takes on a completely different feeL despite the fact that many of the same
concepts appear on the lowest and highest levels. The chunks on the
high-level description are like the chess expert's chunks, and like the
chunked description of the image on the screen: they summarize in capsule
form a number of things which on lower levels are seen as separate. (See
Fig. 57.) Now before things become too abstract, let us pass on to the, ,
, ,
,
, I
I :
I I
\ \
, ,,. I
, I
. - --~ ---- --1-, _ •
\ \:~~ ---.. ~ ---~ .. ~ .--.~. "·;0
, I
\ \ .' '
\ I
\ , ... --' , 'FIGURE 57. The idea of "chunking": a group of items is reperceived as a single "chunk".
The chunk's boundnry is a little like a cell membrane or a national border: it establishes a
separate identity for the cluster within. AC/:ording to context, one may wish to ignore the
chunk's internal structure or to take it into account.concrete facts about computers, beginning with a very quick skim of what a
computer system is like on the lowest leveL The lowest level? Well, not
really, for I am not going to talk about elementary particles-but it is the
lowest level which we wish to think about.
At the conceptual rock-bottom of a computer, we find a memory, a
central processing unit (CPU), and some input-output (I/O) devices. Let us first
describe the memory. It is divided up into distinct physical pieces, called
words. For the sake of concreteness, let us say there are 65,536 words of
memory (a typical number, being 2 to the 16th power). A word is further
divided into what we shall consider the atoms of computer science-bits.
The number of bits in a typical word might be around thirty-six. Physically,
a bit is just a magnetic "switch" that can be in either of two positions.lololxlolxlxlxlolxlololxlxlololxlolxlxlxlxlxlxlolxlxlololxlxlxlololololol
---a word of 36 bits ---
288 Levels of Description, and Computer Systems