64-bit registers using the 'r' prefix: rax, r15
32-bit registers using the 'e' prefix (original registers: e_x) or 'd' suffix (added registers: r__d): eax, r15d
16-bit registers using no prefix (original registers: _x) or a 'w' suffix (added registers: r__w): ax, r15w
8-bit registers using 'h' ("high byte" of 16 bits) suffix (original registers - bits 8-15: _h): ah, bh
8-bit registers using 'l' ("low byte" of 16 bits) suffix (original registers - bits 0-7: _l) or 'b' suffix (added registers: r__b): al, bl, r15b

Usage during syscall/function call:

First six arguments are in rdi, rsi, rdx, rcx, r8d, r9d; remaining arguments are on the stack.
For syscalls, the syscall number is in rax.
Return value is in rax.
The called routine is expected to preserve rsp,rbp, rbx, r12, r13, r14, and r15 but may trample any other registers.

Floating-Point and SIMD Registers

x86_64 also defines a set of large registers for floating-point and single-instruction/multiple-data (SIMD) operations. For details, refer to the Intel or AMD documentation.

Instructions

Starter Kit

These instructions are sufficient to complete the SPO600 Assembler Lab (GAS syntax):

add %r10,%r11    // add r10 and r11, put result in r11
cmp %r10,%r11    // compare register r10 with register r11.  The comparison sets flags in the processor status register which affect conditional jumps.
cmp $99,%r11     // compare the number 99 with register r11.  The comparison sets flags in the processor status register which affect conditional jumps.
div %r10         // divide rax by the given register (r10), places quotient into rax and remainder into rdx (rdx must be zero before this instruction)
inc %r10         // increment r10
jmp label        // jump to label
je  label        // jump to label if equal
jne label        // jump to label if not equal
jl  label        // jump to label if less
jg  label        // jump to label if greater
mov %r10,%r11    // move data from r10 to r11
mov $99,%r10     // put the immediate value 99 into r10
mov %r10,(%r11)  // move data from r10 to address pointed to by r11
mov (%r10),%r11  // move data from address pointed to by r10 to r10
mul %r10         // multiplies rax by r10, places result in rax and overflow in rdx
push %r10        // push r10 onto the stack
pop %r10         // pop r10 off the stack
syscall          // invoke a syscall (in 32-bit mode, use "int $0x80" instead)

Note the syntax:

Register names are prefixed by %
Immediate values are prefixed by $
Indirect memory access is indicated by (parenthesis).
Hexadecimal values are indicated by a 0x prefix.
Character values are indicated by quotation marks. Escapes (such as '\n') are permitted.
Data sources are given as the first argument (mov %r10,%r11 moves FROM r10 INTO r11).

For the MOV instruction:

You can append a suffix indicating the amount of data to be moved -- e.g., q for quadword (64 bits), d for doubleword (32 bits), w for word (16 bits), or b for byte (8 bits).

Resources

CPU Instruction Set and Software Developer Manuals
- AMD: http://developer.amd.com/resources/documentation-articles/developer-guides-manuals/
- Intel: http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html
Web sites
- http://ref.x86asm.net/
- http://sandpile.org/
GAS Manual - Using as, The GNU Assembler: https://sourceware.org/binutils/docs/as/

x86 Registers

原文地址：http://www.eecg.toronto.edu/~amza/www.mindsec.com/files/x86regs.html

The main tools to write programs in x86 assembly are the processor registers. The registers are like variables built in the processor. Using registers instead of memory to store values makes the process faster and cleaner. The problem with the x86 serie of processors is that there are few registers to use. This section describes the main use of each register and ways to use them. That in note that the rules described here are more suggestions than strict rules. Some operations need absolutely some kind of registers but most of the you can use any of the freely.

Here is a list of the available registers on the 386 and higher processors. This list shows the 32 bit registers. Most of the can be broken down to 16 or even 8 bits register.


General registers
EAX EBX ECX EDX

Segment registers
CS DS ES FS GS SS

Index and pointers
ESI EDI EBP EIP ESP

Indicator
EFLAGS

General registers

As the title says, general register are the one we use most of the time Most of the instructions perform on these registers. They all can be broken down into 16 and 8 bit registers.


