotient',
				; edx = (low dword of) remainder'
	mov	ebp, edx	; ebp = (low dword of) remainder'
	mov	ecx, eax	; ebx:ecx = quotient'

ADJUST1:
	add	ecx, -1
	adc	ebx, -1		; ebx:ecx = quotient' - 1
	add	ebp, esi	; ebp = (low dword of) remainder'
				;     + high dword of divisor'
	jc	BREAK1		; remainder' >= 2**32?

AGAIN1:
	test	ebx, ebx
	jnz	ADJUST1		; quotient' >= 2**32?

CHECK1:
	mov	eax, edi	; eax = low dword of divisor'
	mul	ecx		; edx:eax = low dword of divisor'
				;         * low dword of quotient'
ifdef JMPLESS
	cmp	[esp+28], eax
	mov	eax, ebp
	sbb	eax, edx
	jb	ADJUST1
else
	cmp	edx, ebp
	jb	BREAK1
	ja	ADJUST1
	cmp	eax, [esp+28]
	ja	ADJUST1
endif
BREAK1:
	push	ecx		; save (low dword of) quotient'

	mov	eax, edi	; eax = low dword of divisor'
	mul	ecx		; edx:eax = low dword of divisor'
				;         * (low dword of) quotient'
	imul	ecx, esi	; ecx = (low dword of) quotient'
				;     * high dword of divisor'
	add	ecx, edx	; ecx:eax = divisor'
				;         * (low dword of) quotient'
	mov	ebx, eax	; ecx:ebx = divisor'
				;         * (low dword of) quotient'
	mov	eax, [esp+32]
	mov	edx, [esp+36]	; edx:eax = "inner" qword of dividend'
	sub	eax, ebx
	sbb	edx, ecx	; edx:eax = "inner" qword of dividend"
				;         = intermediate (normalised) remainder

	push	eax		; save low dword of "inner" qword of dividend"

DIVISION2:
	cmp	esi, edx
	jna	OVERFLOW2	; overflow with normal division?

				; "normal" division
NORMAL2:
	div	esi		; eax = (low dword of) quotient",
				; edx = (low dword of) remainder"
	mov	ebp, edx	; ebp = (low dword of) remainder"
	mov	ecx, eax
	xor	ebx, ebx	; ebx:ecx = quotient"
	jmp	CHECK2
				; "long" division
OVERFLOW2:
	mov	ecx, eax
	mov	eax, edx
	xor	edx, edx
	div	esi		; eax = high dword of quotient",
				; edx = high dword of remainder"
	mov	ebx, eax	; ebx = high dword of quotient"
	mov	eax, ecx
	div	esi		; eax = low dword of quotient",
				; edx = (low dword of) remainder"
	mov	ebp, edx	; ebp = (low dword of) remainder"
	mov	ecx, eax	; ebx:ecx = quotient"

ADJUST2:
	add	ecx, -1
	adc	ebx, -1		; ebx:ecx = quotient" - 1
	add	ebp, esi	; ebp = (low dword of) remainder"
				;     + high dword of divisor'
	jc	BREAK2		; remainder" >= 2**32?

AGAIN2:
	test	ebx, ebx
	jnz	ADJUST2		; quotient" >= 2**32?

CHECK2:
	mov	eax, edi	; eax = low dword of divisor'
	mul	ecx		; edx:eax = low dword of divisor'
				;         * low dword of quotient"
ifdef JMPLESS
	cmp	[esp+32], eax
	mov	eax, ebp
	sbb	eax, edx
	jb	ADJUST2
else
	cmp	edx, ebp
	jb	BREAK2
	ja	ADJUST2
	cmp	eax, [esp+32]
	ja	ADJUST2
endif
BREAK2:
	pop	ebx		; ebx = low dword of "inner" qword of dividend"
	push	ecx		; save (low dword of) quotient"

	mov	ebp, [esp+56]	; ebp = address of remainder
	test	ebp, ebp
	jz	QUOTIENT	; address of remainder = 0?

REMAINDER:
	mov	eax, edi	; eax = low dword of divisor'
	mul	ecx		; edx:eax = low dword of divisor'
				;         * (low dword of) quotient"
	imul	ecx, esi	; ecx = (low dword of) quotient"
				;     * high dword of divisor'
	add	edx, ecx	; edx:eax = (low dword of) quotient"
				;         * divisor'
	mov	edi, [esp+32]
	mov	esi, ebx
	sub	edi, eax
	sbb	esi, edx	; esi:edi = (normalised) remainder

	mov	ecx, [esp+8]	; ecx = number of leading '0' bits in divisor
				;     = shift count
	jecxz	@F		; shift count = 0?

	shrd	edi, esi, cl
	shr	esi, cl		; esi:edi = remainder

	test	cl, 32
	jz	@F		; shift count < 32?

	mov	edi, esi
	xor	esi, esi	; esi:edi = remainder

@@:
	mov	[ebp], edi
	mov	[ebp+4], esi	; store remainder

QUOTIENT:
	pop	eax
	pop	edx		; edx:eax = quotient

	pop	ecx
	pop	esi
	pop	edi
	pop	ebp
	pop	ebx
	ret

divllu	endp
	end

Contact

If you miss anything here, have additions, comments, corrections, criticism or questions, want to give feedback, hints or tipps, report broken links, bugs, deficiencies, errors, inaccuracies, misrepresentations, omissions, shortcomings, vulnerabilities or weaknesses, …:
don’t hesitate to contact me and feel free to ask, comment, criticise, flame, notify or report!

Notes: I dislike HTML (and even weirder formats too) in email, I prefer to receive plain text.
I also expect to see your full (real) name as sender, not your nickname!
Emails in weird formats and without a proper sender name are likely to be discarded.
I abhor top posts and expect inline quotes in replies.

Terms and Conditions

By using this site, you signify your agreement to these terms and conditions. If you do not agree to these terms and conditions, do not use this site!

Data Protection Declaration

This web page records no data and sets no cookies.

The service provider for *.homepage.t-online.de, Deutsche Telekom AG,


Copyright © 2005–2019 • Stefan Kanthak • <‍stefan‍.‍kanthak‍@‍nexgo‍.‍de‍>