under was not clear to my friends. The idea that "you" know all about
"yourself" is so familiar from interaction with people that it was natural to
extend it to the computer-after all, it was intelligent enough that it could
"talk" to them in English! Their question was not unlike asking a person,
"Why are you making so few red blood cells today?" People do not know
about that level-the "operating system level"-of their bodies.
The main cause of this level-confusion was that communication with all
levels of the computer system was taking place on a single screen, on a
single terminal. Although my friends' naivete might seem rather extreme,
even experienced computer people often make similar errors when several
levels of a complex system are all present at once on the same screen. They
forget "who" they are talking to, and type something which makes no sense
at that level, although it would have made perfect sense on another level. It
might seem desirable, therefore, to have the system itself sort out the
levels-to interpret commands according to what "makes sense". Unfortu-
nately, such interpretation would require the system to have a lot of
common sense, as well as perfect knowledge of the programmer's overall
intent-both of which would require more artificial intelligence than exists
at the present time.The Border between Software and HardwareOne can also be confused by the flexibility of some levels and the rigidity of
others. For instance, on some computers there are marvelous text-editing
systems which allow pieces of text to be "poured" from one format into
another, practically as liquids can be poured from one vessel into another.
A thin page can turn into a wide page, or vice versa. With such power, you
might expect that it would be equally trivial to change from one font to
another-say from roman to italics. Yet there may be only a single font
available on the screen, so that such changes are impossible. Or it may be
feasible on the screen but not printable by the printer-or the other way
around. After dealing with computers for a long time, one gets spoiled, and
thinks that everything should be programmable: no printer should be so
rigid as to have only one character set, or even a finite repertoire of
them-typefaces should be user-specifiable! But once that degree of flexi-
bility has been attained, then one may be annoyed that the printer cannot
print in different colors of ink, or that it cannot accept paper of all shapes
and sizes, or that it does not fix itself when it breaks ...
The trouble is that somewhere, all this flexibility has to "bottom out",
to use the phrase from Chapter V. There must be a hardware level which
underlies it all, and which is inflexible. It may lie deeply hidden, and there
may be so much flexibility on levels above it that few users feel the
hardware limitations-but it is inevitably there.
What is this proverbial distinction between software and hardware? It is
the distinction between programs and machines-between long compli-
cated sequences of instructions, and the physical machines which carryLevels of Description, and Computer Systems 301