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The CPUID instruction (identified by a CPUID opcode) is a processor supplementary instruction (its name derived from CPU IDentification) for the x86 architecture allowing software to discover details of the processor. It was introduced by Intel in 1993 when it introduced the Pentium and SL-enhanced 486 processors.[1]

A program can use the CPUID to determine processor type and whether features such as MMX/SSE are implemented.



Prior to the general availability of the CPUID instruction, programmers would write esoteric machine code which exploited minor differences in CPU behavior in order to determine the processor make and model.[2][3]

Outside the x86 family, developers are mostly still required to use esoteric processes (involving instruction timing or CPU fault triggers) to determine the variations in CPU design that are present.

In the Motorola 680x0 family -- that never had a CPUID instruction of any kind -- certain specific instructions required elevated privileges. These could be used to tell various CPU family members apart.

Because the 68000 offered an unprivileged MOVE from SR the 2 different CPUs could be told apart by a CPU error condition being triggered.

While the CPUID instruction is specific to the x86 architecture, other architectures (like ARM) often provide on-chip registers which can be read in prescribed ways to obtain the same sorts of information provided by the x86 CPUID instruction.

Calling CPUIDEdit

The CPUID opcode is 0Fh, A2h (as two bytes, or A20Fh as a single word).

In assembly language, the CPUID instruction takes no parameters as CPUID implicitly uses the EAX register to determine the main category of information returned. In Intel's more recent terminology, this is called the CPUID leaf. CPUID should be called with EAX = 0 first, as this will store in the EAX register the highest EAX calling parameter (leaf) that the CPU implements.

To obtain extended function information CPUID should be called with the most significant bit of EAX set. To determine the highest extended function calling parameter, call CPUID with EAX = 80000000h.

CPUID leaves greater than 3 but less than 80000000 are accessible only when the model-specific registers have IA32_MISC_DisablE.BOOT_NT4 [bit 22] = 0 (which is so by default). As the name suggests, Windows NT4 did not boot properly unless this bit was set,[4] but later versions of Windows do not need it, so basic leaves greater than 4 can be assumed visible on current Windows systems. As of July 2014, basic valid leaves go up to 14h, but the information returned by some leaves are not disclosed in publicly available documentation, i.e. they are "reserved".

Some of the more recently added leaves also have sub-leaves, which are selected via the ECX register before calling CPUID.

EAX=0: Highest Function Parameter and Manufacturer IDEdit

This returns the CPU's manufacturer ID string – a twelve-character ASCII string stored in EBX, EDX, ECX (in that order). The highest basic calling parameter (largest value that EAX can be set to before calling CPUID) is returned in EAX.

Here is a list of processors and the highest function implemented.

Highest Function Parameter
Processors Basic Extended
Earlier Intel 486 CPUID Not Implemented
Later Intel 486 and Pentium 0x01 Not Implemented
Pentium Pro, Pentium II and Celeron 0x02 Not Implemented
Pentium III 0x03 Not Implemented
Pentium 4 0x02 0x8000 0004
Xeon 0x02 0x8000 0004
Pentium M 0x02 0x8000 0004
Pentium 4 with Hyper-Threading 0x05 0x8000 0008
Pentium D (8xx) 0x05 0x8000 0008
Pentium D (9xx) 0x06 0x8000 0008
Core Duo 0x0A 0x8000 0008
Core 2 Duo 0x0A 0x8000 0008
Xeon 3000, 5100, 5200, 5300, 5400 series 0x0A 0x8000 0008
Core 2 Duo 8000 series 0x0D 0x8000 0008
Xeon 5200, 5400 series 0x0A 0x8000 0008
Atom 0x0A 0x8000 0008
Nehalem-based processors 0x0B 0x8000 0008
IvyBridge-based processors 0x0D 0x8000 0008
Skylake-based processors (proc base & max freq; Bus ref. freq) 0x16 0x8000 0008
System-On-Chip Vendor Attribute Enumeration Main Leaf 0x17 0x8000 0008

The following are known processor manufacturer ID strings:

The following are known ID strings from virtual machines:

For instance, on a GenuineIntel processor values returned in EBX is 0x756e6547, EDX is 0x49656e69 and ECX is 0x6c65746e. The following code is written in GNU Assembler for the x86-64 architecture and displays the vendor ID string as well as the highest calling parameter that the CPU implements.


s0:	.asciz	"CPUID: %x\n"
s1:	.asciz	"Largest basic function number implemented: %i\n"
s2:	.asciz	"Vendor ID: %.12s\n"


