hw/i386.reference/Jcc.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Opcode Jcc</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
   9 <B>prev:</B><A HREF="IRET.htm"> IRET/IRETD Interrupt Return</A><BR>
  10 <B>next:</B><A HREF="JMP.htm"> JMP Jump</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>Jcc -- Jump if Condition is Met</H1>
  15
  16
  17 <PRE>
  18 Opcode         Instruction       Clocks   Description
  19
  20 77  cb         JA rel8           7+m,3    Jump short if above (CF=0 and
  21                                           ZF=0)
  22 73  cb         JAE rel8          7+m,3    Jump short if above or equal
  23                                           (CF=0)
  24 72  cb         JB rel8           7+m,3    Jump short if below (CF=1)
  25 76  cb         JBE rel8          7+m,3    Jump short if below or equal
  26                                           (CF=1 or ZF=1)
  27 72  cb         JC rel8           7+m,3    Jump short if carry (CF=1)
  28 E3  cb         JCXZ rel8         9+m,5    Jump short if CX register is 0
  29 E3  cb         JECXZ rel8        9+m,5    Jump short if ECX register is 0
  30 74  cb         JE rel8           7+m,3    Jump short if equal (ZF=1)
  31 74  cb         JZ rel8           7+m,3    Jump short if 0 (ZF=1)
  32 7F  cb         JG rel8           7+m,3    Jump short if greater (ZF=0 and
  33                                           SF=OF)
  34 7D  cb         JGE rel8          7+m,3    Jump short if greater or equal
  35                                           (SF=OF)
  36 7C  cb         JL rel8           7+m,3    Jump short if less (SF<>OF)
  37 7E  cb         JLE rel8          7+m,3    Jump short if less or equal
  38                                           (ZF=1 and SF<>OF)
  39 76  cb         JNA rel8          7+m,3    Jump short if not above (CF=1 or
  40                                           ZF=1)
  41 72  cb         JNAE rel8         7+m,3    Jump short if not above or equal
  42                                           (CF=1)
  43 73  cb         JNB rel8          7+m,3    Jump short if not below (CF=0)
  44 77  cb         JNBE rel8         7+m,3    Jump short if not below or equal
  45                                           (CF=0 and ZF=0)
  46 73  cb         JNC rel8          7+m,3    Jump short if not carry (CF=0)
  47 75  cb         JNE rel8          7+m,3    Jump short if not equal (ZF=0)
  48 7E  cb         JNG rel8          7+m,3    Jump short if not greater (ZF=1
  49                                           or SF<>OF)
  50 7C  cb         JNGE rel8         7+m,3    Jump short if not greater or
  51                                           equal (SF<>OF)
  52 7D  cb         JNL rel8          7+m,3    Jump short if not less (SF=OF)
  53 7F  cb         JNLE rel8         7+m,3    Jump short if not less or equal
  54                                           (ZF=0 and SF=OF)
  55 71  cb         JNO rel8          7+m,3    Jump short if not overflow
  56                                           (OF=0)
  57 7B  cb         JNP rel8          7+m,3    Jump short if not parity (PF=0)
  58 79  cb         JNS rel8          7+m,3    Jump short if not sign (SF=0)
  59 75  cb         JNZ rel8          7+m,3    Jump short if not zero (ZF=0)
  60 70  cb         JO rel8           7+m,3    Jump short if overflow (OF=1)
  61 7A  cb         JP rel8           7+m,3    Jump short if parity (PF=1)
  62 7A  cb         JPE rel8          7+m,3    Jump short if parity even (PF=1)
  63 7B  cb         JPO rel8          7+m,3    Jump short if parity odd (PF=0)
  64 78  cb         JS rel8           7+m,3    Jump short if sign (SF=1)
  65 74  cb         JZ rel8           7+m,3    Jump short if zero (ZF = 1)
  66 0F  87 cw/cd   JA rel16/32       7+m,3    Jump near if above (CF=0 and
  67                                           ZF=0)
  68 0F  83 cw/cd   JAE rel16/32      7+m,3    Jump near if above or equal
  69                                           (CF=0)
  70 0F  82 cw/cd   JB rel16/32       7+m,3    Jump near if below (CF=1)
  71 0F  86 cw/cd   JBE rel16/32      7+m,3    Jump near if below or equal
  72                                           (CF=1 or ZF=1)
  73 0F  82 cw/cd   JC rel16/32       7+m,3    Jump near if carry (CF=1)
  74 0F  84 cw/cd   JE rel16/32       7+m,3    Jump near if equal (ZF=1)
  75 0F  84 cw/cd   JZ rel16/32       7+m,3    Jump near if 0 (ZF=1)
  76 0F  8F cw/cd   JG rel16/32       7+m,3    Jump near if greater (ZF=0 and
  77                                           SF=OF)
  78 0F  8D cw/cd   JGE rel16/32      7+m,3    Jump near if greater or equal
  79                                           (SF=OF)
  80 0F  8C cw/cd   JL rel16/32       7+m,3    Jump near if less (SF<>OF)
  81 0F  8E cw/cd   JLE rel16/32      7+m,3    Jump near if less or equal (ZF=1
  82                                           and SF<>OF)
  83 0F  86 cw/cd   JNA rel16/32      7+m,3    Jump near if not above (CF=1 or
  84                                           ZF=1)
