/wsr/ - Worksafe Requests » Thread #733753

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Anonymous Sat 02 Nov 2019 15:58:48 No.733753 View View Reply Original Report

How do I convert 16 bit 2'K format numbers to 8 bit 2'K format?
Also, how do I know that if that 16 bit number is not too large for 8 bit one?

Examples that I have are:
8001 (base 16) => FF(base 16)
0025 (base 16) => 02(base 16)
82BC (base 16) => ERROR - too large

Anonymous

Anonymous Sat 02 Nov 2019 16:13:57 No.733756 Report

Quoted By:

It might be worth of mentioning that 2'K means two's complement.

Anonymous

Anonymous Sat 02 Nov 2019 16:51:23 No.733767 Report

Quoted By:

Another correction, the 16 bit number is not in 2'K, but in just signed formant, ie. last bit determines if number is positive (0) or negative (1)

Anonymous

Anonymous Sat 02 Nov 2019 17:59:23 No.733777 Report

Quoted By:

>>733753
a) just drop the first bits, that's what anybody would expect to happen
b) you have to check for ones in the bits to be dropped

Anonymous

Anonymous Sat 02 Nov 2019 20:08:36 No.733798 Report

Quoted By:

>>733753
1) stop making up your own terminology
2) the same way you convert between any number systems:

- check for sign
- check for overflow
- convert the sign bit
- convert the non-sign bits

In your specific case,

if (a && 0x8000) {
//negative
//check for overflow
if (a && 0x7F00) { // watch for edge cases; I suspect there's more tests you need to do than this
throw OverflowException;} else {
// two's complement does negative numbers by the MSB being worth the negative of what it ordinarily should be
// so we can't just copy all the other bits, we have to subtract them from the positive of the MSB
a=(a&&0x7FFF); // chop sign bit
a=0x80 - a // subtraction step
a= a || 0x80; // OR in the negative MSB
return a
}else{
//positive
if (a && 0x700) {throw OverflowException} else {return a} // don't need to do any conversion because two's complement and sign-bit positive numbers are the same
}

There are probably loads of off-by-ones in that pseudocode; finding them is your job.

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