I read somewhere that in C short int range value are -32767 to +32767
http://www.ericgiguere.com/articles/ansi-c-summary.html
And I was wondering why it does not follow the $-1 * 2^{N-1}\ to\ 2^{N-1}-1$
range encoding
Stef
On Dec 23, 2011, at 11:35 AM, Stéphane Ducasse wrote:
> Hi
>
> I'm doing a pass on the two complement chapter and small integer
> implementation and I wanted to check the SmallInteger>>primitiveAdd see below:
>
> stackIntegerValue extracts from a smalltalk integer the 31 encoding number
> bit. I imagine that this is correct.
>
> sqInt
> stackIntegerValue(sqInt offset) {
> DECL_MAYBE_SQ_GLOBAL_STRUCT
> sqInt integerPointer;
> integerPointer = longAt(GIV(stackPointer) + (offset * BytesPerWord));
> /* begin checkedIntegerValueOf: */
> if ((integerPointer & 1)) {
> return (integerPointer >> 1);
> }
> else {
> /* begin primitiveFail */
> if (GIV(primFailCode) == 0) {
> GIV(primFailCode) = 1;
> }
> return 0;
> }
> }
>
> Now I have three questions:
> - is two complement used to manage small integers at VM level?
> I saw digitAdd on Integer and it deals with large integer.
>
> - integerResult = (stackIntegerValue(1)) + (stackIntegerValue(0));
> performs the real add. I imagine that it comes from plain C. Now does
> anybody has an idea
> where I could find the definition of such + procedure?
> In ANSI C
>
> - from the books, performing 54 - 5 is equivalent to adding the two
> complement
> i.e. 54 + 5' (two complement)
> this work well for 54 - 5 correct results and we can follow it looking
> at binary operations:
>
> (2r110110 - 2r101) bitString
> --> '0000000000000000000000000110001'
> (2r110110 bitString)
> --> '0000000000000000000000000110110'
> 2r101 bitString
> --> '0000000000000000000000000000101'
> 2r101 negated bitString
> --> '1111111111111111111111111111011'
>
> Now doing 5 - 15 does not (or I miss something ilke always subtracting the
> smaller one and taking the two complement of the smaller…)
> so does anybody has the answer on how to perform 5 - 15 ?
>
>
>
>
>
> static void
> primitiveAdd(void) {
> DECL_MAYBE_SQ_GLOBAL_STRUCT
> sqInt integerResult;
> char *sp;
>
> /* begin pop2AndPushIntegerIfOK: */
> integerResult = (stackIntegerValue(1)) + (stackIntegerValue(0));
> if (GIV(primFailCode) == 0) {
> if ((integerResult ^ (integerResult << 1)) >= 0) {
> /* begin pop:thenPush: */
> longAtput(sp = GIV(stackPointer) + ((2 - 1) *
> BytesPerWord), ((integerResult << 1) | 1));
> GIV(stackPointer) = sp;
> }
> else {
> /* begin success: */
> if (!(0)) {
> if (GIV(primFailCode) == 0) {
>
> /* Don't overwrite an error code that
> has already been set. */
>
> GIV(primFailCode) = 1;
> }
> }
> }
> }
> }
>
>
> Stef
