idiv

Signed Divide

IDIV r/m

Signed divide (AX / src).

Details

Performs signed division of the implicit accumulator (EDX:EAX for 32-bit or RDX:RAX for 64-bit) by the specified operand; quotient is stored in EAX/RAX and remainder in EDX/RDX. Sets OF, SF, ZF, AF, CF, and PF to undefined values; raises #DE exception if divisor is zero or quotient overflows. Available in 8-bit (AX/AL:AH), 16-bit (DX:AX), 32-bit, and 64-bit variants.

Pseudocode Operation

if (src == 0) raise #DE;
temp_dividend = (src_size == 8) ? AX : ((src_size == 16) ? (DX:AX) : ((src_size == 32) ? (EDX:EAX) : (RDX:RAX)));
if ((temp_dividend / src) > max_signed || (temp_dividend / src) < min_signed) raise #DE;
quotient = temp_dividend / src;
remainder = temp_dividend % src;
if (src_size == 8) { AL ← quotient; AH ← remainder; }
else if (src_size == 16) { AX ← quotient; DX ← remainder; }
else if (src_size == 32) { EAX ← quotient; EDX ← remainder; }
else { RAX ← quotient; RDX ← remainder; }
OF ← undefined; SF ← undefined; ZF ← undefined; AF ← undefined; CF ← undefined; PF ← undefined;

Example

IDIV rbx

Encoding

Binary Layout

ModRM

Format Legacy

Opcode F7 /7

Extension Base

Operands

dest
Register or memory operand

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
F6 /7	IDIV r/m8	M	Valid Valid	Signed divide AX by r/m8, with result stored in: AL := Quotient, AH := Remainder.
F7 /7	IDIV r/m16	M	Valid Valid	Signed divide DX:AX by r/m16, with result stored in AX := Quotient, DX := Remainder.
F7 /7	IDIV r/m32	M	Valid Valid	Signed divide EDX:EAX by r/m32, with result stored in EAX := Quotient, EDX := Remainder.
REX.W + F7 /7	IDIV r/m64	M	Valid N.E.	Signed divide RDX:RAX by r/m64, with result stored in RAX := Quotient, RDX := Remainder.

Description

Divides the (signed) value in the AX, DX:AX, or EDX:EAX (dividend) by the source operand (divisor) and stores the result in the AX (AH:AL), DX:AX, or EDX:EAX registers. The source operand can be a general-purpose register or a memory location. The action of this instruction depends on the operand size (dividend/divisor). Non-integral results are truncated (chopped) towards 0. The remainder is always less than the divisor in magnitude. Overflow is indicated with the #DE (divide error) exception rather than with the CF flag. In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits. In 64-bit mode when REX.W is applied, the instruction divides the signed value in RDX:RAX by the source operand. RAX contains a 64-bit quotient; RDX contains a 64-bit remainder. See the summary chart at the beginning of this section for encoding data and limits. See Table 3-53. Table 3-53. IDIV Results Operand Size Dividend Divisor Quotient Remainder Quotient Range Word/byte AX r/m8 AL AH −128 to +127 Doubleword/word DX:AX r/m16 AX DX −32,768 to +32,767 Quadword/doubleword EDX:EAX r/m32 EAX EDX −231 to 231 − 1 Doublequadword/ quadword RDX:RAX r/m64 RAX RDX −263 to 263 − 1 IDIV—Signed Divide Vol. 2A 3-448

Operation

IF SRC = 0
THEN #DE; (* Divide error *)
FI;
IF OperandSize = 8 (* Word/byte operation *)
THEN
temp := AX / SRC; (* Signed division *)
IF (temp > 7FH) or (temp < 80H)
(* If a positive result is greater than 7FH or a negative result is less than 80H *)
THEN #DE; (* Divide error *)
ELSE
AL := temp;
AH := AX SignedModulus SRC;
FI;
ELSE IF OperandSize = 16 (* Doubleword/word operation *)
THEN
temp := DX:AX / SRC; (* Signed division *)
IF (temp > 7FFFH) or (temp < 8000H)
(* If a positive result is greater than 7FFFH
or a negative result is less than 8000H *)
THEN
#DE; (* Divide error *)
ELSE
AX := temp;
DX := DX:AX SignedModulus SRC;
FI;
FI;
ELSE IF OperandSize = 32 (* Quadword/doubleword operation *)
temp := EDX:EAX / SRC; (* Signed division *)
IF (temp > 7FFFFFFFH) or (temp < 80000000H)
(* If a positive result is greater than 7FFFFFFFH
or a negative result is less than 80000000H *)
THEN
#DE; (* Divide error *)
ELSE
EAX := temp;
EDX := EDXE:AX SignedModulus SRC;
FI;
FI;
ELSE IF OperandSize = 64 (* Doublequadword/quadword operation *)
temp := RDX:RAX / SRC; (* Signed division *)
IF (temp > 7FFFFFFFFFFFFFFFH) or (temp < 8000000000000000H)
(* If a positive result is greater than 7FFFFFFFFFFFFFFFH
or a negative result is less than 8000000000000000H *)
THEN
#DE; (* Divide error *)
ELSE
RAX := temp;
RDX := RDE:RAX SignedModulus SRC;
FI;
FI;
FI;




IDIV—Signed Divide                                                                                                                               Vol. 2A 3-449

Flags Affected

The CF, OF, SF, ZF, AF, and PF flags are undefined.

Exceptions

Protected Mode Exceptions

#DE If the source operand (divisor) is 0. The signed result (quotient) is too large for the destination. #GP(0) 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 is used to access memory and it 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

#DE If the source operand (divisor) is 0. The signed result (quotient) is too large for the destination. #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

#DE If the source operand (divisor) is 0. The signed result (quotient) is too large for the destination. #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. #DE If the source operand (divisor) is 0 If the quotient is too large for the designated register. #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. IDIV—Signed Divide Vol. 2A 3-450