CPU Registers x86-64

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General Purpose Registers

Monikers Description
64-bit 32-bit 16-bit 8 high bits of lower 16 bits 8-bit
RAX EAX AX AH AL Accumulator
RDX EDX DX DH DL Data (commonly extends the A register)
RSI ESI SI N/A SIL Source index for string operations
RDI EDI DI N/A DIL Destination index for string operations
RSP ESP SP N/A SPL Stack Pointer
RBP EBP BP N/A BPL Base Pointer (meant for stack frames)
R8 R8D R8W N/A R8B General purpose
R9 R9D R9W N/A R9B General purpose
R10 R10D R10W N/A R10B General purpose
R11 R11D R11W N/A R11B General purpose
R12 R12D R12W N/A R12B General purpose
R13 R13D R13W N/A R13B General purpose
R14 R14D R14W N/A R14B General purpose
R15 R15D R15W N/A R15B General purpose

Note: you cannot access AH, BH, CH and DH when using the REX.W instruction prefix. This prefix is added (automatically by assemblers) when an operand contains a 64-bit register.

Pointer Registers

Monikers Description
64-bit 32-bit 16-bit
RIP EIP IP Instruction Pointer

Note: The instruction pointer can only be used in RIP-relative addressing, which was introduced with long mode.

Segment Registers

All these are 16 bits long.

Moniker Description
CS Code Segment
DS Data Segment
SS Stack Segment
ES Extra Segment (used for string operations)
FS General-purpose Segment
GS General-purpose Segment

Segments of CS, DS, ES, and SS are treated as if their base was 0 no matter what the segment descriptors in the GDT say. Exceptions are FS and GS which have MSRs to change their base.

Limit checks are disabled for all segments.

RFLAGS Register

Bit(s) Label Description
0 CF Carry Flag
1 1 Reserved
2 PF Parity Flag
3 0 Reserved
4 AF Auxiliary Carry Flag
5 0 Reserved
6 ZF Zero Flag
7 SF Sign Flag
8 TF Trap Flag
9 IF Interrupt Enable Flag
10 DF Direction Flag
11 OF Overflow Flag
12-13 IOPL I/O Privilege Level
14 NT Nested Task
15 0 Reserved
16 RF Resume Flag
17 VM Virtual-8086 Mode
18 AC Alignment Check / Access Control
19 VIF Virtual Interrupt Flag
20 VIP Virtual Interrupt Pending
21 ID ID Flag
22-63 0 Reserved

Control Registers


Bit(s) Label Description
0 PE Protected Mode Enable
1 MP Monitor Co-Processor
2 EM Emulation
3 TS Task Switched
4 ET Extension Type
5 NE Numeric Error
6-15 0 Reserved
16 WP Write Protect
17 0 Reserved
18 AM Alignment Mask
19-28 0 Reserved
29 NW Not-Write Through
30 CD Cache Disable
31 PG Paging
32-63 0 Reserved

NOTE that this register is the only control register that can be written and read via 2 ways unlike the other that can be accessed only via the MOV instruction

;way 1:
mov cr0,reg32(64)
mov reg32(64),cr0 
;way 2:
lmsw reg16(32/64) ; the 'w' in lms(w) stands for word size (16 bit) but the instruction itself can modify the upper 48 bit of cr0 using instruction overrides.

smsw reg16(32/64) ; SAME as above


This control register contains the linear (virtual) address which triggered a page fault, available in the page fault's interrupt handler.


Bit(s) Label Description Condition
0-11 0-2 0 Reserved CR4.PCIDE = 0
3 PWT Page-Level Write Through
5 PCD Page-Level Cache Disable
5-11 0 Reserved
0-11 PCID CR4.PCIDE = 1
12-63 Physical Base Address of the PML4

Note that this must be page aligned


Bit(s) Label Description
0 VME Virtual-8086 Mode Extensions
1 PVI Protected Mode Virtual Interrupts
2 TSD Time Stamp enabled only in ring 0
3 DE Debugging Extensions
4 PSE Page Size Extension
5 PAE Physical Address Extension
6 MCE Machine Check Exception
7 PGE Page Global Enable
8 PCE Performance Monitoring Counter Enable
9 OSFXSR OS support for fxsave and fxrstor instructions
10 OSXMMEXCPT OS Support for unmasked simd floating point exceptions
11 UMIP User-Mode Instruction Prevention (SGDT, SIDT, SLDT, SMSW, and STR are disabled in user mode)
12 0 Reserved
13 VMXE Virtual Machine Extensions Enable
14 SMXE Safer Mode Extensions Enable
15 0 Reserved
17 PCIDE PCID Enable
18 OSXSAVE XSAVE And Processor Extended States Enable
19 0 Reserved
20 SMEP Supervisor Mode Executions Protection Enable
21 SMAP Supervisor Mode Access Protection Enable
22 PKE Enable protection keys for user-mode pages
23 CET Enable Control-flow Enforcement Technology
24 PKS Enable protection keys for supervisor-mode pages
25-63 0 Reserved


