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+Chapter 6\
+ Looking Past Face Value {#Heading1}
+------------------------
+
+### How Machine Instructions May Do More Than You Think {#Heading2}
+
+I first met Jeff Duntemann at an authors’ dinner hosted by *PC Tech
+Journal* at Fall Comdex, back in 1985. Jeff was already reasonably
+well-known as a computer editor and writer, although not as famous as
+*Complete Turbo Pascal*, editions 1 through 672 (or thereabouts), *TURBO
+TECHNIX*, and *PC TECHNIQUES* would soon make him. I was fortunate
+enough to be seated next to Jeff at the dinner table, and, not
+surprisingly, our often animated conversation revolved around computers,
+computer writing, and more computers (not necessarily in that order).
+
+Although I was making a living at computer work and enjoying it at the
+time, I nonetheless harbored vague ambitions of being a science-fiction
+writer when I grew up. (I have since realized that this hardly puts me
+in elite company, especially in the computer world, where it seems that
+every other person has told me they plan to write science fiction
+“someday.” Given that probably fewer than 500—I’m guessing here—original
+science fiction and fantasy short stories, and perhaps a few more novels
+than that, are published each year in this country, I see a few mid-life
+crises coming.)
+
+At any rate, I had accumulated a small collection of rejection slips,
+and fancied myself something of an old hand in the field. At the end of
+the dinner, as the other writers complained half-seriously about how
+little they were paid for writing for *Tech Journal*, I leaned over to
+Jeff and whispered, “You know, the pay isn’t so bad here. You should see
+what they pay for science fiction—even to the guys who win awards!”
+
+To which Jeff replied, “I know. I’ve been nominated for two Hugos.”
+
+Oh.
+
+Had I known I was seated next to a real, live science-fiction writer—an
+*award-nominated* writer, by God!—I would have pumped him for all I was
+worth, but the possibility had never occurred to me. I was at a dinner
+put on by a computer magazine, seated next to an editor who had just
+finished a book about Turbo Pascal, and, gosh, it was *obvious* that the
+appropriate topic was computers.
+
+For once, the moral is *not* “don’t judge a book by its cover.” Jeff is
+in fact what he appeared to be at face value: a computer writer and
+editor. However, he is more, too; face value wasn’t full value. You’ll
+similarly find that face value isn’t always full value in computer
+programming, and especially so when working in assembly language, where
+many instructions have talents above and beyond their obvious abilities.
+
+On the other hand, there are also a number of instructions, such as
+**LOOP**, that are designed to perform specific functions but aren’t
+always the best instructions for those functions. So don’t judge a book
+by its cover, either.
+
+Assembly language for the x86 family isn’t like any other language (for
+which we should, without hesitation, offer our profuse thanks). Assembly
+language reflects the design of the processor rather than the way we
+think, so it’s full of multiple instructions that perform similar
+functions, instructions with odd and often confusing side effects, and
+endless ways to string together different instructions to do much the
+same things, often with seemingly minuscule differences that can turn
+out to be surprisingly important.
+
+To produce the best code, you must decide precisely what you need to
+accomplish, then put together the sequence of instructions that
+accomplishes that end most efficiently, regardless of what the
+instructions are usually used for. That’s why optimization for the PC is
+an art, and it’s why the best assembly language for the x86 family will
+almost always handily outperform compiled code. With that in mind, let’s
+look past face value—and while we’re at it, I’ll toss in a few examples
+of not judging a book by its cover.
+
+The point to all this: You must come to regard the x86 family
+instructions for what they do, not what you’re used to thinking they do.
+Yes, **SHL** shifts a pattern left—but a look-up table can do the same
+thing, and can often do it faster. **ADD** can indeed add two operands,
+but it can’t put the result in a third register; **LEA** can. The
+instruction set is your raw material for writing high-performance code.
+By limiting yourself to thinking only in certain well-established ways
+about the various instructions, you’re putting yourself at a substantial
+disadvantage every time you sit down to program.
+
+In short, the x86 family can do much more than you think—if you’ll use
+everything it has to offer. Give it a shot!
+
+#### Memory Addressing and Arithmetic {#Heading3}
+
+Years ago, I saw a clip on the David Letterman show in which Letterman
+walked into a store by the name of “Just Lamps” and asked, “So what do
+you sell here?”
+
+“Lamps,” he was told. “Just lamps. Can’t you read?”
+
+“Lamps,” he said. “I see. And what else?”
+
+From that bit of sublime idiocy we can learn much about divining the
+full value of an instruction. To wit:
+
+Quick, what do the x86’s memory addressing modes do?
+
+“Calculate memory addresses,” you no doubt replied. And you’re right, of
+course. But what *else* do they do?
+
+They perform arithmetic, that’s what they do, and that’s a distinctly
+different and often useful perspective on memory address calculations.
+
+For example, suppose you have an array base address in BX and an index
+into the array in SI. You could add the two registers together to
+address memory, like this:
+
+    add  bx,si
+    mov  al,[bx]
+
+Or you could let the processor do the arithmetic for you in a single
+instruction:
+
+    mov  al,[bx+si]
+
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+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash