X86 64 registers. Registers The table below lists the commonly used .


  •  X86 64 registers. Reg eax represents the least significant bits (LSB) of rax, ax the LSB of eax, and al is the LSB of ax. The primary defining characteristic of AMD64 is the availability of 64-bit general-purpose processor registers (for example, rax), 64-bit integer arithmetic and logical operations, and 64-bit virtual addresses. Sep 7, 2018 · Due to my working environment is Ubuntu 16. Any argument that doesn't fit in 8 bytes, or isn't 1, 2, 4, or 8 bytes 32-bit x86 processors (starting with the 80386) also include various special/miscellaneous registers such as control registers (CR0 through 4, CR8 for 64-bit only), debug registers (DR0 through 3, plus 6 and 7), test registers (TR3 through 7; 80486 only), and model-specific registers (MSRs, appearing with the Pentium [o]). The can be addressed as 8,16, 32 and 64 bit registers. The register file is as follows: By comparison, the x86-64 System V ABI passes the first 6 integer args in registers, and the first 8 FP args in XMM0. CS107 x86-64 Reference Sheet Registers %rip %rsp %rax %rdi Instruction pointer Stack pointer Return value 1st argument Jul 16, 2025 · We learned the full set of general-purpose registers available in x86-64, along with their smaller aliases which allow us to access just the lower 32, 16, or 8 bits of a larger register. For the first part of the project, we shall use only five of the x86-64’s sixteen general-purpose registers. Feb 2, 2025 · The Base register in x86–64 assembly: understanding RBX, EBX, BX, BH and BL Whether you’re just beginning to learn about assembly language and computer architecture, aiming to deepen your … 1. CR8 CR8 is a new register accessible in 64-bit mode using the REX prefix. Basic understanding of what registers are is a strict minimum required to understand what follows. Given the expanded register set, x64 uses the __fastcall calling convention and a RISC-based exception-handling model. Introduction The x86 instruction set architecture is at the heart of CPU s that power our home computers and remote servers for over two decades. 2. This is because the register is right in the heart of the processor itself. Nov 18, 2009 · Where can I find the names of the new registers for assembly on this architecture? I am referring to registers in X86 like EAX, ESP, EBX, etc. Understanding the x86-64 Architecture Explore the foundational aspects of x86-64 architecture in this introductory article. 2 Registers Modern 64-bit x86 processors have sixteen 64-bit general purpose registers, as depicted in Figure 1. To one-up Intel, they came up with 64-bit extensions to the Intel 32-bit design and produced the first 64-bit chip that could still run 32-bit x86 code. x86 Basics Translation tools: C -> assembly <-> machine code x86 registers, data movement instructions, memory addressing, arithmetic instructions CSAPP book is there are roughly619 registers on the average (relatively recent) x86-64 CPU core. CR8 The # column can be used for numbered registers, e. , EAX for RAX) as distinct registers. If the result of an 8-bit or 16-bit operation is intended for 64-bit address calculation, explicitly sign-extend the register to the full 64-bits. A brief reference is found here (from Standford University). Registers The 64-bit x86 register set consists of 16 general purpose registers, only 8 of which are available in 16-bit and 32-bit mode. Designing an x86-64 Assembler: Segment Registers 1. Registers The table below lists the Jun 22, 2024 · Notes for x86-64 registers and useful pwning info. Notable changes in the 64-bit extensions include: 64-bit integer capability All general-purpose registers The full x86 instruction set is large and complex (Intel's x86 instruction set manuals comprise over 2900 pages), and we do not cover it all in this guide. Therefore, stack is a very important part of the calling conventions. Although the introduction of 64-bit mode significantly reduced their role, segment registers remain relevant for compatibility, certain operating modes Nov 30, 2020 · I will count sub-registers (e. Operands can be registers, immediate values, or memory locations, but not all combinations Jun 9, 2025 · The special purpose registers RIP and RFLAGS are foundational to control flow and processor status in the x86-64 architecture. For example, there is a 16-bit subset of the x86 instruction set. The registers are like variables built in the processor. Priority classes are numbered from 1 to 15, with priority-class 1 being the lowest and priority-class 15 the highest. It mandates the existence of extensions of the 16 SSE 128-bits-wide XMM registers (called XMM0 On x86-64 Linux, the first six function arguments are passed in registers %rdi, %rsi, %rdx, %rcx, %r8, and %r9, respectively. May 31, 2022 · In this article, we will discuss registers for x86 32-bit and 64-bit architecture. Includes example code, a link to a more complete reference, and information on registers, instruction set, stack organization, and calling convention. In addition, stack is also used to save caller-saved and callee-saved registers. I'm in the habit of using this register size, since they also work in 32 bit mode, although