An immediate value (or simply an immediate or imm) is a piece of data that is stored as part of the instruction itself instead of being in a memory location or a register. Immediate values are typically used in instructions that load a value or performs an arithmetic or a logical operation on a constant.

Overview

Under most instruction set architectures, various instructions can also perform operation on constant values. For example consider an ISA that can add two registers and store the result in a third register:

add r3, r2, r1       ; r3 = r2 + r1

The instruction set may also support the common case of adding a constant value (such as in the case of many programming languages that support ++var).

add r1, r1, 1       ; r1 = r1 + 1

In the code above, the value 1 is an immediate value that gets encoded in the add instruction itself.

Implementation restrictions

Because the immediate value is packed into the instruction itself certain ISAs have a restricted range of values that can be used as an immediate value. For example in MIPS32, an immediate value is limited to 16-bits. On some more complex architectures such as ARM, some instructions may accept a 16-bit value, others might accepted a smaller range with a the ability to rate the bits as desired.