CPU MUST DO THE FOLLOWING THINGS : Fetch instruction --- Read instruction from memory Interpret instruction --- The instruction is decoded Fetch data --- Read data from memory or an I/O module Process data --- Perform arithmetic or logical operation Write data --- Write data to memory or an I/O module
REGISTER ORGANIZATION CARLOS GARRIDO & JORGE MONTENEGRO
REGISTERS CPU must have some working space (temporary storage) Called registers Number and function vary between processor designs One of the major design decisions Top level of memory arrangement
USER VISIBLE REGISTERS General Purpose Data Address Condition Codes
HOW MANY GP REGISTERS? Between 8 - 32 Fewer = more memory references More does not reduce memory references and takes up processor real estate See also RISC One cycle execution time Pipelining Large number of registers
HOW BIG? Large enough to hold full address Large enough to hold full word Often possible to combine two data registers C programming double int a; long int a;
CONDITION CODE REGISTERS ADVANTAGES: Because condition codes are set by normal arithmetic and data movement instructions Conditional instructions, such as BRANCH are simplified relative to composite instructions, such as TEST AND BRANCH. Condition codes facilitate multi-way branches. For example, a TEST instruction can be followed by two branches, one on less than or equal to zero and one on greater than zero.
CONDITION CODE REGISTERS DISADVANTAGES: Condition codes add complexity, both to the hardware and software. Condition code bits are often modified in different ways by different instructions. Condition codes are irregular, they are typically not part of the main data path, so they require extra hardware connections. Often condition code machines must add special non-condition-code instructions for special situations anyway. In a pipelined implementation, condition codes require special synchronization to avoid conflicts.
CONTROL AND STATUS REGISTERS Program Counter (PC): Contains the address of an instruction to be fetched. Instruction Decoding Register (IR): Contains the instruction most recently fetched. Memory Address Register (MAR): Contains the address of a location in memory Memory Buffer Register (MBR): Contains a word of data to be written to memory or the most recently read.
PROGRAM STATUS WORD (PSW) Sign: Contains the sign bit of the result of the last arithmetic operation. Zero: Set when the register is “0” Carry: Set if an operation resulted in a carry addition or subtraction of a higher order bit Equal: Set if a logical compare result is equality. Overflow: Used to indicated arithmetic overflow. Interrupt enable/disable: Used to enable or disable interrupts.
SUPERVISOR MODE Supervisor: Indicates whether the processor is executing in supervisor mode or user mode Privilege instruction Address space Memory management Protection domain or Protection Ring Ring Kernel
SECTION 12.3 INSTRUCTION CYCLE AN INSTRUCTION CYCLE (SOMETIMES CALLED FETCH-AND- EXECUTE CYCLE, FETCH-DECODE- EXECUTE CYCLE, OR FDX) IS THE BASIC OPERATION CYCLE OF A COMPUTER. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions. This cycle is repeated continuously by the central processing unit (CPU), from bootup to when the computer is shut down.
The circuits used in the CPU during the cycle are: Program Counter (PC) – Memory Address Register (MAR) - Memory Data Register (MDR) - Instruction register (IR) – Control Unit (CU) - Arithmetic logic unit (ALU) - There are typically four stages of an instruction cycle that the CPU carries out: 1)Fetch the instruction from memory. 2) "Decode" the instruction. 3) "Read the effective address" from memory if the instruction has an indirect address. 4) "Execute" the instruction.
Fetch Reads the next instruction from memory into the processor. Indirect Cycle May require memory access to fetch operands, therefore more memory accesses. Interrupt Save current instruction and service the interrupt. Execute Interpret the opcode and perform the indicated operation. The instruction cycle is the time in which a single instruction is fetched from memory, decoded, and executed. THE FOUR SUB-CYCLES:
There are six fundamental phases of the instruction cycle: 1.) fetch instruction (aka pre-fetch) 2.) decode instruction 3.) evaluate address (address generation) 4.) fetch operands (read memory data) 5.) execute (ALU access) 6.) store result (writeback memory data)
DECODE EVALUATE AND FETCH Decoding the instruction? The decoder interprets what? What is being fetched from memory? What decision is made next? Based on the decision what are the options? What if decision is a direct memory operation? What if decision is an indirect memory operation?
INSTRUCTION CYCLE WITH AND WITHOUT INDIRECT CYCLE…
SAMPLE QUESTION: Given that the instruction cycle is the time in which a single instruction is fetched from memory, decoded, and executed: A microprocessor provides an instruction capable of moving a string of bytes from one area of memory to another. The fetching and initial decoding of the instruction takes 10 clock cycles.Thereafter, it takes 15 clock cycles to transfer each byte.The microprocessor is clocked at a rate of 10 GHz. Determine the length of the instruction cycle for the case of a string of 64 bytes. ANSWER: The length of a clock cycle is 0.1 ns. The length of the instruction cycle for this case is [10 + (15 × 64)] × 0.1 = 960 ns.
ANOTHER EXAMPLE: TOTAL NUMBER OF CYCLES REQUIRED To execute the SAL instruction: add A, B, C 1.) Fetch instruction (add) from memory address PC. 2.) Increment PC to address of next instruction. 3.) Decode the instruction and operands. 4.) Load the operands B and C from memory. 5.) Execute the add operation. 6.) Store the result into memory location A. Execution Time Suppose each memory access (fetch, load, store) requires 10 clock cycles and that the PC update, instruction decode, and execution each require 1 clock cycle. The total number of cycles to execute the add instruction is: 10+1+1+10+10+1+10 = 43 cycles/instruction. A CPU running at 100 Mhz (100,000,000 cycles/sec) can execute add instructions at a rate of 100,000,000/43 = 2,325,581 instructions/sec, or ~2.3 Mips (million instructions/sec).