I'm trying to use the longer rax registers for everything. These range in size from a dozen or so bits, all the way to 1024-byte "tiles" with AMX. See also our x86-64 sheet for a compact one-page reference. Aug 12, 2025 · CPU Architectures: Understanding x86, ARM, 32-bit vs 64-bit, and Registers In today’s world, whether you’re developing applications for mobile, web, or embedded systems, having a good … The main tools to write programs in x86 assembly are the processor registers. Also, the number of incompatible calling conventions has been reduced. All of these registers are 64 bits wide. What I'm confused about is if/what registers are expected to Clobber registers With respect to an API (such as the WinAPI), registers that can be overwritten in a function of this API are called clobber registers. These are commonly used for thread-pointers in user code and CPU-local pointers in kernel code. MSRs are identified by a number, and are not read from or written to directly. But I'd like them in 64bit. This is followed by an examination of x86-64 core architecture, which includes processor register sets, status flags, instruction operands, and memory addressing modes. Intel ended up following AMD's design in their versions of the 64-bit architecture. It provides a legacy 32-bit mode, which is identical to x86, and a new 64-bit mode. The 64-bit design made a number of changes to the register set, while still being backward compatible: A CS107 joint staff effort (Erik, Julie, Nate) x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. It has a segmented memory model, more restrictions on register usage, and so on. Learn about its registers, memory model, instruction set, and the key differences from x86. Could someone please explain to me, what is the difference between MMX and XMM register? I'm very confused in terms of what fun When Intel expanded the x86 architecture to 32 bits in 1986, it doubled the size of all eight registers and gave them new names by prefixing an E in front of each register name, resulting in EAX, EBX, ECX, EDX, EBP, ESI, EDI, and ESP. The register names for the first eight registers are mostly historical in nature; the last eight registers were given sequential numbers. Sep 8, 2023 · Next is a review of x86-64 fundamental, numeric, and SIMD (single instruction multiple data) data types. It includes structural overviews of the instruction format, register set 7. Of these, %rax, %rcx, %rdx, %rdi, %rsi, %rsp, and %r8-r11 are considered caller-save registers, meaning that they are not necessarily saved across function calls. Explore the various registers in the Intel x86 architecture, including general-purpose, segment, and control registers. x86-64 calling conventions take advantage of the added register space to pass more arguments in registers. previous ABI discussion). Registers The table below lists the commonly used Registers Registers are the fastest kind of memory available in the machine. In my point of view, the default call convention is fastcall here and therefore the parameters are passed from right to left. edu> x86-64 overview x86-64 is a 64-bit version of the x86 ISA Registers Registers are for storing data. In the 64-bit WinAPI, all registers are clobber registers except rbp, rbx, rdi, rsi, r12, r13, r14 and r15. Aug 31, 2022 · On x86-64, this performs the same calculation, but since the destination is the 32-bit partial register eax, the operation also zeroes out the upper 32 bits of rax. (Windows x64 passes the 5th arg on the stack, even if it's FP and the first 4 args were all integer. Introduction The Intel x86-64 Instruction Set Architecture (ISA) is defined across multiple detailed volumes in Intel's architecture documentation. The seventh and subsequent arguments are passed on the stack, about which more below. How to access a single register's byte and how to access all the 64-bit register's eight bytes? I'd love attention for both x86-64 (x64) and Itanium processors. The first 8 floating point arguments are passed in registers xmm0 to xmm7. My justification: they have different instruction encodings, and both Intel and AMD optimize/pessimize particular sub-register use patterns in their microcode. There are sixteen 64-bit registers in x86-64: %rax, %rbx, %rcx, %rdx, %rdi, %rsi, %rbp, %rsp, and %r8-r15. Addressing in x86-64 can be relative to the current instruction pointer value. 3. g. This is indicated with the RIP (64-bit) and EIP (32-bit) instruction pointer registers, which are not otherwise exposed to the program and may not exist physically. Registers Registers are the fastest kind of memory available in the machine. This page contains very basic information on the x86_64 architecture: the register layout and naming and the some basic instructions. With x86_64 came another doubling of register size, as well as the addition of some new registers. In the 32-bit WinAPI, the clobber registers are eax, ecx and edx. Jul 10, 2024 · Note: this article is pretty technical. Oct 20, 2025 · MSRs with the addresses 0xC0000100 (for FS) and 0xC0000101 (for GS) contain the base addresses of the FS and GS segment registers. Table of Jun 9, 2025 · The x86-64 design, first introduced by AMD as AMD64, expands upon the