From: David Bishop <dbishop@eda.org>

Date: Mon Aug 11 2014 - 17:39:11 PDT

Date: Mon Aug 11 2014 - 17:39:11 PDT

On 8/11/2014 4:44 PM, Graham, Paul wrote: > > I've hit a problem with the definition of signed division in > numeric_std. With this function: > > function "/" (L: SIGNED; R: INTEGER) return SIGNED is > > if L is "100" (that is, -4), and R is 4, then the function returns, > not -1, but 0. This is because: > > constant R_LENGTH: NATURAL := MAX(L'LENGTH, SIGNED_NUM_BITS(R)); > > ... > > if (R_LENGTH > L'LENGTH) then > > QUOT := (others => '0'); > > return RESIZE(QUOT, L'LENGTH); > > end if; > > The signed_num_bits function says that to represent 4 as a signed > value requires 4 bits. This is correct. Because R's bit > representation is larger than L's bit represented the function > returns zero. > > The result of (-4) / 4 should be -1, not 0. Isn't this a bug in in > numeric_std? > > In the signed math packages the result is the size of the "left" argument. SO: if you give it "100" ( = -4) and 4 (integer = 0100) First the 4 is converted into a signed vector, but this causes an overflow because +4 won't fit into a 3 bit signed number. You need one more bit here to make this work. This function has been there since numeric_std first came out. The fixed point packages behave much better in overflow/saturation issues. -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.Received on Mon Aug 11 17:39:24 2014

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