Re: Signed 16 bit numbers in AD21161 and VisualDSP

kostbill wrote:
My problem is that in the VisualDSP compiler all numbers are represented
in 32 bit format, a signed short is 32 bits. That is a problem in my code
because if I want to represent a negative number, it will not be in the
two's complement until it reaches the 32 bit width, for example: the
number 0xEAFF is -5377 if represented as a 16 bit signed int but is 60159
if represented as a 32 bit signed int.

How will I manipulate the numbers I got from the SPORTS if they are
negative numbers?

You can either convert them to 32-bit numbers by shifting them left by 16 bits, or you can sign extend them. That is:

x = x << 16
or
if (x&0x8000) {x = x | 0xffff0000}

The first approach converts the number from 1.15 fractional format to 1.31. The second converts from 16-bit integer to 32-bit integer.

You might also experiment with short-space addressing. If your number is in internal memory (which I assume it is, since sport dmas go to internal memory IIRC), you can read the lower 16 bits (ignoring the upper 16) by reading from addr*2. You can read the upper by reading from addr*2+1. I might have upper and lower reversed here, but I'm pretty sure that reading from short-space will sign-extend the result. That's why I said "experiment with."


--
Jim Thomas Principal Applications Engineer Bittware, Inc
jthomas@xxxxxxxxxxxx http://www.bittware.com (603) 226-0404 x536
Never ascribe to malice that which is adequately explained by incompetence. -
Napoleon Bonaparte
.

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