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Unformatted text preview: 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, 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 JRCXZ, JECXZ and JCXZ instructions differ from other Jcc instructions because they do not check status flags. Instead, they check RCX, ECX or CX for 0. The register checked is determined by the address-size attribute. These instructions are useful when used at the beginning of a loop that terminates with a conditional loop instruction (such as LOOPNE). They can be used to prevent an instruction sequence from entering a loop when RCX, ECX or CX is 0. This would cause the loop to execute 264, 232 or 64K times (not zero times). All conditional jumps are converted to code fetches of one or two cache lines, regardless of jump address or cacheability. In 64-bit mode, operand size is fixed at 64 bits. JMP Short is RIP = RIP + 8-bit offset sign extended to 64 bits. JMP Near is RIP = RIP + 32-bit offset sign extended to 64-bits. Vol. 2 3-499 INSTRUCTION SET REFERENCE, A-M Operation
IF condition THEN tempEIP EIP + SignExtend(DEST); IF OperandSize = 16 THEN tempEIP tempEIP AND 0000FFFFH; FI; IF tempEIP is not within code segment limit THEN #GP(0); ELSE EIP tempEIP FI; FI; Protected Mode Exceptions
#GP(0) If the offset being jumped to is beyond the limits of the CS segment. Real-Address Mode Exceptions
#GP If the offset being jumped to is beyond the limits of the CS segment or is outside of the effective address space from 0 to FFFFH. This condition can occur if a 32-bit address size override prefix is used. Virtual-8086 Mode Exceptions
Same exceptions as in Real Address Mode. Compatibility Mode Exceptions
Same exceptions as in Protected Mode. 64-Bit Mode Exceptions
#GP(0) If the memory address is in a non-canonical form. 3-500 Vol. 2 INSTRUCTION SET REFERENCE, A-M...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11