instructions

An instruction consists of

• optional up to four in total 1-byte (legacy) prefixes (in any order)
• 1 to three primary opcode bytes (opcode or 0f opcode or 0f 38 opcode or of 3a opcode)
• if required: an addressing form specifier that consists of the ModR/M byte and sometimes also of the scale-index-base byte.
• if required: an address displacement (1, 2 or 4 bytes)
• if required: immediate data (1, 2 or 4 bytes)

For details refer to chapter two (»instruction format«) and appendix B (»Instruction formats and encoings«) in volume two (»instruction set reference«) in Intel® 64 and IA-32 Architectures Software Developer’s Manual.

x86-64: syscall

x86: sysenter.

legacy: int 0x80

In the 80386 days, the fastest way to switch from V86 mode to kernel mode was to execute an invalid instruction.

A hypervisor needs specialized instructions:

• VT-x on Intel
• SVM on AMD