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INSTRUCTION SET REFERENCE
JccJump if Condition Is Met (Continued)
The target instruction is specified with a relative offset (a signed offset relative to the current
value of the instruction pointer in the EIP register). A relative offset (rel8, rel16, or rel32) is
generally specified as a label in assembly code, but at the machine code level, it is encoded as a
signed, 8-bit or 32-bit immediate value, which is added to the instruction pointer. Instruction
coding is most efficient for offsets of 128 to +127. If the operand-size attribute is 16, the upper
two bytes of the EIP register are cleared to 0s, resulting in a maximum instruction pointer size
of 16 bits.
The conditions for each Jcc mnemonic are given in the Description column of the table on the
preceding page. The terms less and greater are used for comparisons of signed integers and
the terms above and below are used for unsigned integers.
Because a particular state of the status flags can sometimes be interpreted in two ways, two
mnemonics are defined for some opcodes. For example, the JA (jump if above) instruction and
the JNBE (jump if not below or equal) instruction are alternate mnemonics for the opcode 77H.
The Jcc instruction does not support far jumps (jumps to other code segments). When the target
for the conditional jump is in a different segment, use the opposite condition from the condition
being tested for the Jcc instruction, and then access the target with an unconditional far jump
(JMP instruction) to the other segment. For example, the following conditional far jump is
illegal:
JZ FARLABEL;
To accomplish this far jump, use the following two instructions:
JNZ BEYOND;
JMP FARLABEL;
BEYOND:
The JECXZ and JCXZ instructions differs from the other Jcc instructions because they do not
check the status flags. Instead they check the contents of the ECX and CX registers, respectively,
for 0. Either the CX or ECX register is chosen according to the address-size attribute. These
instructions are useful at the beginning of a conditional loop that terminates with a conditional
loop instruction (such as LOOPNE). They prevent entering the loop when the ECX or CX
register is equal to 0, which would cause the loop to execute 2
32
or 64K times, respectively,
instead of zero times.
All conditional jumps are converted to code fetches of one or two cache lines, regardless of jump
address or cacheability.
