Semiconductor & Computer Engineering
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== Implementation restrictions == | == Implementation restrictions == | ||
[[File:MIPS32 add and addi instructions.svg|thumb|right|300px|An encoding comparision between the [[MIPS32]] ADD and ADD Immediate instructions. The immediate value is limited to the 0x00-0xFFFF range.]] | [[File:MIPS32 add and addi instructions.svg|thumb|right|300px|An encoding comparision between the [[MIPS32]] ADD and ADD Immediate instructions. The immediate value is limited to the 0x00-0xFFFF range.]] | ||
| − | Because the immediate value is packed into the instruction itself certain [[ISA]]s 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 | + | Because the immediate value is packed into the instruction itself certain [[ISA]]s 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 rotate the bits as desired (see [[LSL - ARM|LSL]]). |
[[Category:microprocessor architecture]] | [[Category:microprocessor architecture]] | ||
[[Category:Instruction set architecture]] | [[Category:Instruction set architecture]] | ||
Revision as of 22:30, 19 July 2014
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 rotate the bits as desired (see LSL).