earlier 32-bit x86 (IA-32) architecture by doubling the register width from 32 bits to 64 bits and increasing the number of general-purpose registers from 8 to 16. eax is the 32-bit, "int" size register. Why do these registers have actual names (compared to just A, etc)? What does "source index" and "destination index" actually mean? And is there some A CS107 joint staff effort (Erik, Julie, Nate) x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. ax is the 16 Feb 4, 2024 · I believe I understand how the linux x86-64 ABI uses registers and stack to pass parameters to a function (cf. Using registers instead of memory to store values makes the process faster and cleaner. There's a strict one-to-one correspondence between a function call's arguments and the registers used for those arguments. In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH. See also our x86-64 sheet for a compact reference. The x86 (and x86-64) architecture contains many different types of registers. ) Floating Point Floating point registers st (0) to st (7) are manipulated by the x87 instructions, which we won't be using. May 17, 2025 · Registers R8–R15 are encoded using REX prefixes in 64-bit mode. x86-64 has 14 general-purpose registers and several special-purpose registers. I will count registers that are present on x86-64 CPUs, but that can’t be used in long mode. Registers The table below lists the commonly used Discover the detailed architecture of x64 registers, including general-purpose, segment, and control registers. Registers The table below lists the commonly used x86-64 processors can operate in a legacy mode, they still start in real mode and 16 and 32 bit protected mode is still available (along with the associated Virtual 8086 mode). Jan 1, 2024 · The x86 architecture has 8 General-Purpose Registers (GPR), 6 Segment Registers, 1 Flags Register and an Instruction Pointer. Win64 This section presents a guide to the X86-64 instruction set and architecture. Any additional parameters get pushed on the stack. Dec 11, 2024 · The x64 architecture is a backwards-compatible extension of x86. Space is allocated on the call stack as a shadow store for callees to save those registers. The result is returned in a specific return register %rax. Note that in NASM, line %use altreg is needed for numbered registers. The core eight 16-bit registers are AX, BX, CX, DX, SI, DI, BP, and SP. 04 x86-64, I found it hard to understand the order of passing parameter. Below is our overview of its features that are relevant to CS107. Any remaining arguments are passed on the stack in reverse order so that they can be popped off the stack in order. 2: Sep 25, 2024 · Bit 13 indicates that the next instruction in the instruction stream accesses one of the debug registers. Registers In the assembly syntax accepted by gcc, register names are always prefixed with %. Firstly, we will discuss the registers for x86 32-bit processors. SIMD: MMX, SSE, AVX We will be using these registers and the instructions that work on them for floating point operations: 1. In general you could use these registers in whatever way you like but there are common conventions. This register is accessible through three overlapping "registers": FLAGS for the lowest 16 bits, EFLAGS for the lower 32, and RFLAGS for the whole register. Oct 23, 2017 · I'm currently learning assembly programming on Intel x86 processor. Assume that the callee function has 8 parameters, 3 . This article describes how x86 and x86-64 instructions are encoded. The x86 instruction set has several times been extended with SIMD (Single instruction, multiple data) instruction set extensions. Segment Registers Overview Segment registers in the x86-64 architecture are legacy components inherited from the earlier x86 designs, originally created to support segmented memory models. It enables you to write faster code, use machine features unavailable in C, and reverse-engineer In this lecture, we briefly overview x86-64 Assembly. EFLAGS Register (Bit Positions and Hex Masks) The EFLAGS register is a 32-bit register, where each flag is a single bit: In Intel x86 64 bit architecture there is the rax rdx registers which are simply AD general purpose registers. In this guide, we will limit our rax is the 64-bit, "long" size register. Difference between x86 and x86-64 registers What is the difference between the "E" and "R" prefixes? Besides one being a 64-bit register and the other 32 bits, the "E" stands Apr 1, 2025 · Two important differences between x86 and x64 are: 64-bit addressing capability Sixteen 64-bit registers for general use. That in note As it is widely advertised, modern x86_64 processors have 64-bit registers that can be used in backward-compatible fashion as 32-bit registers, 16-bit registers and even 8-bit registers, for exampl CheatSheet x86-64 Registers The document provides a detailed overview of various CPU registers, including 64-bit, 32-bit, and 16-bit registers, along with their corresponding names and designations. Most instructions in this architecture have two operands: a source and a destination that specifies the second operand and the location of the result. This table gives all the basic registers, with special-purpose registers