	.align	32
	.globl	main

	pushq	%rbp
	movq	%rsp,%rbp
	subq	$16,%rsp

	movl	$1,%eax

	movq	$s0,%rdi
	movl	%eax,%esi
	xorl	%eax,%eax
	call	printf

	xorl	%eax,%eax

	movl	%ebx,0(%rsp)
	movl	%edx,4(%rsp)
	movl	%ecx,8(%rsp)

	movq	$s1,%rdi
	movl	%eax,%esi
	xorl	%eax,%eax
	call	printf

	movq	$s2,%rdi
	movq	%rsp,%rsi
	xorl	%eax,%eax
	call	printf

	movq	%rbp,%rsp
	popq	%rbp
//	ret
	movl	$1,%eax
	int	$0x80

EAX=1: Processor Info and Feature BitsEdit

This returns the CPU's stepping, model, and family information in register EAX (also called the signature of a CPU), feature flags in registers EDX and ECX, and additional feature info in register EBX.[5]

Processor Version Information
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Reserved Extended Family ID Extended Model ID Reserved Processor Type Family ID Model Stepping ID
  • Stepping ID is a product revision number assigned due to fixed errata or other changes.
  • The actual processor model is derived from the Model, Extended Model ID and Family ID fields. If the Family ID field is either 6 or 15, the model is equal to the sum of the Extended Model ID field shifted left by 4 bits and the Model field. Otherwise, the model is equal to the value of the Model field.
  • The actual processor family is derived from the Family ID and Extended Family ID fields. If the Family ID field is equal to 15, the family is equal to the sum of the Extended Family ID and the Family ID fields. Otherwise, the family is equal to value of the Family ID field.
  • The meaning of the Processor Type field is given by the table below.
Processor Type
Type Encoding in Binary
Original OEM Processor 00
Intel Overdrive Processor 01
Dual processor (not applicable to Intel486 processors) 10
Reserved value 11
Additional Information
Bits EBX Valid
7:0 Brand Index
15:8 CLFLUSH line size (Value . 8 = cache line size in bytes) if CLFLUSH feature flag is set.

CPUID.01.EDX.CLFSH [bit 19]= 1

23:16 Maximum number of addressable IDs for logical processors in this physical package;

The nearest power-of-2 integer that is not smaller than this value is the number of unique initial APIC IDs reserved for addressing different logical processors in a physical package.

Former use: Number of logical processors per physical processor; two for the Pentium 4 processor with Hyper-Threading Technology.[6]

if Hyper-threading feature flag is set.

CPUID.01.EDX.HTT [bit 28]= 1

31:24 Local APIC ID: The initial APIC-ID is used to identify the executing logical processor.

It can also be identified via the cpuid 0BH leaf ( CPUID.0Bh.EDX[x2APIC-ID] ).

Pentium 4 and subsequent processors.

The processor info and feature flags are manufacturer specific but usually the Intel values are used by other manufacturers for the sake of compatibility.

Feature Information
Short Feature Short Feature
0 fpu Onboard x87 FPU sse3 Prescott New Instructions-SSE3 (PNI)
1 vme Virtual 8086 mode extensions (such as VIF, VIP, PIV) pclmulqdq PCLMULQDQ
2 de Debugging extensions (CR4 bit 3) dtes64 64-bit debug store (edx bit 21)
3 pse Page Size Extension monitor MONITOR and MWAIT instructions (SSE3)
4 tsc Time Stamp Counter ds-cpl CPL qualified debug store
5 msr Model-specific registers vmx Virtual Machine eXtensions
6 pae Physical Address Extension smx Safer Mode Extensions (LaGrande)
7 mce Machine Check Exception est Enhanced SpeedStep
8 cx8 CMPXCHG8 (compare-and-swap) instruction tm2 Thermal Monitor 2
9 apic Onboard Advanced Programmable Interrupt Controller ssse3 Supplemental SSE3 instructions
10 (reserved) cnxt-id L1 Context ID
11 sep SYSENTER and SYSEXIT instructions sdbg Silicon Debug interface
12 mtrr Memory Type Range Registers fma Fused multiply-add (FMA3)
13 pge Page Global Enable bit in CR4 cx16 CMPXCHG16B instruction
14 mca Machine check architecture xtpr Can disable sending task priority messages
15 cmov Conditional move and FCMOV instructions pdcm Perfmon & debug capability
16 pat Page Attribute Table (reserved)
17 pse-36 36-bit page size extension pcid Process context identifiers (CR4 bit 17)
18 psn Processor Serial Number dca Direct cache access for DMA writes[7][8]
19 clfsh CLFLUSH instruction (SSE2) sse4.1 SSE4.1 instructions
20 (reserved) sse4.2 SSE4.2 instructions
21 ds Debug store: save trace of executed jumps x2apic x2APIC
22 acpi Onboard thermal control MSRs for ACPI movbe MOVBE instruction (big-endian)
23 mmx MMX instructions popcnt POPCNT instruction
24 fxsr FXSAVE, FXRESTOR instructions, CR4 bit 9 tsc-deadline APIC implements one-shot operation using a TSC deadline value
25 sse SSE instructions (a.k.a. Katmai New Instructions) aes AES instruction set
26 sse2 SSE2 instructions xsave XSAVE, XRESTOR, XSETBV, XGETBV
27 ss CPU cache implements self-snoop osxsave XSAVE enabled by OS
28 htt Hyper-threading avx Advanced Vector Extensions
29 tm Thermal monitor automatically limits temperature f16c F16C (half-precision) FP feature
30 ia64 IA64 processor emulating x86 rdrnd RDRAND (on-chip random number generator) feature
31 pbe Pending Break Enable (PBE# pin) wakeup capability hypervisor Hypervisor present (always zero on physical CPUs)[9][10]