  85 0F  82 cw/cd   JNAE rel16/32     7+m,3    Jump near if not above or equal
  86                                           (CF=1)
  87 0F  83 cw/cd   JNB rel16/32      7+m,3    Jump near if not below (CF=0)
  88 0F  87 cw/cd   JNBE rel16/32     7+m,3    Jump near if not below or equal
  89                                           (CF=0 and ZF=0)
  90 0F  83 cw/cd   JNC rel16/32      7+m,3    Jump near if not carry (CF=0)
  91 0F  85 cw/cd   JNE rel16/32      7+m,3    Jump near if not equal (ZF=0)
  92 0F  8E cw/cd   JNG rel16/32      7+m,3    Jump near if not greater (ZF=1
  93                                           or SF<>OF)
  94 0F  8C cw/cd   JNGE rel16/32     7+m,3    Jump near if not greater or
  95                                           equal (SF<>OF)
  96 0F  8D cw/cd   JNL rel16/32      7+m,3    Jump near if not less (SF=OF)
  97 0F  8F cw/cd   JNLE rel16/32     7+m,3    Jump near if not less or equal
  98                                           (ZF=0 and SF=OF)
  99 0F  81 cw/cd   JNO rel16/32      7+m,3    Jump near if not overflow (OF=0)
 100 0F  8B cw/cd   JNP rel16/32      7+m,3    Jump near if not parity (PF=0)
 101 0F  89 cw/cd   JNS rel16/32      7+m,3    Jump near if not sign (SF=0)
 102 0F  85 cw/cd   JNZ rel16/32      7+m,3    Jump near if not zero (ZF=0)
 103 0F  80 cw/cd   JO rel16/32       7+m,3    Jump near if overflow (OF=1)
 104 0F  8A cw/cd   JP rel16/32       7+m,3    Jump near if parity (PF=1)
 105 0F  8A cw/cd   JPE rel16/32      7+m,3    Jump near if parity even (PF=1)
 106 0F  8B cw/cd   JPO rel16/32      7+m,3    Jump near if parity odd (PF=0)
 107 0F  88 cw/cd   JS rel16/32       7+m,3    Jump near if sign (SF=1)
 108 0F  84 cw/cd   JZ rel16/32       7+m,3    Jump near if 0 (ZF=1)
 109 </PRE>
 110
 111 <EM>
 112 <H3>Notes</H3>
 113   The first clock count is for the true condition (branch taken); the
 114   second clock count is for the false condition (branch not taken). rel16/32
 115   indicates that these instructions map to two; one with a 16-bit relative
 116   displacement, the other with a 32-bit relative displacement, depending on
 117   the operand-size attribute of the instruction.
 118 </EM>
 119
 120 <H2>Operation</H2>
 121
 122 <PRE>
 123 IF condition
 124 THEN
 125    EIP := EIP + SignExtend(rel8/16/32);
 126    IF OperandSize = 16
 127    THEN EIP := EIP AND 0000FFFFH;
 128    FI;
 129 FI;
 130 </PRE>
 131
 132 <H2>Description</H2>
 133
 134 Conditional jumps (except JCXZ) test the flags which have been set by
 135 a previous instruction. The conditions for each mnemonic are given in
 136 parentheses after each description above. The terms "less" and "greater"
 137 are used for comparisons of signed integers; "above" and "below" are
 138 used for unsigned integers.
 139 <P>
 140 If the given condition is true, a jump is made to the location provided as
 141 the operand. Instruction coding is most efficient when the target for the
 142 conditional jump is in the current code segment and within -128 to
 143 +127 bytes of the next instruction's first byte. The jump can also target
 144 -32768 thru +32767 (segment size attribute 16) or -2^(31) thru +2^(31) -1
 145 (segment size attribute 32) relative to the next instruction's first byte.
 146 When the target for the conditional jump is in a different segment, use
 147 the opposite case of the jump instruction (i.e., JE and JNE), and then
 148 access the target with an unconditional far jump to the other segment.
 149 For example, you cannot code--
 150 <PRE>
 151 JZ FARLABEL;
 152 </PRE>
 153 You must instead code--
 154 <PRE>
 155    JNZ BEYOND;
 156    JMP FARLABEL;
 157 BEYOND:
 158 </PRE>
 159 Because there can be several ways to interpret a particular state of the
 160 flags, ASM386 provides more than one mnemonic for most of the
 161 conditional jump opcodes. For example, if you compared two characters in
 162 AX and want to jump if they are equal, use JE; or, if you ANDed AX
 163 with a bit field mask and only want to jump if the result is 0, use JZ, a
 164 synonym for JE.
 165 <P>
 166 JCXZ differs from other conditional jumps because it tests the contents of
 167 the CX or ECX register for 0, not the flags. JCXZ is useful at the beginning
 168 of a conditional loop that terminates with a conditional loop instruction
 169 (such as
 170 <A HREF="LOOP.htm">LOOPNE</A>
 171 TARGET LABEL). The JCXZ prevents entering the loop with CX or
 172 ECX equal to zero, which would cause the loop to execute 64K or 32G times
 173 instead of zero times.
 174
 175 <H2>Flags Affected</H2>
 176
 177 None
 178
 179 <H2>Protected Mode Exceptions</H2>
 180
 181 #GP(0) if the offset jumped to is beyond the limits of the code segment
 182
 183 <H2>Real Address Mode Exceptions</H2>
 184
 185 None
 186
 187 <H2>Virtual 8086 Mode Exceptions</H2>
 188
 189 None
 190
 191
 192 <P>
 193 <HR>
 194 <P>
 195 <B>up:</B> <A HREF="c17.htm">
 196 Chapter 17 -- 80386 Instruction Set</A><BR>
 197 <B>prev:</B><A HREF="IRET.htm"> IRET/IRETD Interrupt Return</A><BR>
 198 <B>next:</B><A HREF="JMP.htm"> JMP Jump</A>
 199 </BODY>