x86-64: 0x0F Prefix - Two-Byte Opcode

A single byte opcode can offer 256 possibilities of instructions, but we have tens of thousands of instructions and extensions now. There's no magic. The way of achieve such goal is called a Two-Byte Opcode. On x86-64, 0x0F indicates that the upcomming instruction's opcode consists of two bytes.

0x0F "Prefix"

An x86-64 instruction is encoded like this

Prefix

Opcode

Other Garbage

You might think that 0x0F is a prefix. But rather than a prefix defined by x86-64, it's an opcode prefix. So the way it works is that once you read an opcode starts with 0x0F, you should read the upfollow 2 bytes as the opcode.

A two-byte opcode is formated as

0x0F Prefix

Primary Opcode

Secondary Opcode

Take an example of vmlaunch from Intel VT, the opcode is 0F 01 C2, so the encoding is

0x0F Prefix	Primary Opcode	Secondary Opcode
0F	01	C2