Monthly Archives: February 2013

Mum, I’m Building a CPU – Performing Subtraction

Andrei’s 8-bit full (ripple) adder is now complete (see below), so Charlie and I have been able to look at subtraction.

Full Adder and Negator

Bottom: Full adder (4 bits on each board) – Top: Negator (now replaced).

To support subtraction, Charlie and I have build a negation board. When a data-value is sent to the addition board, one copy of it will go through a series of not gates whilst the other does not. Both then go through a multiplexer (74245) which selects which one to send on to the adder board. We did realise on Saturday however that the whole thing can be re-built with eight XOR gates, where we can choose whether to invert each bit by putting either a 0 or 1 on one of the inputs.

One thing that has taken some thinking about with this board is how we accurately perform subtractions – the problem lays with Binary. Lets take a simple example (most-significant-bit is a sign bit, b = binary):

00010011b = 19
now let’s swap the 0’s and 1’s
11101100b = 236
236-(2^8) = -20 (since we have 8 bits)

So, you see our result is one less than the desired one. The solution that we are working on is a negation of the carry flag before it is sent to the adder.  Once I’ve looked more at this I’ll update this post with a Truth-Table.

The Original World Wide Web

Over the last 4 years, cloud computing has gone from been something used by very few to something used by very many.  With the advent of products like Dropbox and iCloud, and mobile apps such as Facebook and Evernote, the Internet is growing at a rate the World Wide Web cannot match.  If that means we are seeing the transition from a more traditional World Wide Web to something that’s more of a content provider for Web Applications, the Internet must have changed dramatically in the past also.  It is therefore of the upmost importance that we must protect a record of what it was, in the same manner as we would a historical artifact.

‘The World-Wide Web’ by Berners-Lee et al. is a vitally important classic paper for Computer Scientists because it does just that.  In it, the authors (including Berners-Lee, no-doubt one of the men who has most shaped humanity) explain what their invention is and what sets it apart from other network information projects from the

It is important to think about what we take for granted now, and what was cutting edge at the time.  Having read the paper, the thing that stands out most is Hypertext – text that when clicked takes you to another file.  Without this crucial feature of the web, our interaction with it would never have been the same – we would have had to load web pages as we load documents from our computer – via a directory system.

Even though this paper was published fairly recently (1994), it was written for a world that has changed unimaginably in the last 19 years.  So much of it’s content is still relevant, which is incredible bearing in mind the circumstances.  Surely, it has passed the test of time.