10: Bits, Flags, Branches, and Tables Easing into Mainstream Assembly Programming in .NET

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10: Bits, Flags, Branches, and Tables Easing into Mainstream Assembly Programming
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Overview
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You don't take off until all your flight checks are made That's the reason that we haven't done a lot of instruction arranging in this book up until here, now that we are in the last quarter of the book I've found that machine instructions aren't the most important part of assembly language programming What's most important is understanding your machine and your tools and how everything fits together Higher-level languages such as Pascal and Modula-2 hide much of those essential details from you In assembly language you must see to them yourself For some reason, authors of previous beginner books on assembly language haven't caught on to this fact This fact (in fact) was the major motivation for my writing this book If you've digested everything I've said so far, however, you're ready to get in and understand the remainder of the x86 instruction set I won't teach it all in this book, but the phrase ready to understand is germane You can now find yourself a reference and learn what instructions I don't cover on your own The skills you need to build programming expertise are now yours, and if this book has accomplished that much, I'd say it's accomplished a lot So, let the fun begin
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Bits Is Bits (and Bytes Is Bits)
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Assembly language is big on bits Bits, after all, are what bytes are made of, and one essential assembly language skill is building bytes and taking them apart again A technique called bit mapping is widely used in assembly language Bit mapping assigns special meanings to individual bits within a byte to save space and squeeze the last little bit of utility out of a given amount of memory There is a family of instructions in the x86 instruction set that allows you to manipulate the bits within the bytes by applying Boolean logical operations to the bytes on a bit-by-bit basis These are the bitwise logical instructions: AND, OR, XOR, and NOT Another family of instructions allows you to slide bits back and forth within a single byte or word These are the most-used shift/rotate instructions: ROL, ROR, RCL, RCR, SHL, and SHR (There are a few others that I will not be discussing in this book)
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Bit Numbering
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Dealing with bits requires that we have a way of specifying which bits we're dealing with By convention, bits in assembly language are numbered, starting from 0, at the least-significant bit in the byte, word, or other item we're using as a bit map The least-significant bit is the one with the least value in the binary number system (Return to 2 and reread the material on base 2 if that seems fuzzy to you) It's also the bit on the far right, if you write the value down as a binary number It works best as a visual metaphor See Figure 101
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Figure 101: Bit numbering When you count bits, start with the bit on the right, and number them from 0
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"It's the Logical Thing to Do, Jim "
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Boolean logic sounds arcane and forbidding, but remarkably, it reflects the realities of ordinary thought and action The Boolean operator AND, for instance, pops up in many of the decisions you make every day of your life For example, to write a check that doesn't bounce, you must have money in your checking account AND checks in your checkbook Neither alone will do the job ("How can I be overdrawn " goes the classic question, "I still have checks in my checkbook!") You can't write a check you don't have, and a check without money behind it will bounce People who live out of their checkbooks (and they always end up ahead of me in the checkout line at Safeway) must use the AND operator frequently When mathematicians speak of Boolean logic, they manipulate abstract values called True and False The AND operator works like this Condition1 AND Condition2 will be considered True if both Condition1 and Condition2 are True If either condition is False, the result will be False There are in fact four different combinations of the two input values, so logical operations between two values are usually summarized in a form called a truth table The truth table for the AND operator is shown in Table 101
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Table 101: The AND Truth Table for Formal Logic CONDITION1 False False True True OPERATOR AND AND AND AND CONDITION2 False = True = False = True = RESULT False False False True
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There's nothing mysterious about the truth table It's just a summary of all possibilities of the AND operator as applied to two input conditions The important thing to remember is that only when both input values are True will the result also be True That's the way mathematicians see AND In assembly language terms, the AND instruction looks at two bits and yields a third bit based on the values of the first two bits By convention, we consider a 1 bit to be True and a 0 bit to be False The logic is identical; we're just using different symbols to represent True and False Keeping that in mind, we can rewrite AND's truth table to make it more meaningful for assembly language work See Table 102 Table 102: The AND Truth Table for Assembly Language BIT 1 0 0 1 1 OPERATOR AND AND AND AND BIT 2 0= 1= 0= 1= RESULT BIT 0 0 0 1
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