Programming Examples

3. Dividing a Double-Word Dividend

3. Dividing a Double-Word Dividend

This example contains the code sequence to divide a 64-bit signed dividend by a 32-bit signed divisor using the DS (Divide Step) instruction. Table 7-1 on page 135 lists the registers that this program uses.

start

 

 

 

MOVB,>=

dvdu,rem,check_mag

; Move upper dividend

 

 

;

check for &< 0

ADD

0,dvdl,quo

; Move lower dividend always

SUB

0,quo,quo

; Get absolute value of

SUBB

0,rem,rem

;

the dividend in rem,quo

check_mag

 

 

 

SUBT,=

0,dvr,tp

; Check 0, clear carry,

 

 

;

negate the divisor

 

 

;

and trap if dvr = 0

DS

0,tp,0;

; Set V-bit to the complement

 

 

;

of the divisor sign

ADD

quo,quo,quo

; Shift msb bit into carry

DS,&<<

rem,dvr,rem

 

; 1st divide step, if carry

 

 

;

out msb of quotient = 0

B,n

min_ovfl

; Abs(quotient) > 2**31

 

 

;

deal with elsewhere

ADDC

quo,quo,quo

; Shift quo with/into carry

DS

rem,dvr,rem

; 2nd divide step

; ...

 

 

 

;repeat divide step sequence

 

 

; ...

 

 

 

ADDC

quo,quo,quo

; Shift quo with/into carry

DS

rem,dvr,rem

; 31st divide step

ADDC

quo,quo,quo

; Shift quo with/into carry

DS

rem,dvr,rem

; 32nd divide step,

ADDC

quo,quo,quo

; Shift last quo bit into quo

ADDB,>=,n

rem,0,finish

; Branch if pos. rem

ADD,&<

dvr,0,0

 

; If dvr > 0, add dvr

ADD,tr

rem,dvr,rem

;

for correcting rem.

ADDL

rem,tp,rem

; Else add absolute value dvr

finish

 

 

 

ADD,>=

dvdu,0,0

; Set sign of rem

SUB

0,rem,rem

;

to sign of dividend

XOR,>=

dvdu,dvr,0

; Get correct sign of quo

SUB

0,quo,quo

;

based on operand signs

134

Chapter 7