Subversion Repositories 8051

        mov r5, #0a1h;

        mov r4, #054h;

        mov r1, #001h;

        mov r0, #070h;

        lcall DIV16U;

        mov p0, r5;

        mov p0, r4;

        mov p0, r1;

        mov p0, r0;

;26 Oct 00 added code to zero remainder when dividend is zero

;26 Oct 00 Change labels from duxxx to duaxxx

;19 Dec 99 corrected comments

;16 Dec 99 made from DIV32U

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;

;DIV16U is called to divide (unsigned) a 16-bit dividend using a

; 16-bit divisor.

;

;DIV16U solves for quotient and remainder the equation:

;

; dividend = divisor*quotient + remainder

;

;Call:

;  r5,r4 = dividend

;  r1,r0 = divisor

;  lcall DIV16U

;  jc   divide_by_zero

;

;Return:

; r5,r4 = quotient

; r7,r6 = remainder

; c flag set to 1 if divide by zero attempted

; All registers, acc, b and have been changed.

; Data pointer has not been disturbed

;

;Note:

; (1)Most significant (ms) register always listed first when comma

;  separates two in a comment. Example: r5,r4 (r5 contains the ms bits)

; (2) The algorithm used in this code borrows heavily from work posted

;   by John C. Wren who said he got it from a C complier.

;

;Original author: John Veazey, Ridgecrest, CA, 16 Dec 99

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;

;cseg

DIV16U:

;

;Clear the working quotient

;

      clr   a

      mov   r2,a

      mov   r3,a

;

;b counts the number of places+1 the divisor was initially

; shifted left to align its ms bit set with the ms bit set

; in the dividend

;

      mov   b,#1

;

;Make an error return if trying to divide by zero

;

      mov   a,r1

      orl   a,r0

      jz    dua920

;

;Just return with quotient and remainder zero if dividend is zero

;

      mov   a,r5;

      orl   a,r4;

      jnz   dua200;

      mov   r7,a;

      mov   r6,a;

      ajmp  dua910      ;Make a normal return

;

;Align the msb set in the demoninator with the msb set in the

; numerator. Increment the shift count in b each time a shift left

; is performed.

;

dua200:

      mov   a,r1        ;Stop if MSB set

      rlc   a

      jc    dua600

      clr   c

      mov   a,r5        ;Compare r1 & r5

      subb  a,r1

      jc    dua600      ; jump if r1>r5

      jnz   dua240      ; jump if r1

      mov   a,r4        ;r1=r5, so compare r0 & r4

      subb  a,r0

      jc    dua600      ; jump if r0>r4

dua240:

      clr   c           ;Now shift the denominator

      mov   a,r0        ; left 1 bit position

      rlc   a

      mov   r0,a

      mov   a,r1

      rlc   a

      mov   r1,a

      inc   b           ;Increment b counter and

      sjmp  dua200      ; continue

;

;Compare the shifted divisor with the remainder (what's

; left of the dividend)

;

dua600:

      clr   c

      mov   a,r5

      subb  a,r1

      jc    dua720      ;jump if r1>r5

      jnz   dua700      ;jump if r1

      mov   a,r4

      subb  a,r0

      jc    dua720      ;jump if r0>r4

;

;Divisor is equal or smaller, so subtract it off and

; get a 1 for the quotient

;

dua700:

      mov   a,r4

      clr   c

      subb  a,r0

      mov   r4,a

      mov   a,r5

      subb  a,r1

      mov   r5,a

      clr   c

      cpl   c           ;Get a 1 for the quotient

      sjmp  dua730

;

;Divisor is greater, get a 0 for the quotient

;

dua720:

      clr   c

;

;Shift 0 or 1 into quotient

;

dua730:

      mov   a,r2

      rlc   a

      mov   r2,a

      mov   a,r3

      rlc   a           ;Test for overlow removed here because

      mov   r3,a        ; it can't happen when dividing 16 by 16

;

;Now shift the denominator right 1, decrement the counter

; in b until b = 0

;

dua740:

      clr   c

      mov   a,r1

      rrc   a

      mov   r1,a

      mov   a,r0

      rrc   a

      mov   r0,a

      djnz  b,dua600

;

;Move quotient and remainder so that quotient is returned in the same

; registers as the dividend. This makes it easier to divide repeatedly

; by the same number as you would do when converting to a new radix.

;

      mov   a,r5

      mov   r7,a

      mov   a,r4

      mov   r6,a

      mov   a,r3

      mov   r5,a

      mov   a,r2

      mov   r4,a

;

;Make the normal return

;

dua910:

      clr   c

      ret

;

;Make the error return

;

dua920:

      clr   c

      cpl   c

      ret

;End of DIV16U