Reserved fields should be masked before using them for processor identification purposes.

EAX=2: Cache and TLB Descriptor informationEdit

This returns a list of descriptors indicating cache and TLB capabilities in EAX, EBX, ECX and EDX registers.

EAX=3: Processor Serial NumberEdit

This returns the processor's serial number. The processor serial number was introduced on Intel Pentium III, but due to privacy concerns, this feature is no longer implemented on later models (the PSN feature bit is always cleared). Transmeta's Efficeon and Crusoe processors also provide this feature. AMD CPUs however, do not implement this feature in any CPU models.

For Intel Pentium III CPUs, the serial number is returned in the EDX:ECX registers. For Transmeta Efficeon CPUs, it is returned in the EBX:EAX registers. And for Transmeta Crusoe CPUs, it is returned in the EBX register only.

Note that the processor serial number feature must be enabled in the BIOS setting in order to function.

EAX=4 and EAX=Bh: Intel thread/core and cache topologyEdit

These two leaves are used for processor topology (thread, core, package) and cache hierarchy enumeration in Intel multi-core (and hyperthreaded) processors.[11] As of 2013 AMD does not use these leaves but has alternate ways of doing the core enumeration.[12]

Unlike most other CPUID leaves, leaf Bh will return different values in EDX depending on which logical processor the CPUID instruction runs; the value returned in EDX is actually the x2APIC id of the logical processor. The x2APIC id space is not continuously mapped to logical processors, however; there can be gaps in the mapping, meaning that some intermediate x2APIC ids don't necessarily correspond to any logical processor. Additional information for mapping the x2APIC ids to cores is provided in the other registers. Although the leaf Bh has sub-leaves (selected by ECX as described further below), the value returned in EDX is only affected by the logical processor on which the instruction is running but not by the subleaf.

The processor(s) topology exposed by leaf Bh is a hierarchical one, but with the strange caveat that the order of (logical) levels in this hierarchy doesn't necessarily correspond the order in the physical hierarchy (SMT/core/package). However, every logical level can be queried as an ECX subleaf (of the Bh leaf) for its correspondence to a "level type", which can be either SMT, core, or "invalid". The level id space starts at 0 and is continuous, meaning that if a level id is invalid, all higher level ids will also be invalid. The level type is returned in bits 15:08 of ECX, while the number of logical processors at the level queried is returned in EBX. Finally, the connection between these levels and x2APIC ids is returned in EAX[4:0] as the number of bits that the x2APIC id must be shifted in order to obtain a unique id at the next level.

As an example, a dual-core Westmere processor capable of hyperthreading (thus having two cores and four threads in total) could have x2APIC ids 0, 1, 4 and 5 for its four logical processors. Leaf Bh (=EAX), subleaf 0 (=ECX) of CPUID could for instance return 100h in ECX, meaning that level 0 describes the SMT (hyperthreading) layer, and return 2 in EBX because there are two logical processors (SMT units) per physical core. The value returned in EAX for this 0-subleaf should be 1 in this case, because shifting the aforementioned x2APIC ids to the right by one bit gives a unique core number (at the next level of the level id hierarchy) and erases the SMT id bit inside each core. A simpler way to interpret this information is that the last bit (bit number 0) of the x2APIC id identifies the SMT/hyperthreading unit inside each core in our example. Advancing to subleaf 1 (by making another call to CPUID with EAX=Bh and ECX=1) could for instance return 201h in ECX, meaning that this is a core-type level, and 4 in EBX because there are 4 logical processors in the package; EAX returned could be any value greater than 3, because it so happens that bit number 2 is used to identify the core in the x2APIC id. Note that bit number 1 of the x2APIC id is not used in this example. However EAX returned at this level could well be 4 (and it happens to be so on a Clarkdale Core i3 5x0) because that also gives a unique id at the package level (=0 obviusly) when shifting the x2APIC id by 4 bits. Finally, you may wonder what the EAX=4 leaf can tell us that we didn't find out already. In EAX[31:26] it returns the APIC mask bits reserved for a package; that would be 111b in our example because bits 0 to 2 are used for identifying logical processors inside this package, but bit 1 is also reserved although not used as part of the logical processor identification scheme. In other words, APIC ids 0 to 7 are reserved for the package, even though half of these values don't map to a logical processor.

