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Pipeline1

The component "pipeline1" demonstrates, how pipelining can be implemented by a hardware structure and visualizes how the instructions pass through the different pipeline stages. The component simulates the execution of instructions and shows the dynamic behaviour of the pipelined datapath. It represents a simple pipeline without the dealing of data hazards and branches.

The hardware structure specified in these components is a pipelined implementation of the MIPS (Mixed Instruction Processor Set). The implementation is widely identical to the one described in appendix A.3 of the book "Computer Architecture, A Quantitative Approach (third edition)" from Hennessy & Patterson. A schematic of the hardware structure of the component "pipeline1" is given in illustration 1. For the visualization of an unpipelined implementation of the MIPS see component "MicroProg".


pipeline
Illustration 1: Hardware Structure implemented in pipeline1.hds



The schematic of the component "pipeline1" repesents the hardware structure and at the same time it defines the appearance of the visualization. At the bottom of the component you find also the table "Cycle". Each column of the table represent a pipe stage. During the visualization the active instructions will be displayed in the columns according to their current pipe stage.

The schematic shows the different pipe stages and the pipeline registers "IF/ID", "ID/EX", "EX/MEM" and "MEM/WB". At the top line you can see five controllers, named "Control". There is one controller given for each pipe stage and each of them controls the execution of the corresponding pipe stage.
In the first pipe stage "Instruction Fetch" you can find the program counter "PC", the instruction memory unit "IMem", and the ALU "+" for the address calculation. The pipe stage "Instruction Decode" contains the register file "Reg" and the sign-extension unit "SE". In the pipeline stage "Execution" you can see the arithmetic and logical unit "ALU" and two multiplexers at the ALU inputs. The pipe stage "Memory" includes only the data memory unit "DMem" and the pipe stage "Write Back" shows the multiplexer which serves the result value according to the current instruction type. The clock signal is given by the switch "clock" under the program counter.

With the switches "clock" and "Reset" the user can control the application flow of the visualization. The switch "clock" represents the manual clock cycle. We have not integrated a self-controlled clock signal because the clock cycles should be triggered by the user. New values are loaded into the pipeline registers at rising edges of the clock with the exception of the program counter. The new values of the program counter are loaded at falling edges of the clock. The memory units and the register file serve their output values at falling edges of the clock.
The switch "Reset" realizes a reset signal, by which the program counter is set back to the value '0'.

The light green lines represent the signals between the datapath elements. If they are colored in light green, they are not relevant in the actual cycle. The relevant signals in a cycle will be highlighted with different colors later during the visualization. Each active instruction will get its own color and the signals will be colored according to the instruction they belong to. This is important and quite different to the behaviour of a normal digital simulator system. A conventional simulator highlights all signal lines according to their values. Hence, for learning the basic pipeline concepts the conventional visualization is not useful. With our accentuation method the user can easily follow the way how the pipeline works.



The content of the instruction memory unit (the current MIPS instructions), the content of the data memory unit and the content of the register file are not determined by the component. The user can change them interactively during the visualization. The instructions in the memory unit can be entered either as assembler code or as 32bit strings. The data memory values and the register values have to be entered as 32bit strings. The default instruction and data memory content and the default register file content are given in the file "imem.bin", "dmem.bin" and "reg.bin" respectively. These files are situated in the RaVi directory and will be loaded automatically during the start of the component.






First of all it is important to mention that the hardware structure implemented in these pipeline components only includes a subset of the core MIPS instruction set. With intent to develop training components we have realised only a selected subset in order to explain the execution of MIPS instructions within a pipeline. We have not implemented a complete simulation model of the MIPS-processor.

Differing to the description of Hennessy & Patterson we have implemented an additional multiplexer in the pipe stage "Instruction Decode". The destination register field is in one of two different places depending on the instruction type. So we need a multiplexer to choose the correct portion of the IR in the MEM/WB register to specify the register destination field. The hardware structure described in the book of Hennessy & Patterson does not take this into consideration.

The lack of a zero detection unit in the pipe stage "Execution" is another difference to Hennessy & Patterson. During the execution of a branche the ALU is used for the zero test and for the jump address calculation. Consequently we will get a structure hazard and the components "pipeline1" can not execute branches correctly. Due to the fact that the components "pipeline3" and "pipeline4" deals with branches, we have decided to ignore the structure hazard here in this component.