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Fix division for some large numbers such as 0xFFFF01FF/0x10101FF

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See also https://sourceforge.net/p/freedos/bugs/106/
(Gerd Grosse)
This commit is contained in:
Bart Oldeman 2018-09-06 10:12:41 -04:00
parent fa36d8aa25
commit eda671d079
1 changed files with 9 additions and 5 deletions
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14
kernel/ludivmul.inc
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@ -85,24 +85,28 @@
push dx ; save push dx ; save
push ax ; dividend push ax ; dividend
mov si, bx ; divisor now in mov si, bx ; divisor now in
mov di, cx ; di:bx and cx:si mov di, cx ; di:si and cx:bx
%%shift_loop: %%shift_loop:
shr dx, 1 ; shift both shr dx, 1 ; shift both
rcr ax, 1 ; dividend rcr ax, 1 ; dividend
shr di, 1 ; and divisor shr cx, 1 ; and divisor
rcr bx, 1 ; right by 1 bit rcr bx, 1 ; right by 1 bit
jnz %%shift_loop ; loop if di non-zero (rcr does not touch ZF) jnz %%shift_loop ; loop if di non-zero (rcr does not touch ZF)
mov di, cx ; restore original divisor (di:si)
div bx ; compute quotient dx:ax>>x / cx:bx>>x (stored in ax; remainder in dx not used) div bx ; compute quotient dx:ax>>x / cx:bx>>x (stored in ax; remainder in dx not used)
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 push di ; save divisor hi-word
xchg ax, di ; save in di mov di, ax ; save in di
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 ; dx:ax = quotient * divisor add dx, di ; quotient * divisor !!!! overflow possible !!!!
pop di ; restore divisor hi-word 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 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