The cache hierarchy of the processor is explored by looking at the sub-leaves of leaf 4. The APIC ids are also used in this hierarchy to convey information about how the different levels of cache are shared by the SMT units and cores. To continue our example, the L2 cache, which is shared by SMT units of the same core but not between physical cores on the Westmere is indicated by EAX[26:14] being set to 1, while the information that the L3 cache is shared by the whole package is indicated by setting those bits to (at least) 111b. The cache details, including cache type, size, and associativity are communicated via the other registers on leaf 4.

Beware that older versions of the Intel app note 485 contain some misleading information, particularly with respect to identifying and counting cores in a multi-core processor;[13] errors from misinterpreting this information have even been incorporated in the Microsoft sample code for using cpuid, even for the 2013 edition of Visual Studio,[14] and also in the page for CPUID,[15] but the Intel code sample for identifying processor topology[11] has the correct interpretation, and the current Intel Software Developer’s Manual has more clear language. The (open source) cross-platform production code[16] from Wildfire Games also implements the correct interpretation of the Intel documentation.

Topology detection examples involving older (pre-2010) Intel processors that lack x2APIC (thus don't implement the EAX=Bh leaf) are given in a 2010 Intel presentation.[17] Beware that using that older detection method on 2010 and newer Intel processors may overestimate the number of cores and logical processors because the old detection method assumes there are no gaps in the APIC id space, and this assumption is violated by some newer processors (starting with the Core i3 5x0 series), but these newer processors also come with an x2APIC, so their topology can be correctly determined using the EAX=Bh leaf method.

EAX=7, ECX=0: Extended FeaturesEdit

This returns extended feature flags in EBX, ECX, and EDX.

EAX=7 CPUID feature bits
Short Feature Short Feature
0 fsgsbase Access to base of %fs and %gs prefetchwt1 PREFETCHWT1 instruction
1 IA32_TSC_ADJUST avx512vbmi AVX-512 Vector Bit Manipulation Instructions
2 sgx Software Guard Extensions umip User-mode Instruction Prevention
3 bmi1 Bit Manipulation Instruction Set 1 pku Memory Protection Keys for User-mode pages
4 hle Transactional Synchronization Extensions ospke PKU enabled by OS
5 avx2 Advanced Vector Extensions 2 (reserved)
6 (reserved) avx512vbmi2 AVX-512 Vector Bit Manipulation Instructions 2
7 smep Supervisor Mode Execution Prevention (reserved)
8 bmi2 Bit Manipulation Instruction Set 2 gfni Galois Field instructions
9 erms Enhanced REP MOVSB/STOSB vaes Vector AES instruction set (VEX-256/EVEX)
10 invpcid INVPCID instruction vpclmulqdq CLMUL instruction set (VEX-256/EVEX)
11 rtm Transactional Synchronization Extensions avx512vnni AVX-512 Vector Neural Network Instructions
12 pqm Platform Quality of Service Monitoring avx512bitalg AVX-512 BITALG instructions
13 FPU CS and FPU DS deprecated (reserved)
14 mpx Intel MPX (Memory Protection Extensions) avx512vpopcntdq AVX-512 Vector Population Count Double and Quad-word
15 pqe Platform Quality of Service Enforcement (reserved)
16 avx512f AVX-512 Foundation (reserved)
17 avx512dq AVX-512 Doubleword and Quadword Instructions mawau The value of userspace MPX Address-Width Adjust used by the

