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First, a few definitions: Pipelining : Multiple instructions being executed, each in a different stage of their execution. A form of parallelism. Super Pipelining : Advertising term, just longer pipelines. Super Scalar : Having multiple ALU's. There may be a mix of some integer ALU's and some Floating Point ALU's. Multiple Issue : Starting a few instructions every clock. The CPI can be a fraction, 4 issue gives a CPI of 1/4 . Dynamic Pipeline : This may include all of the above and also can reorder instructions, use data forwarding and hazard workarounds. Pipeline Stages : For our study of the MIPS architecture, IF Instruction Fetch stage ID Instruction Decode stage EX Execute stage MEM Memory access stage WB Write Back into register stage Hyper anything : Generally advertising terminology. Consider the single cycle machine in the previous lecture. The goal is to speed up the execution of programs, long sequences of instructions. Keeping the same manufacturing technology, we can look at speeding up the clock by inserting clocked registers at key points. Note the placement of blue registers that tries to minimize the gate delay time between any pair of registers. Thus, allowing a faster clock. This is called approximate because some additional design must be performed, mostly on "control", that must now be distributed. The next step in the design, for our project, is to pass the instruction along the pipeline and keep the design of each stage of the pipeline simple, just driven by the instruction presently in that stage. pipe1.vhdl implementation moves instruction note clock and reset generation look at register behavioral implementation instruction memory is preloaded pipe1.out just numbers used for demonstrationPipelined Architecture with distributed control
pipe2.vhdl note additional entities equal6 for easy decoding data memory behavioral implementation pipe2.out instructions move through stagesTiming analysis
Consider four instructions being executed. First on the single cycle architecture, needing 8ns per instruction. The time for each part of the circuit is shown. The clock would be: +---------------+ +---------------+ +------ | | | | | -+ +---------------+ +---------------+ Single cycle execution 125MHZ clock 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17ns | | | | | | | | | | | | | | | | | | +-------+---+-------+-------+---+ |IF |ID | EX | MEM |WB | +-------+---+-------+-------+---+ +-------+---+-------+-------+---+ |IF |ID | EX | MEM |WB | +-------+---+-------+-------+---+ +--- |IF ... 24ns +--- ... 32ns The four instructions finished in 32ns. An instruction started every 8ns. An instruction finished every 8ns. Now, the pipelined architecture has the clock determined by the slowest part between clocked registers. Typically, the ALU. Thus use the same ALU time as above, the clock would be: +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ | | | | | | | | | | | | | | | | | | -+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +- Pipelined Execution 500MHZ clock ** +-------+-------+-------+-------+-------+ |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ ** | | | | | | | | | | | | | | | | | | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17ns The four instructions finished in 16ns. (But, the speedup is not 2) An instruction started every 2ns. An instruction finished every 2ns. Thus, the speedup is 8ns/2ns = 4 . Since an instruction finishes every 2ns for the pipelined architecture and every 8ns for the single cycle architecture, the speedup will be 8ns/2ns = 4. The speedup would change with various numbers of instructions if the total time was used. Thus, the time between the start or end of adjacent instructions is used in computing speedup. Note the ** above in the pipeline. The first of the four instructions may load a value in a register. This load takes place on the falling edge of the clock. The fourth instruction is the earliest instruction that could use the register loaded by the first instruction. The use of the register comes after the rising edge of the clock. Thus use of both halves of the clock cycle is important to this architecture and to many modern computer architectures. Remember, every stage of the pipeline must be the same time duration. The system clock is used by all pipeline registers. The slowest stage determines this time duration and thus determines the maximum clock frequency. The worse case delay that does not happen often because of optimizing compilers, is a load word, lw, instruction followed by an instruction that needs the value just loaded. The sequence of instructions, for this unoptimized architecture, would be: lw $1,val($0) load the 32 bit value at location val into register 1 nop nop addi $2,21($1) register 1 is available, add 21 and put result into reg 2 As can be seen in the pipelined timing below, lw would load register 1 by 9ns and register 1 would be used by addi by 10ns (**). The actual add would be finished by 12 ns and register 2 updated sum by 15 ns (***). +-------+-------+-------+-------+-------+ lw $1,val($0)|IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ nop |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ nop |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ +-------+-------+-------+-------+-------+ addi $2,21($1) |IF |ID reg| EX | MEM |reg WB | +-------+-------+-------+-------+-------+ ** *** | | | | | | | | | | | | | | | | | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ns It is interesting to note some similarity to an IBM Power PC that came a few years after the MIPS R3000 architecture that is similar to the above design. IBM Power PC stages and clock usage new IBM Power PC Shipped 2012 at 5.5Ghz
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