The caller uses registers to pass the first 6 arguments to the callee. The parameters are passed in order of declaration as with the pascal convention, and the first three parameters that qualify are passed in the eax, edx, and ecx registers, in that order. Calling convention we will use the standard linux function calling convention. The x64 application binary interface abi uses a fourregister fastcall calling convention by default. Two important differences between x86 and x64 are the 64bit addressing capability and a flat set of 16 64bit registers for general use. Register convention under the register convention, up to three parameters are passed in cpu registers, and the rest if any are passed on the stack. On linux x64, all argument and calleesaved registers must be preserved. The microsoft x64 calling convention is followed on windows and preboot uefi for long mode on x8664. Registers are saved by spilling them to the current functions stack frame. Beware that you must then declare the cdecl or regparm0 attribute for a function that will follow standard gcc calling conventions. An instruction set architecture isa is a very important term that applies to any cpu. In the linux, bsd and mac operating systems, there are fewer compatibility problems because a more or less official standard is defined.
The calling convention of the system v amd64 abi is followed on solaris, linux, freebsd, os x, and other unixlike or posixcompliant operating systems. Mar 27, 2018 the calling convention of the system v amd64 abi is followed on gnu linux. But it does not mean that the whole code of such a system is 64bit. Wiring in the nasmx source code portability to account for both windows and linux in either 32bit or 64bit was quite an undertaking mostly due to the x64 differences. I wanted to redo my early asm vids for a while, and. So i understand that thiscall doesnt really exist in x64 programming.
Instructions, memory addressing, registers, and many other architectural sections of a cpu are. The difference in register usage is interesting given the effort to have only one calling convention in x64 visavis 32bit. In the 64bit calling convention the caller passes the first four arguments in registers but must reserve space on the stack for them. In a pc context, 32bit means x86 and 64bit means x8664. Jul 19, 2018 on windows x64, i believe any volatile integer register can be trashed. That means rcx, rdx, r8, r9 for integer, struct or pointer arguments in that order, and xmm0, xmm1, xmm2, xmm3 for floating point arguments. At this point you should be aware of x64 calling convention refer. On windows x64, i believe any volatile integer register can be trashed. Mar 28, 2017 a new series on x64 assembly language. The language of your answer is asm actually machine code, so treat it as part of a program written in asm, not ccompiledforx86. Note that gcc has options to modify the calling conventions by reserving registers, having arguments in registers, not assuming the fpu, etc. Hardware extensions and all software standards save for a bios calling. Amd64 calling conventions for linux mac osx cse 378 fall 2010, section week 2.
However, i cant really find any definitive explanation as to where the this pointer is put to be passed to the callee is a x64 thiscall like a true cdecl call, where this is put on to the stack last on both gcc and msvc compilers edit. A single cpu architecture always have more than one possible calling convention but the industry has agreed to some general approach across the architectures form different producers. For caller cleanup and callee cleanup conventions these vary in x86 processing but have been unified in x8664. I have never encountered this convention before but the idea is that some parameters are passed through the registers and rest through the stack. Actually it uses different registers for function calls and it would be a great deal of work to rename these registers or. The registers rdi, rsi, rdx, rcx, r8, and r9 are used for integer and memory address arguments and xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6 and xmm7 are used for floating point arguments. Many of you are probably already familiar with the x64 calling convention 1 2 in 64bit windows where generally speaking first four parameters 3 are passed in registers rcx, rdx, r8 and r9 with 32 bytes of spill area reserved on stack just in case callee has to store the parameters on stack in order to free up the registers. The code uses the 64bit registers to build the stack frame. The first six integer or pointer arguments are passed in registers rdi, rsi, rdx, rcx, r8, and r9, while xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6 and xmm7 are used for floating point arguments. Theres a strict onetoone correspondence between the arguments to a function call and the registers used for. Calling conventions allow us to implement recursive functions and call. There is no universal standardeach compiler designer has a different implementation of the convention. Your function doesnt have to be easily callable from c with any standard calling convention. The x86 architecture is used with many different calling conventions.
