Disk images to boot and run: https://copy.sh/v86/
A list of all of the interrupts in the 32-bit x86 CPU architecture. Deep magick lies beyond this link...
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A cheat-sheet for reverse engineering malware, by Lenny Zeltser.
A free and libre textbook of OS design and implementation on the x86 architecture. The full book can be read here or downloaded as a PDF from this site, or you can check out the book from its Github repository (https://github.com/littleosbook/littleosbook)
BinNavi is a tool for performing static and dynamic reverse engineering of executables for a number of platforms - x86, ARM, PowerPC, and MIPS. Can connect to remote debuggers on other systems to examine running code. Extensible with scripts, can annotate call flow graphs. The source is on Github: https://github.com/google/binnavi
Unicorn is a super-CPU emulator which handles multiple common processor core architectures, from x86 to Arm. Based on QEMU, written in pure C for speed and portability. Has multiple API bindings for different languages. Does just-in-time (JIT) translation of opcodes. Designed from the ground up to be thread-safe.
A utility that lets you edit disks and recover data, including entire partitions. Supports multiple operating systems.