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Those assembler-knowledgeable folk who have stuck with me this long may be wondering why I haven't covered conditional jumps until this late in the book I mean, we've explained procedures already and haven't even gotten to jumps yet Indeed That's the whole point I explained procedures before jumps because when people learn those two concepts the other way around, they have a tendency to use jumps for everything, even when procedures are called for Unlike some high-level languages such as Pascal and Modula-2, there is no way around jumpswhat Pascal and Modula people so derisively call "GOTOs"-in assembly language Sadly, some people then assume that jumps are It, and don't bother imposing any structure at all on their assembly language programs By teaching procedures first, I feel that I've at least made possible a more balanced approach on the part of the learner Besides, I felt it wise to teach how to manage complexity before teaching the number one means of creating complexity
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A jump is just that: an abrupt change in the flow of instruction execution Ordinarily, instructions are executed one after the other, in order, moving from low memory toward high memory Jump instructions alter the address of the next instruction to be executed Execute a jump instruction, and zap! All of a sudden you're somewhere else in the code segment A jump instruction can move execution forward in memory or backward It can bend execution back into a loop (And it can tie your program logic in knots ) There are two kinds of jumps: conditional and unconditional An unconditional jump is a jump that always happens It takes this form: JMP <label> When this instruction executes, the sequence of execution moves to the instruction located at the label specified by <label> It's just that simple The unconditional JMP instruction is of limited use by itself It almost always works in conjunction with the conditional jump instructions that test the state of the various x86 flags You'll see how this works in just a little while, once we've gone through conditional jumps too
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A conditional jump instruction is one of those fabled tests I introduced in 1 When executed, a conditional jump tests something, usually one of the flags in the Flags register If the flag being tested happens to be in a particular state, execution may jump to a label somewhere else in the code segment, or it may simply fall through to the next instruction in sequence This two-way nature is important A conditional jump instruction either jumps, or it falls through Jump, or no jump It can't jump to one of two places, or three Whether it jumps or not depends on the current value of one single bit within the CPU For example, there is a flag that is set to 1 by certain instructions when the result of that instruction is zero: the Zero flag ZF The DEC (DECrement) instruction is a good example DEC subtracts one from its operand If by that subtraction the operand becomes zero, ZF is set to 1 One of the conditional jump instructions, JZ (Jump if Zero) tests ZF If ZF is found set to 1, a jump occurs, and execution transfers to a label If ZF is found to be 0, execution falls through to the next instruction in line Here's a simple (and nonoptimal) example, using instructions you should already understand: mov byte [Counter],17 WorkLoop: call DoWork dec byte [Counter] jz AllDone jmp WorkLoop ; ; ; ; ; We're going to do this 17 times Process the data Subtract 1 from the counter If the counter is zero, we're done! Otherwise, go back and execute the loop again
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The label AllDone isn't shown in the example because it's somewhere else in the program, maybe a long way off The important thing is that the JZ instruction is a two-way switch If ZF is equal to 1, execution moves to the location marked by the label AllDone If ZF is equal to 0, execution falls through to the next instruction in sequence Here, that would be the unconditional jump instruction JMP WorkLoop This simple loop is one way to perform a call to a procedure some set number of times A count value is stored in a variable named Counter The procedure is called After control returns from the procedure, Counter is decremented by one If that drops the counter to 0, the procedure has been called the full number of times, and the loop sends execution elsewhere If the counter still has some count in it, execution loops back to the procedure call and begins the loop again Note the use of an unconditional jump instruction to close the loop
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Beware Endless Loops!
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This is a good place to warn you of a common sort of bug that produces the dreaded endless loop, which locks up your machine and forces you to reboot to get out Suppose the preceding code snippet was instead done the following way: WorkLoop: mov byte [Counter],17 call DoWork dec byte [Counter] jz AllDone jmp WorkLoop ; ; ; ; ; We're going to do this 17 times Process the data Subtract 1 from the counter If the counter is zero, we're done! Otherwise, go back and execute the loop again
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This becomes a pretty obvious endless loop (However, you'll be appalled at how often such an obvious bug will dance in your face for hours without being recognized as such ) The key point is that the instruction that loads the initial value to the counter is inside the loop! Every time the loop happens, the counter is counted down by one and then immediately reloaded with the original count value The count value thus never gets smaller than the original value minus one, and the loop (which is waiting for the counter to become zero) never ends You're unlikely to do something like this deliberately, of course But it's very easy to type a label at the wrong place or (easier still!) to type the name of the wrong label, a label that might be at or before the point where a counter is loaded with its initial value Assembly language programming requires concentration and endless attention to detail If you pay attention to what you're doing, you'll make fewer "stupid" errors like the preceding one But I can promise you that you'll still make a few
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