Very curious about something I read.

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Joined: 26 Sep 2006
Posts: 92

2 08 Oct 2006, 07:20

I read somewhere about old processors not having a DIV instruction and using
shifts for multiplication and division.

It's easy to see that you could use shifts for mutliplying by ten,but what about
dividing by ten? I normally wouldn't bother but since we have ten as the
standard base that all non programmers already understand, I was thinking
about it and wondered what the quickest way of doing it was.

So,can you divide by ten with only shifts,addition,and subtraction?

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08 Oct 2006, 07:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 08 Oct 2006, 08:39

Quote:

can you divide by ten with only shifts,addition,and subtraction?

Yes, this is how division is done by the CPU internally. Now days it is a little bit faster with double-bit estimation etc. but basically it is still a compare-subtract-shift algorithm.

For the specific case of division by ten we can do better than the div instruction by multiplying by an approximate reciprocal and then shifting down to get the quotient. There are already some topics on this board showing how it is done.

08 Oct 2006, 08:39

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 08 Oct 2006, 17:36

On the 6502 processor that was used in the C64, Apple II, and various other computers there was not only no divide, but no multiply either. A common way of dividing was doing something like this:

Code:

        LDA     #0
        LDX     #8
divlp:  ASL     number
        ROL
        CMP     #10
        BCC     low
        SBC     #10
        INC     number
low:    DEX
        BNE     divlp

This would be about the x86 equivalent of it (though of course it would be pointless to actually do this)

Code:

        mov     al,0
        mov     cx,16
divlp:  shl     dx,1
        rcl     al,1
        cmp     al,10
        jb      low
        sub     al,10
        inc     dx
low:    loop    divlp

08 Oct 2006, 17:36

2

Joined: 26 Sep 2006
Posts: 92

2 10 Oct 2006, 00:54

That's cool,but I'm a little lost. What does rcl do and why is dx left shifted?

10 Oct 2006, 00:54

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