Intel 253666-024US User Manual

Page 552

3-506 Vol. 2A

Jcc—Jump if Condition Is Met

INSTRUCTION SET REFERENCE, A-M

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 instruc-
tion (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 2

or 64K times (not zero times).

All conditional jumps are converted to code fetches of one or two cache lines, regard-
less 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.

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.

#UD

If the LOCK prefix is used.

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.

#UD

If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Same exceptions as in real address mode.