highlighted in yellow. osdev. On the other hand, registers are highly space-limited. Learn their structure, usage, and role in 64-bit programming. I don't think they are the A CS107 joint staff effort (Erik, Julie, Nate) x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. This section describes the main use of each register and ways to use them. Systems with x86 architecture have only 32-bit registers, while x64 systems have 64-bit registers. Feb 8, 2025 · The System V ABI is one of the major ABIs in use today and is virtually universal among Unix systems. General Purpose Registers Contribute to 0pie/x86-Registers-cheat-sheet development by creating an account on GitHub. It was added in 2003 during the transition to 64-bit processors. . OS X in 64 bit uses the same parameter scheme. There are 8 common registers in assembly and 6 are used for general purpose. Gain a solid understanding of how the CPU processes instructions, setting the stage for writing efficient assembly code. But there are also registers called rsi and rdi which are the "source index" and "destination index" registers respectively. For example, RAX used to be EAX (in the 32-bit machine), which used to be called the “accumulator” since it was used by a number of In x86-64, it is very common to see 32-bit component registers intermixed with the full 64-bit registers. Other registers are all-purpose, but Dec 7, 2023 · When using x86 assembly, it is important to consider the differences between architectures that are 16, 32, and 64 bits. The size of a CPU’s registers is a fundamental aspect that determines the amount of data they can handle. x86-64/32 Registers A CS107 joint staff effort (Erik, Julie, Nate) x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. MSRs are read with the rdmsr instruction and written to with the wrmsr instruction. This appendix distills the most relevant technical highlights, updated after 2020, into a concise summary tailored for assembler developers. It is the calling convention used by toolchains such as i686-elf-gcc and x86_64-elf-gcc. Bit 14 indicates (when set) that the debug exception was triggered by the single-step execution mode (enabled with TF bit in EFLAGS). r3 = rbx. General Purpose Registers For x86 32-bit architecture x86 architecture has 8 general-purpose 32-bit registers. For example, in the adder2 function shown in the previous example, the compiler references component register %eax instead of %rax since int types typically take up 32 bits (four bytes) of space on 64-bit systems. 01h:EDX [bit 5]. Apr 14, 2009 · 1 Introduction You will be generating assembly code for the x86-64 architecture, which is the 64-bit extension to Intel’s venerable x86 architecture. In x86-32 and x86-64 assembly, 16 bit instructions such as mov ax, bx Sep 27, 2023 · x86-64 Registers. A fundamental introduction to x86 assembly programming 0. Same pattern holds for all other registers. RIP governs the flow of program execution and enables advanced addressing modes such as RIP-relative addressing, critical for modern software and system design. 3 x86-64 Calling Conventions In x86-64, the first six arguments are passed in registers, the remaining arguments are passed on the stack. There is more extensive coverage on these topics in Chapter 3 of the B&O textbook. Despite the fact that only bits 0 through 21 are defined, RFLAGS exists for consistency with the rest of the x86-64 general purpose register set. After that, we will discuss the registers for x86 64-bit processors. Mar 12, 2024 · x86-64 Assembly x86 Data Formats & Registers x86-64 has 16 64-bit general purpose registers, as presented in CSAPP Figure 3. Registers The table below lists the commonly used Our focus: The x86-64 Assembly Language x86-64 Targets Intel/AMD chips with 64-bit word size Reminder: 64-bit “word size” size of pointers/addresses Descended from IA32: Intel Architecture 32-bit systems Accessing Model Specific Registers Each MSR that is accessed by the RDMSR and WRMSR group of instructions is identified by a 32-bit integer. Each register is 64 bits wide; the lower 32-, 16- and 8-bit portions are selectable by a pseudo-register name. The processor increments %rip automatically after each instruction, and control flow instructions like branches set the value of %rip to change the next instruction. Given the arguments in left-to-right order, the order of registers used is: %rdi, %rsi, %rdx, %rcx, %r8, and %r9. IA-32 and x86-64 • x86-64 Architecture Diagram • Registers • Instruction Set • Addressing Memory • Flags Register • The System Developer’s Manual Assembly 1: Basics Registers Registers are the fastest kind of memory available in the machine. A CS107 joint staff effort (Erik, Julie, Nate) x86-64 (also known as just x64 and/or AMD64) is the 64-bit version of the x86/IA32 instruction set. x86-64 General-Purpose Registers The x86-64 instruction set extends the legacy IA-32 architecture to 64 bits while preserving full backward compatibility. The __fastcall convention uses registers for the first four arguments, and the stack frame to pass more arguments. Starting from 128-bit XMM registers, extending through 256-bit YMM, and culminating in 512-bit ZMM registers, they offer a scalable platform for parallel data processing across multiple industries and applications. Another helpful resource is the UC Davis AT\&T Assembly Syntax Guide Registers In the assembly syntax accepted by gcc, register names are always prefixed with %. On 64-bit systems like the x86-64, most registers will store and represent values using 64-bits of storage. Summary The SIMD register set—XMM, YMM, and ZMM—is a cornerstone of modern x86-64 vectorized computation. Jul 16, 2025 · Intel x86-64 ISA Manual Summary 1. Using the 16-bit programming model can be quite complex. 