( ESNUG 490 Item 8 ) -------------------------------------------- [04/06/11]
Subject: Oops! Catapult C tool and 10 readers catch Bluebook quiz mistake
> After Round 1, we gradually upped the stakes and increased the difficulty
> level. The 3rd round of questions was definitely the hardest.
>
> Round 1 Round 2 Round 3
> Participants: 1,144 320 106
> Average score: 73% 72% 48%
> # scored 80%+: 715 182 23
> # scored 100%: 362 84 2
>
> As you can see, our contest ultimately had two winners...
>
> - Thomas Bollaert
> Mentor Graphics Corp. Wilsonville, OR
From: Thomas Bollaert <thomas_bollaert=user domain=mentor not mom>
Hi John,
After we published the 2 winners in ESNUG 488 #11 with the answers to the
quiz, 10 of your readers pointed out a mistake to one of our questions.
"What is the optimal bit width of A that keeps the full
precision of the expression?"
ac_int<8,false> B, C; // 8 Bit unsigned
ac_int<9,true> D,E; // 9 Bit signed
A = B*C + D*E;
Our answer was 19 signed bits. But:
Dear Sirs,
May I remark that the correct answer is indeed 18 bits and not 19 bits?
By computing the dynamic range of each operation, it can be found that:
The result of B*C is contained in range [0..65025]
The result of D*E is contained in range [-65280..65536]
Therefore B*C + D*E is contained in range [-65280..130561]
This range can be represented by 18 signed bits, the optimal answer.
- A DeepChip Reader
With this corrected answer, our HLS Blue Book quiz now has 4 "new" winners,
all of whom scored perfectly on the last round:
Lee Bradshaw United States
Philip Chambers Australia
Paul S. D'Urbano United States
Jerome Lachaize France
We decided to award an iPod Touch grand prize to all 4 new winners. And of
course the original 2 winners get to keep their iPod Touches, too.
Congratulations to the winners, and thanks to the DeepChip community for
always keeping us on our toes.
- Thomas Bollaert
Mentor Graphics Corp. Wilsonville, OR
P.S. I wished we checked this question first with our own Catapult C tool.
We thought 19 bits; but it gave the correct signed 18 bits! Ouch!
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