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Chapter 2

Chapter 2. Instructions: Language of the Computer Part III. Representing Instructions. Instructions are encoded as binary strings Called machine code MIPS instructions Encoded as 32-bit instruction words Small number of formats Arithmetic Data transfer Immediate Regularity!.

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Chapter 2

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  1. Chapter 2 Instructions: Language of the Computer Part III

  2. Representing Instructions • Instructions are encoded as binary strings • Called machine code • MIPS instructions • Encoded as 32-bit instruction words • Small number of formats • Arithmetic • Data transfer • Immediate • Regularity! Florida A & M University - Department of Computer and Information Sciences

  3. Big Picture: Stored-Program • Instructions represented as binary numbers, just like data • Programs are stored in memory and can be read or written to just like data • SW/HW simplification • Memory technology for data used for programs • Programs can manipulate other programs (e.g. compiler)

  4. Stored-Program Consequences • Instructions and data have a memory address • Program Counter (PC) : register that keeps address of instruction being executed • Hardware pointer to memory • Intel calls it Instruction Address Pointer • Binary compatibility • Compiled programs can be executed on different computers • Standardized ISAs

  5. Instruction as Numbers • Simplicity • Instruction will be 32-bit word just like data • Word is divided into fields • Different instructions may need different field lengths • 5-bit field can only represent 0-31 • larger values needed (e.g. immediate for addi) • Design Principle: Good design demands good compromises • Multiple formats complicate decoding, but allow 32-bit instructions uniformly • Keep formats as similar as possible

  6. MIPS Instruction Formats • R-format : used for arithmetic instructions that have only register operands • I-format : used for data transfer instructions and arithmetic instructions that have a constant operand • J-format: used for j and jal (later)

  7. op rs rt rd shamt funct 6 bits 5 bits 5 bits 5 bits 5 bits 6 bits MIPS R-format Instructions • Instruction fields • op: operation code • rs: first source register number • rt: second source register number • rd: destination register number • shamt: shift amount (00000 for now) • funct: function code (extends op field) Florida A & M University - Department of Computer and Information Sciences

  8. MIPS R-format Instructions • Each field is viewed as a 5-bit or 6-bit unsigned integer, not as part of a 32-bit integer • Consequence: 5-bit fields can represent integers 0-31 and 6-bit fields can represent integers 0-63 • op: partially specifies what instruction it is • Note: This field is 000000 for all R-format instructions

  9. MIPS R-format Instructions • rs (Source Register): generally denotes register containing first operand • rt (Target Register): generally denotes register containing second operand (note that name is misleading) • rd (Destination Register): generallydenotes register which will receive result of computation • Exceptions: • mult and div have nothing important in the rd field since the destination registers are hi and lo • mfhi and mflo have nothing important in the rs and rt fields since the source is determined by the instruction

  10. MIPS R-format Instructions • shamt: the amount a shift instruction will shift by. • Shifting a 32-bit word by more than 31 is useless, so this field is only 5 bits. • This field is 00000 for all except the shift instructions • funct: combined with op, this number exactly specifies the instruction

  11. MIPS R-format Instructions Decimal values Green insert in textbook is a good resource

  12. R-format Example MIPS Instruction: add $t0,$t1,$t2 op = 0 funct = 32 rs = 9 (first operand - $t1) rt = 10 (second operand - $t2) rd = 8 (destination - $t0) shamt = 0 (not a shift) 000000 01001 01010 01000 00000 100000

  13. op rs rt rd shamt funct 6 bits 5 bits 5 bits 5 bits 5 bits 6 bits R-format Example add $t0, $s1, $s2 special $s1 $s2 $t0 0 add 0 17 18 8 0 32 000000 10001 10010 01000 00000 100000 000000100011001001000000001000002 = 0232402016 Florida A & M University - Department of Computer and Information Sciences

  14. R-format Example • Find the machine language instruction for the following C++ statement: a = b – d; • Recall: # $s0 is a, $s1 is b , $s3 is d sub $s0, $s1, $s3 #a = b – d 0 17 19 16 0 34 000000 10001 10011 10000 00000 100010 What is hexadecimal representation?

  15. op rs rt constant or address 6 bits 5 bits 5 bits 16 bits MIPS I-format Instructions • Immediate arithmetic and load/store instructions • rt: destination or source register number • constant: –215 to +215 – 1 • address: offset added to base address in rs Key Concept: Only one field is inconsistent with R-format. Most importantly, op is still in same location. Florida A & M University - Department of Computer and Information Sciences

  16. I-format Instructions • op: same as R-format except that since there is no funct field, op uniquely specifies an I-format instruction • This answers earlier question :Why aren’t op and funct a single 12-bit field? Consistent with other formats • Tells hardware whether to treat the last 16 bits of instruction as three separate fields (R-format) or one field (I-format) • rs (Source Register): register containing an operand

  17. MIPS I-format Instructions

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