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Implement suggested instead of minimal fix for ludivmul.

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See https://sourceforge.net/p/freedos/bugs/106/
(Gerd Grosse)
This commit is contained in:
Bart Oldeman 2018-09-06 10:30:04 -04:00 committed by Kenneth J Davis
parent 5e31e7fe58
commit 8c45af38c7
1 changed files with 5 additions and 11 deletions
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16
kernel/ludivmul.inc
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@ -96,21 +96,15 @@
pop bx ; get dividend lo-word pop bx ; get dividend lo-word
mov cx, ax ; save quotient mov cx, ax ; save quotient
mul di ; quotient * divisor hi-word (low only) mul di ; quotient * divisor hi-word (low only)
push di ; save divisor hi-word pop dx ; dividend high
mov di, ax ; save in di sub dx,ax ; dividend high - divisor high * quotient, no overflow (carry/borrow) possible here
push dx ; save dividend high
mov ax, cx ; ax=quotient mov ax, cx ; ax=quotient
mul si ; quotient * divisor lo-word mul si ; quotient * divisor lo-word
add dx, di ; quotient * divisor !!!! overflow possible !!!! sub bx, ax ; dividend-lo - (quot.*divisor-lo)-lo
pop di ; restore divisor hi-word
jnc %%skipcorrection
sub ax, si
sbb dx, di
dec cx ; correct quotient
%%skipcorrection:
sub bx, ax ; dividend-lo - (quot.*divisor)-lo
mov ax, cx ; get quotient mov ax, cx ; get quotient
pop cx ; restore dividend hi-word pop cx ; restore dividend hi-word
sbb cx, dx ; subtract divisor * quot. from dividend sbb cx, dx ; subtract (divisor-lo * quot.)-hi from dividend-hi
sbb dx, dx ; 0 if remainder > 0, else FFFFFFFFh sbb dx, dx ; 0 if remainder > 0, else FFFFFFFFh
and si, dx ; nothing to add and si, dx ; nothing to add
and di, dx ; back if remainder positive di:si := di:si(cx:bx) & dx:dx and di, dx ; back if remainder positive di:si := di:si(cx:bx) & dx:dx