In a message dated 28/09/04 18:41:44 GMT Daylight Time,
[EMAIL PROTECTED] writes:
>
> George Gwilt writes:
>
> <>
> >Another place where using 68020+ instructions reduces the number of
> >instructions is in the conversion of long integers to Qdos floating point.
> <>
>
> Demonstration, please!
>
> Per
>
>
To set the long integer in D5 to QDOS fp format in D4.W and D5.L (exponent
and mantissa) the normal 68000/8 way is given in the follwing subroutine.
se_3 move.w d5,d4
move.l d5,d1
beq se_5
move.w #$81f,d4
add.l d1,d1
bvs se_5
subq.w #1,d4
move.l d1,d5
moveq #16,d0
se_6 move.l d5,d1
asl.l d0,d1
bvs se_7
sub.w d0,d4
move.l d1,d5
se_7 asr.w #1,d0
bne se_6
se_5 rts
By using the 68020+ instruction BFFFO, which returns the first non zero bit
in the range indicated) a good reduction is obtained. Not only is the number of
instructions reduced but also the looping is eliminated.
t1 move.l d5,d4
beq m1
bpl m2
neg.l d5
m2 bfffo d5{1:31},d0
subq.l #1,d0
lsl.l d0,d5
move.w #$81f,d4
sub.w d0,d4
tst.l d4
bpl m1
neg.l d5
m1 rts
George
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