I was reading the Intel CPU Architect AMA over on Reddit (
http://www.reddit.com/r/IAmA/comments/15iaet/iama_cpu_architect_and_designer_at_intel_ama/), and someone asked him if the CPU design is laid out by hand.
Here's his reply:
That begs
for the question
to be asked, how do you tell which parts were laid out by hand and which were done by computer?
EDIT: I've always thought of "begs the question" as being short for, "That previous statement begs for the following question to be asked."
It's
really obvious once you know what you're looking for. Hand designs will generally have the design broken into many small pieces (because humans have to use a "divide and conquer" approach for large, complex things), and you'll see a lot of things neatly lined up. Auto-placed designs will either look almost entirely even, or will have blob-shaped patterns. Sometimes, designs are still broken into smaller pieces which get auto-placed, but that's less common (Intel seems to like that technique).
It's easiest to start with really old chips, so let's start with an Intel 486:
See how things are lined up? The second row, and the stuff at the lower right are probably the best examples of what to look for in newer designs when you're trying to identify hand-designed modules
Jumping forward to Apple's A6:
The blue part ("Dual ARM cores") is done by hand. Most of the rest is auto-placed (note the very even appearance).
Here's AMD's Llano; the GPU at the top is auto-placed, while the cores at the bottom are largely done by hand. Note the blobs in the GPU:
The little rectangular things scattered all over are memory arrays ("SRAMs"), and they'll always look like rectangles.
If you want to understand how the hand-placed designs end up looking the way they do, there are some examples showing how the engineers actually work in
this PhD thesis. Page 137 (page 150 of the PDF) shows an excellent example (there's a color key on the next page). You can see how the design is "bit-sliced": each major chunk is made up of a one-bit slice that gets repeated left to right. Of course, the slices can't always be identical (most real logic functions aren't identical the whole way across the datapath), so it's not
exactly the same in every slice, and sometimes the differences are significant (see the purple/pink stuff that's much narrower than the blue stuff? See the warts hanging off the right end?). You can tell that the designer picked pieces to do separately, and then assembled into the final result.
I wonder how they blow them.
Hand them to an intern that wears wool socks and shuffles feet when walking?
Just like a fuse in your car... you run a lot of current through the fuse, and it stops conducting (although the actual method of "failure" may be different - rather than just melting, it can be related to changes in crystal structure, or electromigration moving atoms around). You need a high voltage to blow fuses, so that usually comes from an external power supply connected to (possibly undocumented) pins in the socket, or gets generated on-die by a special circuit.