shld

Double Precision Shift Left

SHLD r/m, r, imm8

Shifts dest left, filling with bits from src.

Details

Shifts the destination operand left by a count, filling the vacated bits from the most-significant bits of the source operand. Supports 16/32/64-bit operands; count comes from the immediate or implicitly from CL. Sets CF to the last bit shifted out, OF if count=1, SF/ZF/PF based on the result.

Pseudocode Operation

if count > operand_width:
  count ← count mod (operand_width + 1)
if count == 0:
  skip
else:
  CF ← bit_shifted_out_from_left
  dest ← (dest << count) | (src >> (operand_width - count))
  if count == 1:
    OF ← (dest_sign_bit_after != CF)
  else:
    OF ← undefined
  ZF ← (dest == 0)
  SF ← (dest_sign_bit == 1)
  PF ← popcount(dest & 0xFF) is even

Example

SHLD rbx, rax, 3

Encoding

Binary Layout

Format Legacy

Opcode 0F A4

Extension Base

Operands

dest
Reg/Mem
fill
Reg
count
Imm

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
0F A4 /r ib	SHLD r/m16, r16, imm8	MRI	Valid Valid		Shift r/m16 to left imm8 places while shifting bits from r16 in from the right.
0F A5 /r	SHLD r/m16, r16, CL		Valid Valid	MRC	Shift r/m16 to left CL places while shifting bits from r16 in from the right.
0F A4 /r ib	SHLD r/m32, r32, imm8	MRI	Valid Valid		Shift r/m32 to left imm8 places while shifting bits from r32 in from the right.
REX.W + 0F A4 /r ib	SHLD r/m64, r64, imm8	MRI	Valid N.E.		Shift r/m64 to left imm8 places while shifting bits from r64 in from the right.
0F A5 /r	SHLD r/m32, r32, CL		Valid Valid	MRC	Shift r/m32 to left CL places while shifting bits from r32 in from the right.
REX.W + 0F A5 /r	SHLD r/m64, r64, CL		Valid N.E.	MRC	Shift r/m64 to left CL places while shifting bits from r64 in from the right.

Description

The SHLD instruction is used for multi-precision shifts of 64 bits or more. The instruction shifts the first operand (destination operand) to the left the number of bits specified by the third operand (count operand). The second operand (source operand) provides bits to shift in from the right (starting with bit 0 of the destination operand). The destination operand can be a register or a memory location; the source operand is a register. The count operand is an unsigned integer that can be stored in an immediate byte or in the CL register. If the count operand is CL, the shift count is the logical AND of CL and a count mask. In non-64-bit modes and default 64-bit mode; only bits 0 through 4 of the count are used. This masks the count to a value between 0 and 31. If a count is greater than the operand size, the result is undefined. If the count is 1 or greater, the CF flag is filled with the last bit shifted out of the destination operand. For a 1-bit shift, the OF flag is set if a sign change occurred; otherwise, it is cleared. If the count operand is 0, flags are not affected. In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bits (upgrading the count mask to 6 bits). See the summary chart at the beginning of this section for encoding data and limits.

Operation

IF (In 64-Bit Mode and REX.W = 1)
THEN COUNT := COUNT MOD 64;
ELSE COUNT := COUNT MOD 32;
FI
SIZE := OperandSize;
tempDEST := DEST;
IF COUNT > SIZE
THEN (* Bad parameters *)
tempDEST is undefined;


SHLD—Double Precision Shift Left                                                                                                                Vol. 2B 4-639
CF, OF, SF, ZF, AF, PF are undefined;
ELSE IF COUNT > 0 (* Perform the shift *)
CF := BIT[tempDEST, SIZE – COUNT];
(* Last bit shifted out on exit *)
FOR i := SIZE – 1 DOWN TO COUNT
DO
Bit(tempDEST, i) := Bit(tempDEST, i – COUNT);
OD;
FOR i := COUNT – 1 DOWN TO 0
DO
BIT[tempDEST, i] := BIT[SRC, i – COUNT + SIZE];
OD;
FI;
DEST := tempDEST;

Flags Affected

If the count is 1 or greater, the CF flag is filled with the last bit shifted out of the destination operand and the SF, ZF, and PF flags are set according to the value of the result. For a 1-bit shift, the OF flag is set if a sign change occurred; otherwise, it is cleared. For shifts greater than 1 bit, the OF flag is undefined. If a shift occurs, the AF flag is undefined. If the count operand is 0, the flags are not affected. If the count is greater than the operand size, the flags are undefined.

Exceptions

Protected Mode Exceptions

#GP(0) If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit. #UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. SHLD—Double Precision Shift Left Vol. 2B 4-640 #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used. SHLD—Double Precision Shift Left Vol. 2B 4-641