BNDLDX and BNDSTX Intel MPX instructions in 64-bit mode

18 rdseed RDSEED instruction
19 adx Intel ADX (Multi-Precision Add-Carry Instruction Extensions)
20 smap Supervisor Mode Access Prevention
21 avx512ifma AVX-512 Integer Fused Multiply-Add Instructions
22 pcommit PCOMMIT instruction rdpid Read Processor ID
23 clflushopt CLFLUSHOPT instruction (reserved)
24 clwb CLWB instruction (reserved)
25 intel_pt Intel Processor Trace (reserved)
26 avx512pf AVX-512 Prefetch Instructions (reserved)
27 avx512er AVX-512 Exponential and Reciprocal Instructions (reserved)
28 avx512cd AVX-512 Conflict Detection Instructions (reserved)
29 sha Intel SHA extensions (reserved)
30 avx512bw AVX-512 Byte and Word Instructions sgx_lc SGX Launch Configuration
31 avx512vl AVX-512 Vector Length Extensions (reserved)
Short Feature
0 (reserved)
1 (reserved)
2 avx512_4vnniw AVX-512 4-register Neural Network Instructions
3 avx512_4fmaps AVX-512 4-register Multiply Accumulation Single precision
4 (reserved)
5 (reserved)
6 (reserved)
7 (reserved)
8 (reserved)
9 (reserved)
10 (reserved)
11 (reserved)
12 (reserved)
13 (reserved)
14 (reserved)
15 (reserved)
16 (reserved)
17 (reserved)
18 pconfig Platform configuration (Memory Encryption Technologies Instructions)
19 (reserved)
20 (reserved)
21 (reserved)
22 (reserved)
23 (reserved)
24 (reserved)
25 (reserved)
26 spec_ctrl, part of Indirect Branch Control (IBC) Speculation Control:
Indirect Branch Restricted Speculation (IBRS) and
Indirect Branch Prediction Barrier (IBPB)[18][19]
27 part of Indirect Branch Control (IBC) Single Thread Indirect Branch Predictor (STIBP)[18]
28 (reserved)


30 (reserved)
31 Speculative Store Bypass Disable (SSBD),[18] as mitigation for Speculative Store Bypass

EAX=80000000h: Get Highest Extended Function ImplementedEdit

The highest calling parameter is returned in EAX.

EAX=80000001h: Extended Processor Info and Feature BitsEdit

This returns extended feature flags in EDX and ECX.

AMD feature flags are as follows:[20][21]

EAX=80000001h CPUID feature bits
Short Feature Short Feature
0 fpu Onboard x87 FPU lahf_lm LAHF/SAHF in long mode
1 vme Virtual mode extensions (VIF) cmp_legacy Hyperthreading not valid
2 de Debugging extensions (CR4 bit 3) svm Secure Virtual Machine
3 pse Page Size Extension extapic Extended APIC space
4 tsc Time Stamp Counter cr8_legacy CR8 in 32-bit mode
5 msr Model-specific registers abm Advanced bit manipulation (lzcnt and popcnt)
6 pae Physical Address Extension sse4a SSE4a
7 mce Machine Check Exception misalignsse Misaligned SSE mode
8 cx8 CMPXCHG8 (compare-and-swap) instruction 3dnowprefetch PREFETCH and PREFETCHW instructions
9 apic Onboard Advanced Programmable Interrupt Controller osvw OS Visible Workaround
10 (reserved) ibs Instruction Based Sampling
11 syscall SYSCALL and SYSRET instructions xop XOP instruction set
12 mtrr Memory Type Range Registers skinit SKINIT/STGI instructions
13 pge Page Global Enable bit in CR4 wdt Watchdog timer
14 mca Machine check architecture (reserved)
15 cmov Conditional move and FCMOV instructions lwp Light Weight Profiling[22]
16 pat Page Attribute Table fma4 4 operands fused multiply-add
17 pse36 36-bit page size extension tce Translation Cache Extension
18 (reserved)
19 mp Multiprocessor Capable nodeid_msr NodeID MSR
20 nx NX bit (reserved)
21 (reserved) tbm Trailing Bit Manipulation
22 mmxext Extended MMX topoext Topology Extensions
23 mmx MMX instructions perfctr_core Core performance counter extensions
24 fxsr FXSAVE, FXRSTOR instructions, CR4 bit 9 perfctr_nb NB performance counter extensions
25 fxsr_opt FXSAVE/FXRSTOR optimizations (reserved)
26 pdpe1gb Gibibyte pages dbx Data breakpoint extensions
27 rdtscp RDTSCP instruction perftsc Performance TSC
28 (reserved) pcx_l2i L2I perf counter extensions
29 lm Long mode (reserved)
30 3dnowext Extended 3DNow! (reserved)
31 3dnow 3DNow! (reserved)

EAX=80000002h,80000003h,80000004h: Processor Brand StringEdit

These return the processor brand string in EAX, EBX, ECX and EDX. CPUID must be issued with each parameter in sequence to get the entire 48-byte null-terminated ASCII processor brand string.[23] It is necessary to check whether the feature is present in the CPU by issuing CPUID with EAX = 80000000h first and checking if the returned value is greater or equal to 80000004h.

