Clock cycle

A pipelined floating-point multiply unit might accomplish a floating-point multiply by performing four independent suboperations, labeled a, b, c, and d, on the operands. The suboperations can be envisioned as the four workers on a four-person assembly line. The floating-point multiply pipeline could accept a new set of operands every clock cycle. The pipeline occupancy of this code fragment would look like... 

In other words, X is being computed during cycles 0—3, Z is being computed during cycles 1—4, and lU is being computed during cycles 5—8. The code fragment is complete after nine clock cycles. 

Now the computation of IF can begin on the fifth clock cycle, rather than on the sixth. 

The code fragment is now finished after the eighth clock cycle. Note that there are stiU three clock cycles during which there are idle stages in the multiplication pipeline. The compiler would look for other statements in the code that could be overlapped with those already in process. 

On a vector computer having vector registers that hold 64 floating-point numbers, this loop would be processed 64 elements at a time. The first 64 elements of Y would be fetched from memory and stored in a vector register. Each iteration of the loop is independent of the previous iteration, so this loop can be fliUy pipelined, with successive iterations started every clock cycle. Once the pipeline is filled, the result, X, will be produced one element per clock cycle and will be stored in another vector register. The results in the vector register will then be stored back into main memory or used as input to a subsequent vector operation. 

Banked Memory. Another characteristic of many vector supercomputers is banked memory. The main memory is usually divided into a small number of electronically separate banks. A given memory bank can absorb or supply operands at a much slower rate than the rate at which the central processing unit (CPU) can produce or use data. If the data can be spread across multiple memory banks, the effective memory bandwidth, or rate at which memory can absorb or supply data, is increased. For example, if a single memory bank can supply one operand every 16 clock cycles, then 16 memory banks would enable the entire memory subsystem to deflver one operand per clock cycle, assuming that the data come sequentially from different memory banks. 

The most common loop has stride = 1. Typically X(l) would be stored in memory bank 1, X(2) in memory bank 2, X(16) in memory bank 16, and X(17) in memory bank 1. In the loop in the example, if stride = 1, then the elements of X can be deflvered to the CPU at the maximum rate, one per clock cycle. 

The RISC versus CISC conundmm has led to the much abused and ultimately extremely confusiag term MIPS (millions of iastmctions per second). Measures of performance that can be more directiy related to a computer s abiUty to perform usehil work should always be preferred over machine MIPS. The throughput of a computer is a function of the number of iastmctions to be executed, the average number of iastmctions that can be executed per clock cycle, and the time per clock cycle. 

Real time. The clock cycle for the collection and transfer of process data and the optimization calculations is the same. 

Giebultowicz JM, Stanewsky R, Hall JC, Hege DM 2000 Transplanted Drosophila excretory tubules maintain circadian clock cycling out of phase with the host. Curr Biol 10 ... 

Run a Transient Analysis for several clock cycles and display the results in Probe. 

Each slot was further divided into 16 clock cycles (of 64 ns each). At IRCAM, P. diGiugno designed an early machined named the "4A . 

MLBSs are generated by a shift register of length L with feedback as shown in Fig. 27. The bit position to which the output is added depends on L. Initially, all bits are loaded with 1. At every clock cycle, the register contents is shifted to the right, the rightmost bit is applied to the output and the result of the modulo 2 addition is fed into the leftmost bit. The output sequence is repeated after 2L-l steps. For practical purposes, the elements bj. of the MLBS can be computed from a recursion formula ... 

The tasks cannot be pre-empted. Each task is characterized by its execution time and deadline. The execution time is measured in number of clock cycles, NC. Assume that the processor has two modes an active mode and a power saving mode [Kar 99]. The task deadline Tdl accommodates both the active period Tact and the power-saving period Tps... 

The "pipeline11 structure allows instructions to be processed concurrently in all levels of the pipe in both scalar and vector mode. The eight levels of the MBU-AU pair under optimum conditions can each produce an output every CPU clock cycle (80 nsec). Pipe levels unnecessary to a particular instruction are bypassed. Figure 1 also illustrates how different sections of the arithmetic pipeline are utilized for execution of a particular instruction, i.e., floating-point addition and fixed-point multiplication. 

The pipelines achieve their highest sustained flow rate in vector mode. In this situation, a single instruction is interpreted and a single operation performed on many pairs of operands. For example, if A, B, and C are arrays, only one vector instruction is required for computing the sum C(I) = A(I) + B(I), 1=1,100. The A and B values stream continuously into the pipes, additions are performed in discrete steps and results flow back to CM at the rate of one per CPU clock cycle per pipe. This is in contrast to scalar execution which requires five instructions to be executed 100 times 2 fetches, 1 add, 1 store, and 1 counter incremention. 

Finally, most doubly or triply subscripted array operations can execute as a single vector instruction on the ASC. To demonstrate the hardware capabilities of the ASC,the vector dot product matrix multiplication instruction, which utilizes one of the most powerful pieces of hardware on the ASC, is compared to similar code on an IBM 360/91 and the CDC 7600 and Cyber 174. Table IV lists the Fortran pattern, which is recognized by the ASC compiler and collapsed into a single vector dot product instruction, the basic instructions required and the hardware speeds obtained when executing the same matrix operations on all four machines. Since many vector instructions in a CP pipe produce one result every clock cycle (80 nanoseconds), ordinary vector multiplications and additions (together) execute at the rate of 24 million floating point operations per second (MFLOPS). For the vector dot product instruction however, each output value produced represents a multiplication and an addition. Thus, vector dot product on the ASC attains a speed of 48 million floating point operations per second. 

What happens in the above case if we switch the order of the statements around In this case, since the value of Temp is used before its assignment, its value needs to be retained across multiple clock cycles, thereby inferring flip-flops for Temp. Temp models the internal state of the always statement. This is shown in the following example, where Temp is used before its assignment. 

The synthesized netlist is the same as in Figure 2-48. Notice that on every clock edge, NextState always get the value of Temp assigned in the previous clock cycle, but not so in the synthesized netlist. The recommendation here is to avoid using locally declared variables in this fashion. Hopefully a synthesis tool will issue a warning if no flip-flops are inferred for Temp. 

Here is a synthesizable model for the divider block DIV. This circuit produces a pulse every sixteen clock cycles. If input TESTN is 0, ENA is set to a 1. Variable COUNT is inferred as flip-flops. 

Figure 4. HOMO/LUMO scheme for operation of a proposed molecular shift register [48]. a) The clock cycle is initiated by photoexcitation of the donor moiety, resulting in the electronic configuration shown. Decay pathways from this excited state are forward electron transfer within the same monomer unit (solid), back electron transfer to the adjacent monomer unit (dash-dot), and fluorescence (dot), b) Electronic configuration resulting from successive forward electron transfer steps. The charge-separated state [D -Ai-A2 ] can recombine charge within a single monomer unit (dot-dash) or with the adjacent monomer unit (solid).

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