LOOP/LOOPcond -- Loop Control with CX Counter

Opcode   Instruction  Clocks  Description

E2  cb   LOOP rel8    11+m    DEC count; jump short if count <> 0
E1  cb   LOOPE rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
E1  cb   LOOPZ rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
E0  cb   LOOPNE rel8  11+m    DEC count; jump short if count <> 0 and ZF=0
E0  cb   LOOPNZ rel8  11+m    DEC count; jump short if count <> 0 and ZF=0

Operation

IF AddressSize = 16 THEN CountReg is CX ELSE CountReg is ECX; FI;
CountReg := CountReg - 1;
IF instruction <> LOOP
THEN
   IF (instruction = LOOPE) OR (instruction = LOOPZ)
   THEN BranchCond := (ZF = 1) AND (CountReg <> 0);
   FI;
   IF (instruction = LOOPNE) OR (instruction = LOOPNZ)
   THEN BranchCond := (ZF = 0) AND (CountReg <> 0);
   FI;
FI;

IF BranchCond
THEN
   IF OperandSize = 16
   THEN
      IP := IP + SignExtend(rel8);
   ELSE (* OperandSize = 32 *)
      EIP := EIP + SignExtend(rel8);
   FI;
FI;

Description

LOOP decrements the count register without changing any of the flags. Conditions are then checked for the form of LOOP being used. If the conditions are met, a short jump is made to the label given by the operand to LOOP. If the address-size attribute is 16 bits, the CX register is used as the count register; otherwise the ECX register is used. The operand of LOOP must be in the range from 128 (decimal) bytes before the instruction to 127 bytes ahead of the instruction.

The LOOP instructions provide iteration control and combine loop index management with conditional branching. Use the LOOP instruction by loading an unsigned iteration count into the count register, then code the LOOP at the end of a series of instructions to be iterated. The destination of LOOP is a label that points to the beginning of the iteration.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the offset jumped to is beyond the limits of the current code segment

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None

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