.section .data

s0 : .asciz "Processor Brand String: %.48s\n"
err : .asciz "Feature not implemented.\n"

.section .text

.global main
.type main,@function
.align 32
	pushq	%rbp
	movq	%rsp,	%rbp
	subq	$48,	%rsp
	pushq	%rbx

	movl	$0x80000000,	%eax

	cmpl	$0x80000004,	%eax
	jl	error

	movl	$0x80000002,	%esi
	movq	%rsp,	%rdi

.align 16
	movl	%esi,	%eax

	movl	%eax,	(%rdi)
	movl	%ebx,	4(%rdi)
	movl	%ecx,	8(%rdi)
	movl	%edx,	12(%rdi)

	addl	$1,	%esi
	addq	$16,	%rdi
	cmpl	$0x80000004,	%esi
	jle	get_brand

	movq	$s0,	%rdi
	movq	%rsp,	%rsi
	xorb	%al,	%al
	call	printf

	jmp	end

.align 16
	movq	$err,	%rdi
	xorb	%al,	%al
	call	printf

.align 16
	popq	%rbx
	movq	%rbp,	%rsp
	popq	%rbp
	xorl	%eax,	%eax

EAX=80000005h: L1 Cache and TLB IdentifiersEdit

This function contains the processor’s L1 cache and TLB characteristics.

EAX=80000006h: Extended L2 Cache FeaturesEdit

Returns details of the L2 cache in ECX, including the line size in bytes, type of associativity (encoded by a 4 bits) and the cache size.

.section .data

info : .ascii "L2 Cache Size : %u KB\nLine size : %u bytes\n"
.asciz "Associativity : %02xh\n"
err : .asciz "Feature not implemented.\n"

.section .text

.global main
.type main,@function
.align 32
	pushq	%rbp
	movq	%rsp,	%rbp
	pushq	%rbx

	movl	$0x80000000,	%eax

	cmpl	$0x80000006,	%eax
	jl	error

	movl	$0x80000006,	%eax

	movl	%ecx,	%eax

	movl	%eax,	%edx
	andl	$0xff,	%edx

	movl	%eax,	%ecx
	shrl	$12,	%ecx
	andl	$0xf,	%ecx

	movl	%eax,	%esi
	shrl	$16,	%esi
	andl	$0xffff,%esi

	movq	$info,	%rdi
	xorb	%al,	%al
	call	printf

	jmp end

.align 16
	movq	$err,	%rdi
	xorb	%al,	%al
	call	printf

.align 16
	popq	%rbx
	movq	%rbp,	%rsp
	popq	%rbp
	xorl	%eax,	%eax

EAX=80000007h: Advanced Power Management InformationEdit

This function provides advanced power management feature identifiers.

EAX=80000008h: Virtual and Physical address SizesEdit

Returns largest virtual and physical address sizes in EAX. Bits 07-00: #Physical Address Bits. Bits 15-8: #Linear Address Bits. Bits 31-16: Reserved = 0. It could be used by the hypervisor in a virtual machine system to report physical/virtual address sizes possible with the virtual CPU.

EAX=8FFFFFFFh: AMD Easter EggEdit

Specific to AMD K7 and K8 CPUs, this returns the string "IT'S HAMMER TIME" in EAX, EBX, ECX and EDX.[24]

CPUID usage from high-level languagesEdit

This information is easy to access from other languages as well. For instance, the C code for gcc below prints the first five values, returned by the cpuid:

#include <stdio.h>

int main()
  int a, b;

  for (a = 0; a < 5; a++)
            :"=a"(b)                 // EAX into b (output)
            :"0"(a)                  // a into EAX (input)
            :"%ebx","%ecx","%edx");  // clobbered registers

    printf("The code %i gives %i\n", a, b);

  return 0;

Or, a generally useful C implementation that works on 32- and 64-bit systems:

#include <stdio.h>

int main() {
    int i;
    unsigned int index = 0;
    unsigned int regs[4];
    int sum;
    __asm__ __volatile__(
#if defined(__x86_64__) || defined(_M_AMD64) || defined (_M_X64)
        "pushq %%rbx     \n\t" /* save %rbx */
        "pushl %%ebx     \n\t" /* save %ebx */
        "cpuid            \n\t"
        "movl %%ebx ,%[ebx]  \n\t" /* write the result into output var */
#if defined(__x86_64__) || defined(_M_AMD64) || defined (_M_X64)
        "popq %%rbx \n\t"
        "popl %%ebx \n\t"
        : "=a"(regs[0]), [ebx] "=r"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
        : "a"(index));
    for (i=4; i<8; i++) {
        printf("%c" ,((char *)regs)[i]);
    for (i=12; i<16; i++) {
        printf("%c" ,((char *)regs)[i]);
    for (i=8; i<12; i++) {
        printf("%c" ,((char *)regs)[i]);

