tyler.durden
/
kernel
mirror of https://github.com/FDOS/kernel.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

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

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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
kernel/ludivmul.inc
View File

@ -85,24 +85,28 @@
push dx ; save
push ax ; dividend
mov si, bx ; divisor now in
mov di, cx ; di:bx and cx:si
mov di, cx ; di:si and cx:bx
%%shift_loop:
shr dx, 1 ; shift both
rcr ax, 1 ; dividend
shr di, 1 ; and divisor
shr cx, 1 ; and divisor
rcr bx, 1 ; right by 1 bit
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)
pop bx ; get dividend lo-word
mov cx, ax ; save quotient
mul di ; quotient * divisor hi-word (low only)
push di ; save divisor hi-word
xchg ax, di ; save in di
mov di, ax ; save in di
mov ax, cx ; ax=quotient
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
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
pop cx ; restore dividend hi-word