Amd64 calling conventions for linux mac osx calling. The most important difference between x86 and x64 is that the former is the 32 bit architecture and the latter is the 64 bit instruction set architecture. So its not weird that functions are allowed to clobber the registers their args are in. The first six integer or pointer arguments are passed in registers rdi, rsi, rdx, rcx, r8, and r9 xmm0xmm7 are used for floating point arguments to make a system call in 64bit linux, place the system call number in rax, then its arguments, in order, in rdi, rsi, rdx, r10, r8, and r9, then invoke syscall. For these fpu stack registers, there is no calling convention, and they are preserved across context switches. Dec 17, 2018 the x64 application binary interface abi uses a fourregister fastcall calling convention by default. The calling convention of the system v amd64 abi is followed on gnulinux. And any parameters after that are pushed on the stack. I think everybody agrees that such an axe of calling conventions is noble.
Clarify calling conventions for profiler enter callback. Actually it uses different registers for function calls and it would be a great deal of work to rename these registers or even clone and rewrite the calls. In x8632 parameters for linux system call are passed using registers. The system v application binary interface is a set of specifications that detail calling conventions, object file formats, executable file formats, dynamic linking semantics, and much more for systems that complies with the xopen common application environment specification and the system v interface definition. Adhering to calling conventions ensures that your functions wont step. Due to the small number of architectural registers, the x86 calling conventions mostly pass arguments on the stack, while the return value or a pointer to it is passed in a register. There are 16 generalpurpose registers, and 16 xmm floating point registers. But yes, push imm8 pop reg is 3 bytes, and is fantastic for 64bit constants on x8664, where dec inc is 2 bytes. Large unix developers also ship 64bit versions, for example, linux debian 3. Where in the linux kernel source can i see it defined. The calling convention on the x8664 platform resembles the fastcall convention existing in x86. A simplified version of the most important points to keep in mind about the system v amd abi, which both mac and linux use, are.
System v amd64 abi system v amd64 abi dominating in linux and other unix systems, and mac os x. In computer science, a calling convention is an implementationlevel lowlevel scheme for how subroutines receive parameters from their caller and how they return a result. In each case functions with a small number of arguments 4 or 6 pass all arguments in registers. The focus will be on linux and other oses following the official system v amd64 abi. From what i read in wikipedia, i would have expected the arguments to open and read to be passed in rregisters rdi, rsi, rdx. When a unixlinux program does a file io, network data transfer or invo. This could be converted to a reference to a specific compiler my guess is borland. As a general rule, a function which follows the c calling conventions, and is appropriately declared see below in the c headers, can be called as a normal c function. In x8664, the number of calling conventions was greatly reduced. The x64 application binary interface abi uses a four register fast call calling convention by default. The first four arguments are placed onto the registers.
Note the calling convention covered here is different than the one used on x64 linux systems. The calling convention is defined in detail in system v application binary interfaceamd64 architecture processor supplement. Space is allocated on the call stack as a shadow store for callees to save those registers. Nearly all modern operating systems have versions for intel 64 architecture. Two important differences between x86 and x64 are the 64 bit addressing capability and a flat set of 16 64 bit registers for general use. Because the x86 instruction set uses 32bit registers and the x8664 instruction set uses 64bit registers, and because for each instruction set there is a single abi used by all linux installations. Platform, return value, parameter registers, additional parameters. Register usage and saving schema is also different to x64 abi clobber list difference to be described later. The x64 calling convention was able to get rid of vestigial pieces like calling convention options and nearly drop x87, but this bit of history sticks with us. This reduces the cost of a function call, because operations with registers are faster than with the stack. Linux 64abi, calling convention a passionate techie. On x86 64 linux, the first six function arguments are passed in registers %rdi. Note there are no calleesaved floatingpoint registers.
