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New version DRDOS 7.01.7 & 7.01.8 (Announce)

posted by boeckmann, Aachen, Germany, 12.12.2023, 21:15

> > I made some changes to the latest EDR-DOS sources maintained by ecm:
> >
> > - fixed bug in DRDOS.SYS div32 function, leading to division errors on
> > partitions with more than 66535 FAT sectors. There is also a div32
> function
> > in DRBIO.SYS present, but it did not contain the bug.
>
> https://github.com/SvarDOS/edrdos/commit/167fe71ec058f80b7da47f11f2a34fc66938ad57
>
> div16:
> mov       ax,12[bp]
> mov  dx,14[bp]
>
> Surely you can xor dx, dx here if you already checked that
> word [bp + 14] is zero?
>
> More to the point if the divisor high word is zero, a two-step division of
> two 32/16=16 divisions can be done on a 32-bit dividend (with 32-bit
> quotient) which would be faster than the full 32-bit division (that fully
> supports 32-bit divisors in addition to dividends/quotients).
>
> The division functions in DRBIO both do not offer the fast path for smaller
> numbers:
>
> https://github.com/SvarDOS/edrdos/blob/f6584d40d6dda553e63f7682ad672536f53ae6ff/drbio/disk.asm#L3286
>
> https://github.com/SvarDOS/edrdos/blob/f6584d40d6d...e63f7682ad672536f53ae6ff/drbio/bdosldr.a86#L772
Yes, by re-arranging a little bit, the unconditional jump can also be eliminated, like so:

div32:                                  ; 32-bit division
;--------
; On Entry:
;       32-bit dividend & divisor on stack
;       space for 32-bit quotient & remainder reserved on stack
;       SP-16
; On Exit:
;       32-bit quotient & remainder on stack
;       SP-16
; Modified registers:
;       AX,CX,DX,BP
        mov     bp,sp                   ; base address of temporary variables
        add     bp,2
        cmp     word ptr 10[bp],0       ; if divisor high => 32bit div
         jne    div32_full
        cmp     word ptr 14[bp],0       ; if dividend high => 32bit div
         jne    div32_full
div16:  mov     ax,12[bp]               ; AX=low 16 bits of dividend
        xor     dx,dx
        div     word ptr 8[bp]
        xor     cx,cx
        mov     [bp],dx
        mov     2[bp],cx                ; clear high part of remainder
        mov     4[bp],ax
        mov     6[bp],cx                ; and quotient result
        ret
div32_full:
        ...

I first thought that the xor cx,cx can be eliminated too, by clearing the result after the xor dx,dx. But the xor cx,cx leaves the flags in a defined state, which the div does not. So I think it is safer to leave this xor cx,cx in.

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