ARM Immediate Value Encoding

ARM Immediate Value Encoding

• ARM instruction set encodes immediate values in an unusual way, using 12 bits of instruction space to represent a useful set of 32-bit constants.

Machine Code and ARM Processors

• Machine code is the binary representation of simple instructions that computer processors run on.
• ARM processors (like those in iPhones and other devices) have 16 basic data processing instructions.

Data Processing Instructions

• Each data processing instruction can work with several combinations of operands.
• Examples of ADD instructions include:
  • ADD r0, r2, r3 (r0 = r2 + r3)
  • ADD r0, r2, r3, LSL #4 (r0 = r2 + (r3 << 4))

Immediate Value Encoding

• The immediate value is represented using 12 bits of instruction space.
• The encoding scheme allows for a useful set of 32-bit constants to be represented.

Example Immediate Value

• The immediate value 0x000000D7 can be represented in the instruction space.
• The binary representation of this value is: 00000000 00000000 00000000 11010111.
• The value can be rotated right by 0 bits to get 215 (0xD7).