DATA FLOW: EXECUTE CYCLE The execute cycle: Takes many forms the form depends on which of the various machine instructions is in the IR. This cycle may involve transferring data among registers read or write from memory I/O invocation of the ALU
INSTRUCTION PIPELINE ADRIAN SUAREZ & DIEGO ARIAS
INSTRUCTION PIPELINING By separating an instruction cycle into stages, multiple instructions at different stages can be worked on at the same time. For example, Stage 2 of the current instruction can be overlapped with Stage 1 of the next instruction.
A TWO-STAGE PIPELINE An instruction cycle can be divided into two stages: Fetch: get an op-code from main memory and put it in a register Execute: decode an op-code and execute the instruction The execute stage of the current instruction would overlap with the fetch stage of the next instruction. Assuming that fetch and execute use the same number of clock cycles, this would double the speed (in reality, execute takes longer).
A SIX-STAGE PIPELINE Fetch instruction (FI): get op-code from memory and put it in a register Decode instruction (DI): decode op-code and determine addressing mode Calculate operand (CO): get effective address of source operands Fetch operands (FO): get operands from memory and put them in registers Execute instruction (EI): execute instruction and write result to a register Write operand (WO): store the result in memory This pipeline is more typical of modern computers, especially RISC computers (e.g. MIPS, SPARC, and DLX). Each stage occupies about the same number of clock cycles.
WHY NOT A 100-STAGE PIPELINE? If 1 instruction per cycle can be achieved with a 5-stage pipeline, adding more stages would only increase the number registers without increasing speed (it might actually make the computer less efficient). Overlapping of instructions requires additional logic to account for dependencies between instructions (i.e. a memory read after a memory write to the same location).
PIPELINE HAZARDS Resource hazards: when two instructions in the pipeline need to use the same resource Data hazards: when two instructions must be executed in sequence (i.e. a memory write followed by a memory read) Branch hazards: when a conditional branch occurs and the pipeline fetches the wrong instructions
RESOURCE HAZARDS A resource hazard occurs when two stages in the pipeline need to use the same resource at the same time. For example, two stages need to read from main memory (assuming that the data hasn’t been cached) when an operand fetch is overlapped with an instruction fetch. In this case, the two stages must be executed in series rather than in parallel, and a delay must be introduced in the pipeline.
DATA HAZARDS A data hazard occurs whenever data is fetched from a location before it contains the correct value. The “correct” value is whatever value it would contain if the instructions were executed in sequence. Whenever the fetch stage must access data that hasn’t yet been written, the pipeline must be delayed at the fetch stage.
ADD EAX, EBX SUB ECX, EAX ; I3 ; I4 DATA HAZARDS
PIPELINE IMPLEMENTATION A pipeline is implemented as a series of sequential circuits, with each stage taking its input from the output of the previous stage
DEALING WITH BRANCHES The most difficult part in designing an instruction pipeline is assuring a steady flow of instructions to the initial stages of the pipeline. Several approaches have been taken for dealing with conditional branches. Multiple streams Prefetch branch target Loop buffer Branch prediction Delayed branch.
MULTIPLE STREAMS A pipeline has disadvantages for a branch instruction because it must choose one of two instructions to fetch the next instruction and may take the wrong choice. One way of dealing with this is to allow the pipeline to fetch both instructions, making use of both streams. With multiple pipelines there are delays for accessing register and memory. Additional branch instructions may enter the pipeline before the original branch decision is resolved.
PREFETCH BRANCH TARGET The target of the branch is prefetched when a conditional branch is recognized in addition to the instruction following the branch. The target is saved until execution, if a branch is taken that means that it has already been prefetched.
LOOP BUFFER A loop buffer is high speed memory that works in sequence with the instruction fetch stage of the pipeline and it contains the most recently fetched instruction. Instructions fetched in sequence will be available without the usual memory access time. If a branch occurs to be ahead of the address of the branch instruction, the target will already be in the buffer. If the loop buffer is large enough to contain all the instruction in the loop, then the instructions will only have to be fetched once.
BRANCH PREDICTION The are several techniques to predict whether or not a branch will be taken. Predict never taken Predict always taken Predict by opcode Taken/ not taken switch Branch history table
DELAYED BRANCH It’s possible to improve the performance of a pipeline be rearranging instructions within a program, so that the instructions occurs later than actually desired. This branch will not take effect until after the execution of the following instruction.
INTEL 80486 PIPELINING The Intel 80486 implements a five stage pipeline. Fetch Decode stage 1 Decode stage 2 Execute Write back
QUESTIONS 1 What’s the function of internal processing bus? 2 What’s the similarity between the internal structure as a whole and the internal structure of the CPU? 3. What is an instruction cycle? 4. What are the four sub cycles of an instruction cycle? 5. Is the fetch or execute cycle the same for all CPU? 6. What is the sequence of an interrupt cycle? 7. How does pipelining increase processor speed? 8. What are some pipeline hazards? 9. Which computers use a 5-stage pipeline? 10. What are the five ways to deal with conditional branches? 11. What happens in the fetch cycle inside an Intel 80486?
REFERENCES Computer Organization and Architecture, Designing for Performance, 8/E, Stallings, William Embedded System Design: A Unified Hardware/Software Approach, Vahid, Frank, and Givargis, Tony Wikipedia, “Instruction Cycle” http://en.wikipedia.org/wiki/Instruction_cycle CIS-77 Introduction to Computer Systems http://www.c-jump.com/CIS77/CPU/InstrCycle/lecture.html