# RDRAND — Read Random Number

Opcode*/Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
NFx 0F C7 /6 RDRAND r16 | M | V/V | RDRAND | Read a 16-bit random number and store in the destination register.
NFx 0F C7 /6 RDRAND r32 | M | V/V | RDRAND | Read a 32-bit random number and store in the destination register.
NFx REX.W + 0F C7 /6 RDRAND r64 | M | V/I | RDRAND | Read a 64-bit random number and store in the destination register.

## Instruction Operand Encoding ¶

Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | ---
M | ModRM:r/m (w) | N/A | N/A | N/A

## Description ¶

Loads a hardware generated random value and store it in the destination register. The size of the random value is determined by the destination register size and operating mode. The Carry Flag indicates whether a random value is available at the time the instruction is executed. CF=1 indicates that the data in the destination is valid. Otherwise CF=0 and the data in the destination operand will be returned as zeros for the specified width. All other flags are forced to 0 in either situation. Software must check the state of CF=1 for determining if a valid random value has been returned, otherwise it is expected to loop and retry execution of RDRAND (see Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, Section 7.3.17, “Random Number Generator Instructions”).

This instruction is available at all privilege levels.

In 64-bit mode, the instruction's default operand size is 32 bits. Using a REX prefix in the form of REX.B permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bit operands. See the summary chart at the beginning of this section for encoding data and limits.

## Operation ¶

    IF HW_RND_GEN.ready = 1
        THEN
            CASE of
                operand size is 64: DEST[63:0] := HW_RND_GEN.data;
                operand size is 32: DEST[31:0] := HW_RND_GEN.data;
                operand size is 16: DEST[15:0] := HW_RND_GEN.data;
            ESAC
            CF := 1;
        ELSE
            CASE of
                operand size is 64: DEST[63:0] := 0;
                operand size is 32: DEST[31:0] := 0;
                operand size is 16: DEST[15:0] := 0;
            ESAC
            CF := 0;
    FI
    OF, SF, ZF, AF, PF := 0;

## Flags Affected ¶

The CF flag is set according to the result (see the “Operation” section above). The OF, SF, ZF, AF, and PF flags are set to 0.

## Intel C/C++ Compiler Intrinsic Equivalent ¶

    RDRAND int _rdrand16_step( unsigned short * );

    RDRAND int _rdrand32_step( unsigned int * );

    RDRAND int _rdrand64_step( unsigned __int64 *);

## Protected Mode Exceptions ¶

#UD | If the LOCK prefix is used.
If CPUID.01H:ECX.RDRAND[bit 30] = 0.

## Real-Address Mode Exceptions ¶

Same exceptions as in protected mode.

## Virtual-8086 Mode Exceptions ¶

Same exceptions as in protected mode.

## Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

## 64-Bit Mode Exceptions ¶

Same exceptions as in protected mode.