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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode Jcc</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="IRET.htm"> IRET/IRETD Interrupt Return</A><BR><B>next:</B><A HREF="JMP.htm"> JMP Jump</A><P><HR><P><H1>Jcc -- Jump if Condition is Met</H1><PRE>Opcode Instruction Clocks Description77 cb JA rel8 7+m,3 Jump short if above (CF=0 and ZF=0)73 cb JAE rel8 7+m,3 Jump short if above or equal (CF=0)72 cb JB rel8 7+m,3 Jump short if below (CF=1)76 cb JBE rel8 7+m,3 Jump short if below or equal (CF=1 or ZF=1)72 cb JC rel8 7+m,3 Jump short if carry (CF=1)E3 cb JCXZ rel8 9+m,5 Jump short if CX register is 0E3 cb JECXZ rel8 9+m,5 Jump short if ECX register is 074 cb JE rel8 7+m,3 Jump short if equal (ZF=1)74 cb JZ rel8 7+m,3 Jump short if 0 (ZF=1)7F cb JG rel8 7+m,3 Jump short if greater (ZF=0 and SF=OF)7D cb JGE rel8 7+m,3 Jump short if greater or equal (SF=OF)7C cb JL rel8 7+m,3 Jump short if less (SF<>OF)7E cb JLE rel8 7+m,3 Jump short if less or equal (ZF=1 and SF<>OF)76 cb JNA rel8 7+m,3 Jump short if not above (CF=1 or ZF=1)72 cb JNAE rel8 7+m,3 Jump short if not above or equal (CF=1)73 cb JNB rel8 7+m,3 Jump short if not below (CF=0)77 cb JNBE rel8 7+m,3 Jump short if not below or equal (CF=0 and ZF=0)73 cb JNC rel8 7+m,3 Jump short if not carry (CF=0)75 cb JNE rel8 7+m,3 Jump short if not equal (ZF=0)7E cb JNG rel8 7+m,3 Jump short if not greater (ZF=1 or SF<>OF)7C cb JNGE rel8 7+m,3 Jump short if not greater or equal (SF<>OF)7D cb JNL rel8 7+m,3 Jump short if not less (SF=OF)7F cb JNLE rel8 7+m,3 Jump short if not less or equal (ZF=0 and SF=OF)71 cb JNO rel8 7+m,3 Jump short if not overflow (OF=0)7B cb JNP rel8 7+m,3 Jump short if not parity (PF=0)79 cb JNS rel8 7+m,3 Jump short if not sign (SF=0)75 cb JNZ rel8 7+m,3 Jump short if not zero (ZF=0)70 cb JO rel8 7+m,3 Jump short if overflow (OF=1)7A cb JP rel8 7+m,3 Jump short if parity (PF=1)7A cb JPE rel8 7+m,3 Jump short if parity even (PF=1)7B cb JPO rel8 7+m,3 Jump short if parity odd (PF=0)78 cb JS rel8 7+m,3 Jump short if sign (SF=1)74 cb JZ rel8 7+m,3 Jump short if zero (ZF = 1)0F 87 cw/cd JA rel16/32 7+m,3 Jump near if above (CF=0 and ZF=0)0F 83 cw/cd JAE rel16/32 7+m,3 Jump near if above or equal (CF=0)0F 82 cw/cd JB rel16/32 7+m,3 Jump near if below (CF=1)0F 86 cw/cd JBE rel16/32 7+m,3 Jump near if below or equal (CF=1 or ZF=1)0F 82 cw/cd JC rel16/32 7+m,3 Jump near if carry (CF=1)0F 84 cw/cd JE rel16/32 7+m,3 Jump near if equal (ZF=1)0F 84 cw/cd JZ rel16/32 7+m,3 Jump near if 0 (ZF=1)0F 8F cw/cd JG rel16/32 7+m,3 Jump near if greater (ZF=0 and SF=OF)0F 8D cw/cd JGE rel16/32 7+m,3 Jump near if greater or equal (SF=OF)0F 8C cw/cd JL rel16/32 7+m,3 Jump near if less (SF<>OF)0F 8E cw/cd JLE rel16/32 7+m,3 Jump near if less or equal (ZF=1 and SF<>OF)0F 86 cw/cd JNA rel16/32 7+m,3 Jump near if not above (CF=1 or ZF=1)0F 82 cw/cd JNAE rel16/32 7+m,3 Jump near if not above or equal (CF=1)0F 83 cw/cd JNB rel16/32 7+m,3 Jump near if not below (CF=0)0F 87 cw/cd JNBE rel16/32 7+m,3 Jump near if not below or equal (CF=0 and ZF=0)0F 83 cw/cd JNC rel16/32 7+m,3 Jump near if not carry (CF=0)0F 85 cw/cd JNE rel16/32 7+m,3 Jump near if not equal (ZF=0)0F 8E cw/cd JNG rel16/32 7+m,3 Jump near if not greater (ZF=1 or SF<>OF)0F 8C cw/cd JNGE rel16/32 7+m,3 Jump near if not greater or equal (SF<>OF)0F 8D cw/cd JNL rel16/32 7+m,3 Jump near if not less (SF=OF)0F 8F cw/cd JNLE rel16/32 