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An extension for PHP adding inline Assembly support.

The sources are hosted on GitHub.

What the..?

Seriously. This extension for PHP adds an asm() function that will assemble, link and run Assembly code from within a PHP script.

What for, you ask? Well I'm glad you did! Here's a list of good reasons to use this feature:

  • uhh..

(end of list)

Actually I was writing another piece of Assembly code (chASM, just about as practical as this) and the idea of adding inline Assembly to PHP came up repeatedly. The thought made me laugh, so eventually I gave in and just implemented it.

You won't get any performance benefit from small chunks of Assembly code (quite the opposite, actually) because the overhead is significant: this extension uses just-in-time assembly.

For the record, I haven't seen the words "just-in-time assembly" used anywhere in 20+ years of software development. If the phrase is up for grabs I'm calling dibs.


  • a 64-bit x86 processor (physical or virtual)
  • a 64-bit operating system using the System V calling convention, e.g. Linux (tested with Ubuntu 17.04)
  • a 64-bit version of PHP 7 (tested with 7.0.18)
  • NASM (tested with 2.12.02)
  • build tools for PHP


You can either compile this as a static extension into new php binaries, or compile it as a shared extension that can be loaded with the extension=<file> directive in PHP's .ini files.

Static Extension


You'll need to extract the PHP sources somewhere, e.g. /usr/src/php7/. All paths in the installation instructions below are relative to this sources root.

It's recommended you successfully build the PHP sources before adding this extension. This will make any troubleshooting faster and easier.

You need to copy this code into a new directory, ext/chiasm/.


You'll need to tell PHP to look for and include the new extension:

$ buildconf -force
$ ./configure --enable-chiasm

If configure reports that the --enable-chiasm parameter is invalid then you might not have extracted the extension to ext/chiasm/. Once configure works you won't have to run buildconf again (unless you e.g. rename the extension).

You can now build PHP with the new module.

$ make

If you haven't built this PHP version before this might take a few minutes, otherwise it should finish quickly. Once make is done the new binaries are ready.


You can run the .phpt tests (make test TESTS=ext/chiasm, see "Tests" below) to make sure everything works as expected. There's a small demonstration script, you can run it with:

$ sapi/cli/php -f ext/chiasm/demo.php

As of writing this it should output this:

result should be true: true
out1 should be 5<<3-6=34: 34
out2 should be 0.5*0.5=0.25: 0.25
out3 should be false: false

Check out demo.php for how this is done.

Shared Extension

Please consider carefully whether you really want to do this. If you go this route on e.g. a web server the served PHP code gets access to the inline Assembly function.


You'll need:

  • PHP and the phpize tool installed, e.g. phpize --help should output its usage information
  • root/sudo access to install the new module globally
  • this extension's sources somewhere, the following commands are executed in that directory


Run these commands:

$ phpize && ./configure && make

This should show a bit of output without any error messages, the end should say something like:

Build complete.
Don't forget to run 'make test'.

The tests won't work yet, you still need to first install the module:

$ sudo make install

and then include it by adding this line somewhere in the .ini file(s):

The exact method of doing this depends on the OS, for example on Ubuntu 17.04 there's a /etc/php/7.0/cli/conf.d/ directory that contains individual .ini files to add/configure modules for the CLI version of PHP. In there create a new file, chiasm.ini, and add the above line.


After that the extension should be loaded and you can run the tests with:

$ make tests

This should report all tests passed.

If you added the extension to a web server (and you're really, really sure that's what you want), you may have to restart the web server before the module is available.

The demo script should work too - currently it should output this:

$ php -f demo.php
result should be true: true
out1 should be 5<<3-6=34: 34
out2 should be 0.5*0.5=0.25: 0.25
out3 should be false: false



If the extension is loaded there's a new PHP function asm:

mixed asm(string $code, ?array $inputs=NULL, ?array $outputs=NULL)

$code is the Assembly code to execute, in Intel syntax (NASM is used to assemble it).

$inputs is an optional array of input variables. They're passed by value or by reference into the Assembly code. For example:


asm("mov r8,[rax]    ;copy $in1's value into r8
     mov r9,[rax+8]  ;copy $in1's type into r9
     mov r10,rbx     ;copy $in2's value into r10",
    [&$in1,$in2]);   //pass $in1 by reference, $in2 by value

Note that booleans and NULLs are differentiated in a ZVAL's type field, so they need to be passed as references.

$outputs is an optional array of output variables. They're passed by reference into the Assembly code.

Some internal errors will make asm() return a string with the error message.


Variables, regardless of whether they're by value or by reference, are passed in these 13 GPRs:

rax, rbx, rcx, rdx, rbp, r8, r9, r10, r11, r12, r13, r14, r15

These registers are used left to right for input variables (rax is the first input, rbx the second and so on) and right to left for output variables (r15 being the first output, r14 the second and so on). If more than 13 inputs and outputs total are passed to the asm() function it will error out.

The other 3 GPRs (rsp, rsi and rdi) have special meanings:

rsi points to the return value ZVAL.

rdi points to the zend_execute_data context.

rsp needs to be preserved, previous register values are saved/restored on the stack.

Volatile Registers

The asm() function will check your Assembly code for all used GPRs. This list of GPRs, in addition to all input and output GPRs will be pushed to the stack before your code, and popped from the stack after your code.

Currently no other registers are preserved.

Implicitly modified GPRs will not be detected, for example the CPUID instruction changes the rax-rdx registers by design. If you call such a function you'll have to manually mark those registers as volatile by including them in your inline assembly code somewhere. If you aren't already using those registers anyway (in which case they're already recognized as volatile) you can just add a comment line that lists the remaining registers.

Stack Alignment

The generated assembly preamble will automatically align the stack to 128 bits. The first line of inline assembly code can assume that rsp%16 == 0. For example it's perfectly valid to have the first code line perform a 128bit-aligned AVX move like MOVDQA.

The generated assembly trailer will undo the automatic alignment, i.e. the inline assembly code should leave rsp the way it found it.


The code:

  mov qword [rsi+8],3   ;set return type to true

will output:



This extension contains .phpt tests. How to execute them depends on whether you built a shared or static extension, see Installation above.

All tests should pass.



Do not use this PHP extension in production or another environment where user input, transmitted files or any other kind of data not 100% under the developer's control finds its way to the asm() function.

Abusing this extension can crash PHP, your web server, be used as an attack vector in privilege escalations, compromise your network, crash your entire system and potentially physically damage your hardware.



Copyright (C) 2017-2018 Richard Nusser


This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see


an extension for PHP adding inline Assembly support





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