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For years, PC programmers used x86 assembly to write performance-critical code. However, bit PCs are being replaced with bit ones, and the underlying assembly code has changed. This white paper is an introduction to x64 assembly. No prior knowledge of x86 code is needed, although it makes the transition easier.

We call this intersection flavor x This white paper won't cover hardware details such as caches, branch prediction, and other advanced topics. Several references will be given at the end of the article for further reading in these areas. Assembly knowledge is useful for debugging code - sometimes a compiler makes incorrect assembly code and stepping through the code in a debugger helps locate the cause. Code optimizers sometimes make mistakes. Another use for assembly is interfacing with or fixing code for which you have no source code.

Assembly is necessary if you want to know how your language of choice works under the hood - why some things are slow and others are fast. Finally, assembly code knowledge is indispensable when diagnosing malware. When learning assembly for a given platform, the first place to start is to learn the register set.

General Architecture Since the bit registers allow access for many sizes and locations, we define a byte as 8 bits, a word as 16 bits, a double word as 32 bits, a quadword as 64 bits, and a double quadword as bits. Intel stores bytes "little endian," meaning lower significant bytes are stored in lower memory addresses. The second eight are named R8-R Note there is no R8H.

The bit instruction pointer RIP points to the next instruction to be executed, and supports a bit flat memory model. Memory address layout in current operating systems is covered later. The stack pointer RSP points to the last item pushed onto the stack, which grows toward lower addresses.

This is formed from the x86 bit register EFLAGS by adding a higher 32 bits which are reserved and currently unused. Table 1 lists the most useful flags.

x64 assembly

Most of the other flags are used for operating system level tasks and should always be set to the value previously read. Table 1 - Common Flags. FPR can each store one value of the types shown in Table 2.

Accessing Windows API with Plain x64 Assembly Language

Floating point operations conform to IEEE These registers share space with the eight bit MMX registers. Table 2 - Floating Point Types. Binary Coded Decimal BCD is supported by a few 8-bit instructions, and an oddball format supported on the floating point registers gives an 80 bit, 17 digit BCD type. The sixteen bit XMM registers eight more than x86 are covered in more detail.

The most notable performance opcode is RDTSC, which is used to count processor cycles for profiling small pieces of code. They are available for free download as PDF, order on CD, and often can be ordered for free as a hardcover set when listed. For example, eight bytes can be added to eight bytes in one instruction using MMX. The MMX instructions operate on integer types, allowing byte, word, and doubleword operations to be performed on values in the MMX registers in parallel. Most MMX instructions begin with 'P' for "packed".

The sixteen bit XMM registers allow parallel operations on four single or two double precision values per instruction. Some instructions also work on packed byte, word, doubleword, and quadword integers.You can find a more exhaustive list of contributers on the wiki.

X86 Assembly/Control Flow

Check out the blog! A familiar, yet new interface. Fully customizable color schemes. One GUI, two platforms: x64 and x More information about jump targets and register values. Smart, content-sensitive register view. Memory map Symbol view Thread view Source code view Content-sensitive register view Fully customizable color scheme Dynamically recognize modules and strings Import reconstructor integrated Scylla Fast disassembler Zydis User database JSON for comments, labels, bookmarks, etc.

Overview Active development x64dbg is under constant active development. GPLv3 We provide both the executable and the source.

x64 assembly

Feel free to contribute. There is only one interface. Scriptable x64dbg has an integrated, debuggable, ASM-like scripting language. Community-aware x64dbg has many features thought of or implemented by the reversing community. Extendable Write plugins to add script commands or to integrate your tools. Contact x64dbg on Twitter x64dbg on Google Groups x64dbg [at] googlegroups.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. NASM examples.

x64 assembly

Then the code would look like this. Can't be run "directly" transparently under any bit Windows, because an x kernel can't use vm86 mode. Build this into a. You can find more details in this article. Why does x64 Windows need to reserve 28h bytes of stack space before a call?

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That's 32 bytes 0x20 of shadow space aka home space, as required by the calling convention. And another 8 bytes to re-align the stack by 16, because the calling convention requires RSP be byte aligned before a call. Our main 's caller in the CRT startup code did that. The 8-byte return address means that RSP is 8 bytes away from a byte boundary on entry to a function.