GCC also provides a header called <cpuid.h> on systems that have CPUID. The __cpuid is a macro expanding to inline assembly. Typical usage would be:

#include <cpuid.h>
#include <stdio.h>

main (void)
  int a, b, c, d;
  __cpuid (0 /* vendor string */, a, b, c, d);
  printf ("EAX: %x\nEBX: %x\nECX: %x\nEDX: %x\n", a, b, c, d);
  return 0;

But if one requested an extended feature not present on this CPU, they would not notice and might get random, unexpected results. Safer version is also provided in <cpuid.h>. It checks for extended features and does some more safety checks. The output values are not passed using reference-like macro parameters, but more conventional pointers.

#include <cpuid.h>
#include <stdio.h>

main (void)
  int a, b, c, d;
  if (!__get_cpuid (0x81234567 /* nonexistent, but assume it exists */, &a, &b, &c, &d))
      fprintf (stderr, "Warning: CPUID request 0x81234567 not valid!\n");
  printf ("EAX: %x\nEBX: %x\nECX: %x\nEDX: %x\n", a, b, c, d);
  return 0;

Notice the ampersands in &a, &b, &c, &d and the conditional statement. If the __get_cpuid call receives a correct request, it will return a non-zero value, if it fails, zero.[25]

Microsoft Visual C compiler has builtin function __cpuid() so the cpuid instruction may be embedded without using inline assembly, which is handy since the x86-64 version of MSVC does not allow inline assembly at all. The same program for MSVC would be:

#include <iostream>
#include <intrin.h>

int main()
  int b[4];

  for (int a = 0; a < 5; a++)
    __cpuid(b, a);
    std::cout << "The code " << a << " gives " << b[0] << ", " << b[1] << ", " << b[2] << ", " << b[3] << '\n';

  return 0;

For Borland/Embarcadero C compilers (bcc32), native asm function calls are necessary, as there is no asm() implementation. The pseudo code:

  unsigned int a, b, c, d;
  unsigned int InfoType = 0;
  __asm xor EBX, EBX;
  __asm xor ECX, ECX;
  __asm xor EDX, EDX;
  __asm mov EAX, InfoType;
  __asm cpuid;
  __asm mov a, EAX;
  __asm mov b, EBX;
  __asm mov c, ECX;
  __asm mov d, EDX;

Many interpreted or compiled scripting languages are capable of using CPUID via an FFI library. One such implementation shows usage of the Ruby FFI module to execute assembly language that includes the CPUID opcode.

CPU-specific information outside x86Edit

Some of the non-x86 CPU architectures also provide certain forms of structured information about the processor's abilities, commonly as a set of special registers:

  • ARM architectures have a CPUID coprocessor register which requires EL1 or above to access.[26]
  • The IBM System z mainframe processors have a Store CPU ID (STIDP) instruction since the 1983 IBM 4381[27] for querying the processor ID.[28]
  • The MIPS32/64 architecture defines a mandatory Processor Identification (PrId) and a series of daisy-chained Configuration Registers.[29]
  • The PowerPC processor has the 32-bit read-only Processor Version Register (PVR) identifying the processor model in use. The instruction requires supervisor access level.[30]

DSP and transputer-like chip families have not taken up the instruction in any noticeable way, in spite of having (in relative terms) as many variations in design. Alternate ways of silicon identification might be present; for example, DSPs from Texas Instruments contain a memory-based register set for each functional unit that starts with identifiers determining the unit type and model, its ASIC design revision and features selected at the design phase, and continues with unit-specific control and data registers. Access to these areas is performed by simply using the existing load and store instructions; thus, for such devices there is no need for extending the register set for the device identification purposes.[citation needed]