Most of this standard is followed by gnu compilers version 3. Here is a quick overview of common calling conventions. There are only two calling conventions in use, effectively windows and unix. This article describes the calling conventions used when programming x86 architecture. The documentation of calling conventions and binary interfaces of compilers and operating systems is often shamefully poor and sometimes completely absent. For concreteness, we learn the x8664 calling conventions for linux.
For the microsoft x64 calling convention, the additional register space let fastcall be the only calling convention under x86 there were many. The biggest problem i now encounter is that i learned that linux uses a different calling convention for 64bit than windows does. Tips for golfing in x86x64 machine code code golf stack. The x87 register stack is unused, and may be used by the callee, but must be. Under linux this would be called an application binary interface abi. The system v abi is one of the major abis in use today and is virtually universal among unix systems. Amd64 calling conventions for linux mac osx cse 378 fall 2010, section week 2 calling conventions calling conventions are a scheme for how functions receive parameters from their caller and how they return a result. Choose your calling convention to put args where you want them. The registers rdi, rsi, rdx, rcx, r8, and r9 are used for.
Feb 06, 2018 im reading intel manual about calling convention and which register has which purpose. I read here that the arguments after rdx are esi, edi, ebp or for x64 rsi, rdi, rbp, i dont see it documented in wikipedias page for calling conventions, but int80h seems to indicate that windows also uses this convention. That still doesnt tell you which has the this pointer. Which registers the called function must preserve for the caller. Difference between x86 and x64 compare the difference. A common x64 calling convention is the microsoft 64 calling convention used for c style function calling see msdn, chen, and pietrek. The calling convention of the system v amd64 abi is followed on gnu linux. Im reading intel manual about calling convention and which register has which purpose. Userlevel applications use as integer registers for passing the sequence %rdi, %rsi, %rdx, %rcx, %r8. Theres a strict onetoone correspondence between the arguments to a function call and the registers used for those arguments. Amd64 calling conventions for linux mac osx calling conventions.
From what i read in wikipedia, i would have expected the arguments to open and read to be passed in r registers rdi, rsi, rdx. The language never specified a calling convention, any more than the c language did. In this article i will examine the stack frame layout of the newer 64bit version of the x86 architecture, x64. Btw, in most stackargs calling conventions including args that dont fit in registers in the x8664 system v convention used on linux, functions are allowed to modify their args on the stack.
See the excellent wikipedia article on calling conventions. We will summarize the calling convention as it applies to decaf. The registers rdi, rsi, rdx, rcx, r8, and r9 are used for integer and memory address arguments and xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6 and xmm7 are used for floating point arguments for system calls, r10 is used instead of rcx. And push r64 pop 64 2 bytes can even replace a 3 byte mov r64, r64 3 bytes with rex. Difference between windows native and system v x64 abis. Writing small programs to demonstrate these rules is a pleasant exercise. The 32bit architecture has 32 registers while x64 extends x86s 8 generalpurpose registers to be 64 bit. After the called function has returned some calling conventions have one more instruction in the call sequence to clean the stack.
To understand the c calling convention, you should. In fact, this subroutine was quite simple it did not modify any registers except eax or. The article concluded with a useful diagram presenting the stack frame layout of a typical function call. Stack frame layout on x8664 eli benderskys website. I flip back and forth between the two calling conventions when discussing which is better as each plays to the strengths of its respective target os. Is there much difference between x86 assembly language on. In the x64convention, the first four integer arguments left to right are passed in 64bit registers chosen specially for this purpose. Since some of x86s registers have special implicit meanings and arent really used as generalpurpose most notably ebp and esp, the effective increase is even larger. As a general rule, a function which follows the c calling conventions, and is. Oct 30, 20 as far as i know there is only one calling convention in x64 fastcall. In this vid, well look at few general aspects of asm, before diving in and coding a few simple examples.
The microsoft x64 calling convention is followed on windows and pre boot uefi for long mode on x86. Is an x64 assembly function responsible for preserving. This provides a welldefined location for the called program to save and restore the corresponding argument if the value passed in the register would otherwise be overwritten. X64 calling convention this article describes the calling.
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