Shadow space can be used by a function to dump its register args next to where any stack args if any would be. A system call requires 30h 48 bytes to also reserve space for r10 and r11 in addition to the previously mentioned 4 registers.

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But DLL calls are just function calls, even if they're wrappers around syscall instructions. Fun fact: non-Windows, i. Using MASM's invoke directive which knows the calling conventionyou can use one ifdef to make a version of this which can be built as bit or bit. The macro variant is the same for both, but you won't learn assembly this way. You'll learn C-style asm instead. The assembler knows which to use. Flat Assembler does not need an extra linker.

This makes assembler programming quite easy. It is also available for Linux. Also, as it is done here, if Visual's linker is invoked from command prompt, it is necessary to setup the environment previously run once vcvarsall. Unless you call some function this is not at all trivial. And, seriously, there's no real difference in complexity between calling printf and calling a win32 api function. If you want to do it without help you need to talk to your video hardware directly, likely writing bitmaps of the letters of "Hello world" into a framebuffer.

Note that, really, none of this stuff all the way down to the hardware is any more interesting in ASM than in C. A "hello world" program boils down to a function call. The best examples are those with fasm, because fasm doesn't use a linker, which hides the complexity of windows programming by another opaque layer of complexity.

If you're content with a program that writes into a gui window, then there is an example for that in fasm's example directory. If you want a console program, that allows redirection of standard in and standard out that is also possible.In a logical shift instruction also referred to as unsigned shiftthe bits that slide off the end disappear except for the last, which goes into the carry flagand the spaces are always filled with zeros.

Logical shifts are best used with unsigned numbers. Examples GAS Syntax :. In an arithmetic shift also referred to as signed shiftlike a logical shift, the bits that slide off the end disappear except for the last, which goes into the carry flag.

But in an arithmetic shift, the spaces are filled in such a way to preserve the sign of the number being slid. For this reason, arithmetic shifts are better suited for signed numbers in two's complement format. Arithmetic shift dest to the right by src bits. Spaces are filled with sign bit to maintain sign of original valuewhich is the original highest bit. Arithmetic shift dest to the left by src bits. The bottom bits do not affect the sign, so the bottom bits are filled with zeros.

This instruction is synonymous with SHL. The names of the double precision shift operations are somewhat misleading, hence they are listed as extended shift instructions on this page.

The src operand is always a register, the dest operand can be a register or memory location, the cnt operand is an immediate byte value or the CL register. In bit mode it is possible to address bit data as well.

x64 assembly

The operation performed by shld is to shift the most significant cnt bits out of destbut instead of filling up the least significant bits with zeros, they are filled with the most significant cnt bits of src. Likewise, the shrd operation shifts the least significant cnt bits out of destand fills up the most significant cnt bits with the least significant bits of the src operand.

Intel's nomenclature is misleading, in that the shift does not operate on double the basic operand size i. Also, Intel's manual [1] states that the results are undefined when cnt is greater than the operand size, but at least for and bit data sizes it has been observed that shift operations are performed by cnt mod nwith n being the data size.

In a rotate instruction, the bits that slide off the end of the register are fed back into the spaces. Unless stated, these instructions can take either one or two arguments. From Wikibooks, open books for an open world.Jump to navigation. Electronic versions of these documents allow you to quickly get to the information you need and print only the pages you want.

All content is identical in each set; see details below.

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At present, downloadable PDFs of all volumes are at version Additional related specifications, application notes, and white papers are also available for download.

Volume 2: Includes the full instruction set reference, A-Z. Describes the format of the instruction and provides reference pages for instructions. Volume 3: Includes the full system programming guide, parts 1, 2, 3, and 4. This set allows for easier navigation of the instruction set reference and system programming guide through functional cross-volume table of contents, references, and index.

This document contains the full instruction set reference, A-Z, in one volume. This document allows for easy navigation of the instruction set reference through functional cross-volume table of contents, references, and index. This document contains the full system programming guide, parts 1, 2, 3, and 4, in one volume.

This document allows for easy navigation of the system programming guide through functional cross-volume table of contents, references, and index. This set contains the same information as the four-volume set, but separated into ten smaller PDFs: volume 1, volume 2A, volume 2B, volume 2C, volume 2D, volume 3A, volume 3B, volume 3C, volume 3D, and volume 4.

