Control Instructions In Assembly Language

The last major set of instructions that imaginary machine implemented are the control instructions. These are the assembly language corresponding of the control instructions in C. In other words, these are used instead of for loops, if..Else constructs, and while loops.

Control instructions are extremely primitive in assembly language, though there is one instruction which does not come into view in a lot of of the higher level languages: the goto or jmp (standing for jump) instruction. This instruction permits a program to branch off and continue execution as of an arbitrary address. One provides it with the address of a location in memory (as an operand) and the processor changes its instruction pointer to that latest address, thus that execution continues from that new position.

Of course, the JMP instruction is not much use on its own. A program needs to make decisions concerning when to jump and when to continue execution at the current location without branching off. Therefore, the JMP instruction is balanced with numerous conditional jump instructions. These jump just when a certain condition (generally based on the state of th flags) is true. They make a type of primitive if..else statement. If the flags are set one way, then branch, or else continue execution at the present location.

So as to set the flags in a significant way, one frequently calls a compare instruction before making a conditional jump. A compare just subtracts two values (without storing the result) and sets the flags suitably. If a compare instruction is followed immediately by a conditional jump instruction, the jump will occur based on the worth of the subtraction that is completed by the compare. Particularly, if one value is superior to the other, or the values are very similar, different flags will be set. If the two values were the same for instance, the result of the subtraction would be zero, setting the zero flag. If the first is bigger than the second, the sign flag will be set, and if neither is the case, nor of the sign and zero flags will be set. In its place they will be cleared to zero by the compare instruction. Of course one needs a range of compare instructions for comparing different types of values (memory with accumulator, register by means of accumulator, immidiate value with accumulator, etc).

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