1 / 17

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!.

acton
Download Presentation

Chapter 2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related