64-bit x86 has additional registers. Both instructions read the MSR to interact with from the ECX register, and read data into Feb 26, 2022 · General-Purpose Registers For AArch64 The aarch64 registers are named: r0 through r30 - to refer generally to the registers x0 through x30 - for 64-bit-wide access (same registers) w0 through w30 - for 32-bit-wide access (same registers - upper 32 bits are either cleared on load or sign-extended (set to the value of the most significant bit of the loaded value)). This is Jorgensen Chapter 2. Jan 31, 2025 · The Accumulator register in x86–64 assembly: understanding RAX, EAX, AX, AH, and AL If you’ve just started learning about assembly and computer architecture, whether to deepen your The %rip register on x86-64 is a special-purpose register that always holds the memory address of the next instruction to execute in the program's code segment. 7. A 64 bit linux machine passes function parameters in rdi, rsi, rdx, rcx, r8, and r9. These extensions, starting from the MMX instruction set extension introduced with Pentium MMX in 1997, typically define sets of wide registers and instructions that subdivide these registers into fixed-size lanes and perform a computation for each lane in parallel. 1 CPU Registers The current X86 family has 16 general purpose registers. Register '31' is one of two 21. It was added in 1985 during the transition to 32-bit processors with the 80386 CPU. What is AVX? AVX (and its extension AVX2) are complex x86_64 instruction sets which extends the SSE4 instruction set. MSRs are 64-bit wide. For details on the x64 calling Clobber registers With respect to an API (such as the WinAPI), registers that can be overwritten in a function of this API are called clobber registers. The presence of MSRs on your processor is indicated by CPUID. x86-64 CS 351: Systems Programming Michael Lee <lee@iit. CR8 is used to prioritize external interrupts and is referred to as the task-priority register (TPR). The problem with the x86 serie of processors is that there are few registers to use. References https://wiki. Being able to read and write code in low-level assembly language is a powerful skill to have. Registers The table below lists the commonly used registers (sixteen general-purpose plus two special). [16] The designers took the opportunity to make other improvements as well. org/CPU_Registers_x86-64 CPU Registers Registers Notes 64-bit 32-bit 16-bit 8-bit high 8-bit low RAX EAX AX AH AL Accumulator. 1. The x86_64 instruction set architechture includes 16 general purpose regisers, each of which can be addressed in full, or by the lower 32, 16 and 8 bits. This page will talk about some of the basic differences between architectures with different bit widths. Example function call: extern putchar mov rdi,'H' ; function parameter: one char to print call putchar Windows in 64 bit is quite different: Win64 function parameters go in registers rcx, rdx, r8, and r9. Mar 19, 2025 · The x64 Application Binary Interface (ABI) uses a four-register, fast-call calling convention by default. You’ll notice different naming conventions, a side effect of the long history of the x86 architecture (the 8086 was first released in 1978). The least significant 8 bits of the first four of these registers are accessible via the AL, BL, CL, and DL in all execution modes. Stack Frame If the callee has more than 6 parameters, except for the first 6 parameters which are passed through registers, the remaining parameters are passed through stack. Some registers are designated for a certain purpose, such as %rsp being used as the stack pointer or %rax for the return value from a function. Registers The table below lists the commonly used Jun 24, 2012 · The upper 56 bits or 48 bits (respectively) of the destination general-purpose register are not be modified by the operation. Oct 29, 2025 · 2. Registers The table below lists the commonly used CR1 CR2 CR3 CR4 Model-Specific Registers Model-specific registers are 64-bit registers whose availability or location depends on the CPU model. Feb 14, 2017 · It's common to compile code for x86_64 target in PIC-compatible way (in some OS like OS X for Mac it is mandatory), while 32b x86 targets were usually using absolute code expecting particular position in memory. It lists the registers such as RAX, RCX, RDX, RBX, RSP, RBP, RSI, and RDI, along with their aliases in different bit formats. They are: EAX EBX ECX EDX EBP […] In the x86-64 architecture CPUs, there are a few groups of registers: general-purpose, segment, and EFLAGS registers. 2bl xyebnp qx91gd 2ybgi9pn xbblj nmdxv k72kzip jqdmjyj6 ov c97zc
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