See alsoEdit


  1. ^ "Intel 64 and IA-32 Architectures Software Developer's Manual" (PDF). Retrieved 2013-04-11.
  2. ^ "Detecting Intel Processors - Knowing the generation of a system CPU". Retrieved 2013-04-11.
  3. ^ "LXR linux-old/arch/i386/kernel/head.S". Archived from the original on 2012-07-13. Retrieved 2013-04-11.
  4. ^ "CPUID, EAX=4 - Strange results (Solved)". Retrieved 2014-07-10.
  5. ^ "Chapter 3 Instruction Set Reference, A-L" (PDF). Intel® 64 and IA-32 Architectures Software Developer's Manual. Intel Corporation. 2018-12-20. Retrieved 2018-12-20.
  6. ^
  7. ^ Huggahalli, Ram; Iyer, Ravi; Tetrick, Scott (2005). "Direct Cache Access for High Bandwidth Network I/O". ACM SIGARCH Computer Architecture News. 33 (2): 50–59. doi:10.1145/1080695.1069976. CiteSeerX:
  8. ^ Drepper, Ulrich (2007), What Every Programmer Should Know About Memory, CiteSeerX:
  9. ^ "Mechanisms to determine if software is running in a VMware virtual machine". VMware Knowledge Base. VMWare. 2015-05-01. Intel and AMD CPUs have reserved bit 31 of ECX of CPUID leaf 0x1 as the hypervisor present bit. This bit allows hypervisors to indicate their presence to the guest operating system. Hypervisors set this bit and physical CPUs (all existing and future CPUs) set this bit to zero. Guest operating systems can test bit 31 to detect if they are running inside a virtual machine.
  10. ^ Kataria, Alok; Hecht, Dan (2008-10-01). "Hypervisor CPUID Interface Proposal". LKML Archive on Archived from the original on 2019-03-15. Bit 31 of ECX of CPUID leaf 0x1. This bit has been reserved by Intel & AMD for use by hypervisors, and indicates the presence of a hypervisor. Virtual CPU's (hypervisors) set this bit to 1 and physical CPU's (all existing and future cpu's) set this bit to zero. This bit can be probed by the guest software to detect whether they are running inside a virtual machine.
  11. ^ a b Shih Kuo (Jan 27, 2012). "Intel® 64 Architecture Processor Topology Enumeration".
  12. ^ "Processor and Core Enumeration Using CPUID | AMD". Archived from the original on 2014-07-14. Retrieved 2014-07-10.
  13. ^ "Sandybridge processors report incorrect core number?". 2012-12-29. Retrieved 2014-07-10.
  14. ^ "cpuid, __cpuidex". 2014-06-20. Retrieved 2014-07-10.
  15. ^ "x86 architecture - CPUID". Retrieved 2014-07-10.
  16. ^ "topology.cpp in ps/trunk/source/lib/sysdep/arch/x86_x64 – Wildfire Games". 2011-12-27. Retrieved 2014-07-10.
  17. ^ Hyper-Threading Technology and Multi-Core Processor Detection
  18. ^ a b c d "Speculative Execution Side Channel Mitigations" (PDF). Revision 2.0. Intel. May 2018 [January 2018]. Document Number: 336996-002. Retrieved 2018-05-26.
  19. ^ "IBRS patch series []".
  20. ^ CPUID Specification (PDF), AMD, September 2010, retrieved 2013-04-02
  21. ^ Linux kernel source code
  22. ^ Lightweight Profiling Specification (PDF), AMD, August 2010, retrieved 2013-04-03
  23. ^ "Intel® Processor Identification and the CPUID Instruction" (PDF). 2012-03-06. Retrieved 2013-04-11.
  24. ^ Ferrie, Peter. "Attacks on Virtual Machine Emulators" (PDF). Symantec Advanced Threat Research. Retrieved 15 March 2017.
  25. ^;a=blob;f=gcc/config/i386/cpuid.h[permanent dead link]
  26. ^ "ARM Information Center". Retrieved 2013-04-11.
  27. ^ "Processor version codes and SRM constants". Archived from the original on 2014-09-08. Retrieved 2014-09-08.
  28. ^ "IBM System z10 Enterprise Class Technical Guide" (PDF).
  29. ^ "MIPS32 Architecture For Programmers, Volume III: The MIPS32 Privileged Resource Architecture" (PDF). MIPS Technologies, Inc. 2001-03-12.
  30. ^ "PowerPC Operating Environment Architecture, book III" (PDF).

Further readingEdit

External linksEdit

According to this note, the former Intel app note 485, which was specifically about CPUID, is now incorporated in the Intel® 64 and IA-32 Architectures Software Developer’s Manual. As of July 2014 the manual however still directs the reader to the app note 485 for further information. The latest published version of the app note 485, dating to May 2012, is available via App note 485 contains some information that can be and was easily misinterpreted though, particularly with respect to processor topology identification.

The big Intel manuals tend to lag behind the Intel ISA document, available at the top of this page, which is updated even for processors not yet publicly available, and thus usually contains more CPUID bits. For example, as of this writing the ISA book (at revision 19, dated May 2014) documents the CLFLUSHOPT bit in leaf 7, but the big manuals although apparently more up-to-date (at revision 51, dated June 2014) don't mention it.