Understanding Low-Level Logic in .NET

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Understanding Low-Level Logic
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The most basic element in software that distinguishes your average pocket calculator from a full-blown computer is the ability to execute a sequence of logical and conditional instructions. The following sections demonstrate the most common types of low-level logical constructs frequently encountered while
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Appendix A
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reversing, and explain their exact meanings. I begin by going over the process of comparing two operands in assembly language, which is a significant building block used in almost every logical statement. I then proceed to discuss the conditional codes in IA-32 assembly language, which are employed in every conditional instruction in the instruction set.
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Comparing Operands
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The vast majority of logical statements involve the comparison of two or more operands. This is usually followed by code that can act differently based on the result of the comparison. The following sections demonstrate the operand comparison mechanism in IA-32 assembly language. This process is somewhat different for signed and unsigned operands. The fundamental instruction for comparing operands is the CMP instruction. CMP essentially subtracts the second operand from the first and discards the result. The processor s flags are used for notifying the instructions that follow on the result of the subtraction. As with many other instructions, flags are read differently depending on whether the operands are signed or unsigned.
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If you re not familiar with the subtleties of IA-32 flags, it is highly recommended that you go over the Arithmetic Flags section in Appendix B before reading further.
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Signed Comparisons
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Table A.1 demonstrates the behavior of the CMP instruction when comparing signed operands. Remember that the following table also applies to the SUB instruction.
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Table A.1 Signed Subtraction Outcome Table for CMP and SUB Instructions (X represents the left operand, while Y represents the right operand) LEFT OPERAND X >= 0 RIGHT OPERAND Y >= 0 RELATION BETWEEN OPERANDS X=Y FLAGS AFFECTED
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OF = 0 SF = 0 ZF = 1 The two operands are equal, so the result is zero. OF = 0 SF = 0 ZF = 0 Flags are all zero, indicating a positive result, with no overflow.
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Y >= 0
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OF = 0 SF = 0 ZF = 0 This is the same as the preceding case, with both X and Y containing negative integers. OF = 0 SF = 1 ZF = 0 An SF = 1 represents a negative result, which (with OF being unset) indicates that Y is larger than X. OF = 0 SF = 1 ZF = 0 This is the same as the preceding case, except that X is negative and Y is positive. Again, the combination of SF = 1 with OF = 0 represents that Y is greater than X. OF = 1 SF = 0 ZF = 0 This is another similar case where X is negative and Y is positive, except that here an overflow is generated, and the result is positive. OF = 1 SF = 1 ZF = 0 When X is positive and Y is a negative integer low enough to generate a positive overflow, both OF and SF are set.
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Appendix A
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In looking at Table A.1, the ground rules for identifying the results of signed integer comparisons become clear. Here s a quick summary of the basic rules:
Anytime ZF is set you know that the subtraction resulted in a zero, which means that the operands are equal. When all three flags are zero, you know that the first operand is greater than the second, because you have a positive result and no overflow. When there is a negative result and no overflow (SF=1 and OF=0), you know that the second operand is larger than the first. When there is an overflow and a positive result, the second operand must be larger than the first, because you essentially have a negative result that is too small to be represented by the destination operand (hence the overflow). When you have an overflow and a negative result, the first operand must be larger than the second, because you essentially have a positive result that is too large to be represented by the destination operand (hence the overflow).
While it is not generally necessary to memorize the comparison outcome tables (tables A.1 and A.2), it still makes sense to go over them and make sure that you properly understand how each flag is used in the operand comparison process. This will be helpful in some cases while reversing when flags are used in unconventional ways. Knowing how flags are set during comparison and subtraction is very helpful for properly understanding logical sequences and quickly deciphering their meaning.