[Vm-dev] positive32BitValueOf: fail with LargePositiveInteger

Mariano Martinez Peck marianopeck at gmail.com
Fri Oct 8 22:02:35 UTC 2010

On Fri, Oct 8, 2010 at 11:47 PM, David T. Lewis <lewis at mail.msen.com> wrote:

> On Fri, Oct 08, 2010 at 07:12:13PM +0200, Mariano Martinez Peck wrote:
> >
> > So....I don't understand...how can I have a LargePositiveInteger but that
> is
> > less than 32 bits?  Wouldn't that  be a SmallInteger?
> SmallInteger maxVal hex ==> '16r3FFFFFFF'.
> The largest positive two-compliment integer that fits into 32 bits is
> 16rEFFFFFFF, so any Integer in the range 16r4000000 through 16rEFFFFFFF
> is a LargePositiveInteger that can fit into a signed 32-bit C int.
> Thus the number of LargePositiveInteger values that fit into a 32-bit
> twos-compliment representation is (16r4000000 to: 16rEFFFFFFF) size ==>
> 3959422976
> Note that a SmallInteger is represented internally as a 31-bit value,
> which accounts for the difference. It's a bit confusing when you are used
> to thinking of 32-bit int values.

Thanks Dave for the clarification. This was exactly my question and my
I do know that SmallInteger are 31 bits sine the last bit is used to
distinguish from oop.
What I didn't understand is how can I have a LargePositiveIntener in 32
bits.....because less than that it would be a SmallIntegr.

Then, I didn't understand why positive32BitValueOf:  has this part:

  self assertClassOf: oop is: (self splObj: ClassLargePositiveInteger).
    successFlag ifTrue: [
        sz := self lengthOf: oop.
        sz = 4 ifFalse: [^ self primitiveFail]].
    successFlag ifTrue: [
        ^ (self fetchByte: 0 ofObject: oop) +
          ((self fetchByte: 1 ofObject: oop) <<  8) +
          ((self fetchByte: 2 ofObject: oop) << 16) +
          ((self fetchByte: 3 ofObject: oop) << 24) ].

In summary, I forgot about this bit of difference and the possible rang of
values between being 31 and 32 bits and this is how I can have LargePositive
integer with 32 bits.

Thanks Dave.


> Dave
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