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This set is better suited to those with slower connection speeds. This instruction pointer value is loaded into the EIP register to re-start the calling task. Found a typo on the page Vol. CPUID h returns eax, ebx, ecx, edx : 0 0 0x 0. Documentation says that bits of ecx is L2 cache size in K units, which translates to K. My question is: which method is correct? Multiply the packed single-precision floating-point values This is corroborated by the pseudo-code for the same entry:.

Modern Code Documentation. Home What is Code Modernization? Share Tweet Share Send.Say hello to x64 Assembly [part 1]. August 29, Introduction There are many developers between us. We write a tons of code every day. Sometime, it is even not a bad code : Every of us can easily write the simplest code like this: Every of us can understand what's this C code does. How this code works at low level? I think that not all of us can answer on this question, and me too.

I thought that i can write code on high level programming languages like Haskell, Erlang, Go and etc So I decided to take a few deep steps down, to assembly, and to describe my learning way about this. Hope it will be interesting, not only for me.

Something about 5 - 6 years ago I already used assembly for writing simple programs, it was in university and i used Turbo assembly and DOS operating system.

Introduction to writing x64 assembly in Visual Studio

Now I use Linux-x operating system. Yes, must be big difference between Linux 64 bit and DOS 16 bit. So let's start. Different CPU supports different set of instructions. I use Intel Core i7 processor, and all code will be written processor. Also i will use nasm assembly. You can install it with: sudo apt-get install nasm It's version must be 2.

I use NASM version 2. And the last part, you will need in text editor where you will write you assembly code. I use Emacs with nasm-mode. It is not mandatory, of course you can use your favourite text editor. If you use Emacs as me you can download nasm-mode.

Bluff your way in x64 assembler - Roger Orr [ACCU 2017]

Other tools will be describe in next posts. Usually NASM program divided into sections. In this post we'll meet 2 following sections: data section text section The data section is used for declaring constants. This data does not change at runtime.The x64 architecture is a backwards-compatible extension of x It provides a legacy bit mode, which is identical to x86, and a new bit mode.

The term "x64" includes both AMD 64 and Intel The instruction sets are close to identical. The bit registers have names beginning with "r", so for example the bit extension of eax is called rax.

The new registers are named r8 through r The lower 32 bits, 16 bits, and 8 bits of each register are directly addressable in operands. This includes registers, like esiwhose lower 8 bits were not previously addressable. The following table specifies the assembly-language names for the lower portions of bit registers. Operations that output to a bit subregister are automatically zero-extended to the entire bit register. Operations that output to 8-bit or bit subregisters are not zero-extended this is compatible x86 behavior.

The high 8 bits of axbxcxand dx are still addressable as ahbhchdhbut cannot be used with all types of operands. The instruction pointer, eipand flags register have been extended to 64 bits rip and rflagsrespectively as well.

This calling convention takes advantage of the increased number of registers available on x The first four integer or pointer parameters are passed in the rcxrdxr8and r9 registers. The first four floating-point parameters are passed in the first four SSE registers, xmm0 - xmm3. The caller reserves space on the stack for arguments passed in registers. The called function can use this space to spill the contents of registers to the stack. An integer or pointer return value is returned in the rax register, while a floating-point return value is returned in xmm0.

The next three parameters are passed in registers, while the rest are passed on the stack. Instructions that refer to bit registers are automatically performed with bit precision. For example mov rax, [rbx] moves 8 bytes beginning at rbx into rax. A special form of the mov instruction has been added for bit immediate constants or constant addresses. For all other instructions, immediate constants or constant addresses are still 32 bits. Instructions that refer to a single constant address are encoded as offsets from rip.

Instructions, such as jmpcallpushand popthat implicitly refer to the instruction pointer and the stack pointer treat them as 64 bits registers on x AMD 64 Developer Resources. You may also leave feedback directly on GitHub. Skip to main content. Exit focus mode. Registers x64 extends x86's 8 general-purpose registers to be bit, and adds 8 new bit registers. The x64 processor also provides several sets of floating-point registers: Eight bit x87 registers.

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