CR8 is a new register accessible in 64-bit mode using the REX prefix. CR8 is used to prioritize external interrupts and is referred to as the task-priority register (TPR).

The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. Priority classes are numbered from 1 to 15, with priority-class 1 being the lowest and priority-class 15 the highest. CR8 uses the four low-order bits for specifying a task priority and the remaining 60 bits are reserved and must be written with zeros.

System software can use the TPR register to temporarily block low-priority interrupts from interrupting a high-priority task. This is accomplished by loading TPR with a value corresponding to the highest-priority interrupt that is to be blocked. For example, loading TPR with a value of 9 (1001b) blocks all interrupts with a priority class of 9 or less, while allowing all interrupts with a priority class of 10 or more to be recognized. Loading TPR with 0 enables all external interrupts. Loading TPR with 15 (1111b) disables all external interrupts.

The TPR is cleared to 0 on reset.

Bit Purpose
0-3 Priority
4-63 Reserved

CR1, CR5-7, CR9-15

Reserved, the cpu will throw a #ud exeption when trying to access them.



Extended Feature Enable Register (EFER) is a model-specific register added in the AMD K6 processor, to allow enabling the SYSCALL/SYSRET instruction, and later for entering and exiting long mode. This register becomes architectural in AMD64 and has been adopted by Intel. Its MSR number is 0xC0000080.

Bit(s) Label Description
0 SCE System Call Extensions
1-7 0 Reserved
8 LME Long Mode Enable
10 LMA Long Mode Active
11 NXE No-Execute Enable
12 SVME Secure Virtual Machine Enable
13 LMSLE Long Mode Segment Limit Enable
15 TCE Translation Cache Extension
16-63 0 Reserved

FS.base, GS.base

MSRs with the addresses 0xC0000100 (for FS) and 0xC0000101 (for GS) contain the base addresses of the FS and GS segment registers. These are commonly used for thread-pointers in user code and CPU-local pointers in kernel code. Safe to contain anything, since use of a segment does not confer additional privileges to user code.

In newer CPUs, these can also be written with WRFSBASE and WRGSBASE instructions at any privilege level.


MSR with the address 0xC0000102. Is basically a buffer that gets exchanged with GS.base after a swapgs instruction. Usually used to seperate kernel and user use of the GS register.

Debug Registers

DR0 - DR3

Contain linear addresses of up to 4 breakpoints. If paging is enabled, they are translated to physical addresses.


It permits the debugger to determine which debug conditions have occurred. When an enabled debug exception is triggered, low order bits 0-3 are set before entering debug exception handler.


Bit Description
0 Local DR0 Breakpoint
1 Global DR0 Breakpoint
2 Local DR1 Breakpoint
3 Global DR1 Breakpoint
4 Local DR2 Breakpoint
5 Global DR2 Breakpoint
6 Local DR3 Breakpoint
7 Global DR3 Breakpoint
16-17 Conditions for DR0
18-19 Size of DR0 Breakpoint
20-21 Conditions for DR1
22-23 Size of DR1 Breakpoint
24-25 Conditions for DR2
26-27 Size of DR2 Breakpoint
28-29 Conditions for DR3
30-31 Size of DR3 Breakpoint

A local breakpoint bit deactivates on hardware task switches, while a global does not.
00b condition means execution break, 01b means a write watchpoint, and 11b means an R/W watchpoint. 10b is reserved for I/O R/W (unsupported).

Test Registers

Name Description
TR3 - TR5 Undocumented
TR6 Test Command Register
TR7 Test Data Register

Protected Mode Registers


Operand Size Label Description
64-bit 32-bit
Bits 0-15 Limit Size of GDT
Bits 16-79 Bits 16-47 Base Starting Address of GDT


Stores the segment selector of the LDT.


Stores the segment selector of the TSS.


Operand Size Label Description
64-bit 32-bit
Bits 0-15 Limit Size of IDT
Bits 16-79 Bits 16-47 Base Starting Address of IDT