7+m,3 Jump near if not less or equal (ZF=0 and SF=OF)0F 81 cw/cd JNO rel16/32 7+m,3 Jump near if not overflow (OF=0)0F 8B cw/cd JNP rel16/32 7+m,3 Jump near if not parity (PF=0)0F 89 cw/cd JNS rel16/32 7+m,3 Jump near if not sign (SF=0)0F 85 cw/cd JNZ rel16/32 7+m,3 Jump near if not zero (ZF=0)0F 80 cw/cd JO rel16/32 7+m,3 Jump near if overflow (OF=1)0F 8A cw/cd JP rel16/32 7+m,3 Jump near if parity (PF=1)0F 8A cw/cd JPE rel16/32 7+m,3 Jump near if parity even (PF=1)0F 8B cw/cd JPO rel16/32 7+m,3 Jump near if parity odd (PF=0)0F 88 cw/cd JS rel16/32 7+m,3 Jump near if sign (SF=1)0F 84 cw/cd JZ rel16/32 7+m,3 Jump near if 0 (ZF=1)</PRE><EM><H3>Notes</H3> The first clock count is for the true condition (branch taken); the second clock count is for the false condition (branch not taken). rel16/32 indicates that these instructions map to two; one with a 16-bit relative displacement, the other with a 32-bit relative displacement, depending on the operand-size attribute of the instruction.</EM><H2>Operation</H2><PRE>IF conditionTHEN EIP := EIP + SignExtend(rel8/16/32); IF OperandSize = 16 THEN EIP := EIP AND 0000FFFFH; FI;FI;</PRE><H2>Description</H2>Conditional jumps (except JCXZ) test the flags which have been set bya previous instruction. The conditions for each mnemonic are given inparentheses after each description above. The terms "less" and "greater"are used for comparisons of signed integers; "above" and "below" areused for unsigned integers.<P>If the given condition is true, a jump is made to the location provided asthe operand. Instruction coding is most efficient when the target for theconditional jump is in the current code segment and within -128 to+127 bytes of the next instruction's first byte. The jump can also target-32768 thru +32767 (segment size attribute 16) or -2^(31) thru +2^(31) -1(segment size attribute 32) relative to the next instruction's first byte.When the target for the conditional jump is in a different segment, usethe opposite case of the jump instruction (i.e., JE and JNE), and thenaccess the target with an unconditional far jump to the other segment.For example, you cannot code--<PRE>JZ FARLABEL;</PRE>You must instead code--<PRE> JNZ BEYOND; JMP FARLABEL;BEYOND:</PRE>Because there can be several ways to interpret a particular state of theflags, ASM386 provides more than one mnemonic for most of theconditional jump opcodes. For example, if you compared two characters inAX and want to jump if they are equal, use JE; or, if you ANDed AXwith a bit field mask and only want to jump if the result is 0, use JZ, asynonym for JE.<P>JCXZ differs from other conditional jumps because it tests the contents ofthe CX or ECX register for 0, not the flags. JCXZ is useful at the beginningof a conditional loop that terminates with a conditional loop instruction(such as <A HREF="LOOP.htm">LOOPNE</A> TARGET LABEL). The JCXZ prevents entering the loop with CX orECX equal to zero, which would cause the loop to execute 64K or 32G timesinstead of zero times.<H2>Flags Affected</H2>None<H2>Protected Mode Exceptions</H2>#GP(0) if the offset jumped to is beyond the limits of the code segment<H2>Real Address Mode Exceptions</H2>None<H2>Virtual 8086 Mode Exceptions</H2>None<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="IRET.htm"> IRET/IRETD Interrupt Return</A><BR><B>next:</B><A HREF="JMP.htm"> JMP Jump</A></BODY>?? 快捷鍵說明
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