32 bits :  EAX EBX ECX EDX
16 bits : AX BX CX DX
 8 bits : AH AL BH BL CH CL DH DL

The "H" and "L" suffix on the 8 bit registers stand for high byte and low byte. With this out of the way, let's see their individual main use


EAX,AX,AH,AL : Called the Accumulator register. 
               It is used for I/O port access, arithmetic, interrupt calls,
               etc...

EBX,BX,BH,BL : Called the Base register
               It is used as a base pointer for memory access
               Gets some interrupt return values

ECX,CX,CH,CL : Called the Counter register
               It is used as a loop counter and for shifts
               Gets some interrupt values

EDX,DX,DH,DL : Called the Data register
               It is used for I/O port access, arithmetic, some interrupt 
               calls.

Segment registers

Segment registers hold the segment address of various items. They are only available in 16 values. They can only be set by a general register or special instructions. Some of them are critical for the good execution of the program and you might want to consider playing with them when you'll be ready for multi-segment programming


CS         : Holds the Code segment in which your program runs.
             Changing its value might make the computer hang.

DS         : Holds the Data segment that your program accesses.
             Changing its value might give erronous data.

ES,FS,GS   : These are extra segment registers available for
             far pointer addressing like video memory and such.

SS         : Holds the Stack segment your program uses.
             Sometimes has the same value as DS.
             Changing its value can give unpredictable results,
             mostly data related.

Indexes and pointers

Indexes and pointer and the offset part of and address. They have various uses but each register has a specific function. They some time used with a segment register to point to far address (in a 1Mb range). The register with an "E" prefix can only be used in protected mode.


ES:EDI EDI DI : Destination index register
                Used for string, memory array copying and setting and
                for far pointer addressing with ES

DS:ESI EDI SI : Source index register
                Used for string and memory array copying

SS:EBP EBP BP : Stack Base pointer register
                Holds the base address of the stack
                
SS:ESP ESP SP : Stack pointer register
                Holds the top address of the stack

CS:EIP EIP IP : Index Pointer
                Holds the offset of the next instruction
                It can only be read

The EFLAGS register

The EFLAGS register hold the state of the processor. It is modified by many intructions and is used for comparing some parameters, conditional loops and conditionnal jumps. Each bit holds the state of specific parameter of the last instruction. Here is a listing :


Bit   Label    Desciption
---------------------------
0      CF      Carry flag
2      PF      Parity flag
4      AF      Auxiliary carry flag
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 Priviledge level
14     NT      Nested task flag
16     RF      Resume flag
17     VM      Virtual 8086 mode flag
18     AC      Alignment check flag (486+)
19     VIF     Virutal interrupt flag
20     VIP     Virtual interrupt pending flag
21     ID      ID flag

Those that are not listed are reserved by Intel.

Undocumented registers

There are registers on the 80386 and higher processors that are not well documented by Intel. These are divided in control registers, debug registers, test registers and protected mode segmentation registers. As far as I know, the control registers, along with the segmentation registers, are used in protected mode programming, all of these registers are available on 80386 and higher processors except the test registers that have been removed on the pentium. Control registers are CR0 to CR4, Debug registers are DR0 to DR7, test registers are TR3 to TR7 and the protected mode segmentation registers are GDTR (Global Descriptor Table Register), IDTR (Interrupt Descriptor Table Register), LDTR (Local DTR), and TR.

CPU Registers x86-64

原文地址：https://wiki.osdev.org/CPU_Registers_x86-64

General Purpose Registers

Monikers					Description
64-bit	32-bit	16-bit	8 high bits of lower 16 bits	8-bit	Description
RAX	EAX	AX	AH	AL	Accumulator
RBX	EBX	BX	BH	BL	Base
RCX	ECX	CX	CH	CL	Counter
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	Description
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

CR0

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:
;write:
mov cr0,reg32(64)
;read:
mov reg32(64),cr0 
;----------------------
;way 2:
;write:
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.
 
;read:
smsw reg16(32/64) ; SAME as above

CR2

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

CR3

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

CR4

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
22-63	0	Reserved

CR8

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.

MSRs

IA32_EFER

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
14	FFXSR	Fast FXSAVE/FXRSTOR
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.

KernelGSBase

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.

DR6

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

DR7

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

GDTR

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

LDTR

Stores the segment selector of the LDT.

TR

Stores the segment selector of the TSS.

IDTR

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

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Registers

General-Purpose Registers