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ARM instruction encoding

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• ARM processor
  • ARM:

    ARM = ACORN/Advanced RISC Machine is a family of Reduced Instruction Set Computing architecture for computer processors Wiki: click here

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  • The ARM processor nowadays is used in many appliances:

    Mobile phones Tablet computers EBook readers Smart TV Cable TV boxes Blue Ray/DVD players              Microwave oven Washing machines etc. etc (See: click here)

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• ARM instruction codes
  • Fixed length:

    Each ARM instruction is encoded (= represented) in 32 bits

    Here is the encoding of the ARM instructions:

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  • Example 1:

    The instruction add r1, r2, r3 (r1 = r2 + r3) is represented (encoded) as: 11100000100000100001000000000011 Break down the bit pattern according to Data processing instruction type: Cond 00 I OpCo S SrR1 Dest Operand-2 ---------------------------------------------------- 1110 00 0 0100 0 0010 0001 000000000011 ^ ----- ^ ---- ---- ------------ | add | r2 r1 reg #3 I S Cond 1110 mean: don't set the conditional flags (explained later) I = 0 means: use Operand-2 as a register number

  • Example 2:

    The instruction add r1, r2, 3 (r1 = r2 + 3) is represented (encoded) as: 11100010100000100001000000000011 (The only difference between this and the previous instruction is the red 1 bit) Break down the bit pattern according to Data processing instruction type: Cond 00 I OpCo S SrR1 Dest Operand-2 ---------------------------------------------------- 1110 00 1 0100 0 0010 0001 000000000011 ^ ----- ^ ---- ---- ------------ | add | r2 r1 int value 3 I S I = 1 means: use Operand-2 as an integer value

    Note:

    You don't need to know how each computer instruction is encoded !!! I just want to show you that every instruction/command that a computer executes is represented by a unique bit pattern

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/instr-code.s

    How to run the demo:

    Compile with as255 instr-code Look in a.lst for the instruction codes          Alert: little endian !!!

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  • Here is a full description of the ARM instructions with their encoding if you want to know more: click here

    (We will learn a partial (sub)set in this course to understand what's going on when a computer execute a program):