CIL nop, break, localloc and the reserved opcode bytes

Introduction

Three real instructions that belong with no group, and the eight reserved opcode bytes.

Exceptions and verifiability, for the whole page. nop and break throw nothing; localloc can fail as described below.

Instructions

Opcode	Instruction	Stack	What it does	Exceptions · verifiability
`00`	`nop`	… → …	Does nothing. Compilers emit it in debug builds so that a breakpoint has an instruction to stop on, and to pad a branch target.	None.
`01`	`break`	… → …	Tells the debugger that a breakpoint was reached, so it stops here (the instruction behind `Debugger.Break`). Without a debugger it does nothing.	None.
`FE 0F`	`localloc`	…, size → …, pointer	Pops a size in bytes (a native unsigned integer) and allocates a block of that size on the stack frame, pushing an unmanaged pointer to it. The block lives until the method returns. It is zeroed if the body’s InitLocals flag is set. It must be used with only the size on the stack, and cannot be in a filter, catch, finally or fault block. C# `stackalloc` compiles to it.	StackOverflowException if there is no room; OutOfMemoryException. Not verifiable.

Reserved bytes

The runtime library’s opcode table (System.Reflection.Emit.OpCodes) lists eight more single-byte opcodes, all with no operand and no stack effect and of a type it calls Nternal: they are the bytes F8 to FF. They are reserved for the runtime’s own use and are not instructions a runtime accepts in a body. The explorer decodes them as the instructions the table names and reports each one as a problem on the body. FE, the one that matters, is the first byte of the two-byte opcodes (see the opcode map) and is never an instruction alone.

Opcode	Name in the table	Meaning
`F8`	`prefix7`	Reserved.
`F9`	`prefix6`	Reserved.
`FA`	`prefix5`	Reserved.
`FB`	`prefix4`	Reserved.
`FC`	`prefix3`	Reserved.
`FD`	`prefix2`	Reserved.
`FE`	`prefix1`	The table’s name for the byte that starts every two-byte opcode.
`FF`	`prefixref`	Reserved.

Not described

Known, not yet covered

What the reserved bytes were meant for: the table gives their names only; their purpose is not documented here.

Unknown

Whether any tool uses the reserved bytes in practice.

.CIL File Format – Other instructions