Intel 80286, 80287 manual Instruction SET

THE 80286 INSTRUCTION SET

If the given condition is true, then a short jump is made to the label provided as the operand. Instruc- tion encoding is most efficient when the target for the conditional jump is in the current code segment and within -128 to +127 bytes of the first byte of the next instruction. Alternatively, the opposite sense (e.g., JNZ has opposite sense to that of JZ) of the conditional jump can skip around an uncon- ditional jump to the destination.

This range is necessary for the assembler to construct a one-byte signed displilcement from the end of the current instruction. If the label is out-of-range, or if the label is a FAR label, then you must perform a jump with the opposite condition around an unconditional jump to the non-short label.

Because there are, in many instances, several ways to interpret a particular state of the flags, ASM286 provides more than one mnemonic for most of the conditional jump opcodes. For example, consider that a programmer who has just compared a character to another in AL might wish to jump if the two were equal (JE), while another programmer who had just ANDed AX with a bit field mask would prefer to consider only whether the result was zero or not (he would use JZ, it synonym for JE).

JCXZ differs from the other conditional jumps in that it actually tests the contents of the CX register for zero, rather than interrogating the flags. This instruction is useful following a conditionally repeated string operation (REPE SCASB, for exampl!) or a conditional loop instruction (such as LOOPNE TARGETLABEL). These instructions implicitly use a limiting count in the CX register. Looping (repeating) ends when either the CX register goes to zero or the condition specified in the instruction (flags indicating equals in both of the above cases) occurs. JCXZ is useful when the terminations must be handled differently.

PROTECTED MODE EXCEPTIONS

#GP(O) if the offset jumped to is beyond the limits of the code segment.

REAL ADDRESS MODE EXCEPTIONS

None

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