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(hardware description language).
VHDL has a Pascal or ADA based syntax and it includes several extensions which make it useful for modeling of digital circuits . VHDL processes and modules execute in parallel. The WAIT statement suspends a process until the specified condition is true. A WAIT statement which waits for the clock signal to change is used to model and synthesize latches and flip-flops. Another VHDL extension is the AFTER keyword. The keyword, AFTER, followed by a time delay can be used to model the physical time delays present in digital circuits. These features permit VHDL to model the parallel operation of digital hardware even including physical time delays, if so desired. Behavioral VHDL based modeling of digital systems has been in use for several years. VHDL based logic synthesis is a newer development. In VHDL synthesis models, the time delays in the synthesized schematic are automatically simulated using special libraries for the target logic technology.
A full 32-bit version of the MIPS requires several hours to synthesize on the PC platform. A smaller version with an 8-bit data path with full 32-bit instructions and control was developed to permit the design to synthesize and simulate in just a few minutes for student laboratory assignments. This version has minimal VHDL source level modifications from the 32-bit version. Changes are limited to the bit vector array size declarations for variables and signals in the data path. If fast systems are available and long run times are not an issue, the width of the data path could be easily changed back to 32-bits. The smaller model requires machine language test programs that use 8-bit integer values. Since the hardware test programs in the textbook are just a few instructions long and relatively simple, this has not affected the pedagogical value of the MIPS VHDL model.
What does the term "Verilog Synthesis" mean? - Stack …
A VHDL synthesis based laboratory course was developed to accompany our undergraduate computer architecture course sequence. Computer and electrical engineering majors take this two-quarter course sequence. Our goal was to enable students to design and develop a realistic computer system simulation using many of the recent advances in CAD tools for logic synthesis and simulation. One of these recent developments, VHDL, Very High Speed Integrated Circuit Hardware Description Language, is now widely used to model and simulate digital circuits. VHDL has syntax similar to Pascal or ADA. VHDL Synthesis tools are available which automatically translate a high level VHDL behavioral model into a gate level digital logic schematic. Increasing the level of abstraction using such automatic synthesis tools produces substantial reductions in development time for complex digital circuits and is now widely used in industry.
Computer Architecture I and II is taught every quarter and has an enrollment of around 400 students per year. Course lectures are held three hours a week and three hours of laboratory work per week are required. Several tools such as assemblers, compilers, and CAD tools were collected from a variety of sources for this laboratory and many are available free of charge to educational users via the Internet. Links to many of these tools are included in the reference section. The lecture and laboratory outline for Computer Architecture I and II is shown in Table 1. The MIPS 32-bit RISC assembly language, integer and floating point computer arithmetic, and several hardware implementations of the MIPS processor are contained in the course textbook .
What does the term “Verilog Synthesis” mean
This contribution describes and contains the necessary VHDL files to synthesize and simulate a MIPS 32-bit RISC processor core for use in introductory computer architecture classes. This MIPS processor core is based on the design presented in chapters 5 and 6 of the widely used text, Computer Organization & Design the Hardware/ Software Interface by David Patterson and John Hennessy. IEEE Standard Logic 1164 is used in the VHDL model and versions are provided for several popular CAD tools. Our experiences in using this model in our introductory computer architecture classes, CmpE 2510 and CmpE 3510, during the past two years are described along with typical laboratory assignments.
New instructions and features are added to the VHDL model of the MIPS processor in the two VHDL based laboratory assignments. Detailed block diagrams of the MIPS processor architecture are presented in the course textbook . Changes and additions to the block diagram are made as new instructions are added to the processor. Students determine the hardware changes or additions needed and modify the appropriate source code in the MIPS VHDL model. Typical assignments have included the addition of unconditional branch instructions and unsigned add operations. The entire design is resynthesized to a gate level schematic, and a gate level timing simulation running a short machine language test program is used to verify correct operation of the new instructions. An example timing diagram with the MIPS model running a short test program is shown in Fig. 3. Tutorial and batch command files are included to make the process simple enough for first time users of the CAD tools.
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Synthesis | definition of synthesis by Medical dictionary
VHDL synthesis tools have predefined VHDL based implementations of adders, subtractors, and making the design process easier. Hardware for addition and subtraction can be automatically generated by operator inferencing in the VHDL code. With current VHDL synthesis tools, IEEE Standard Logic 1164 is the most widely used standard. Some portions of the VHDL model would need to be modified if a different VHDL synthesis CAD tool is used. Typically, only small changes are needed in the library and use statements at the beginning of each source code module to convert to a different CAD tool that supports VHDL Synthesis and IEEE Standard Logic 1164. It is possible for students to examine the synthesized schematics and to experiment with different hardware design tradeoffs such as time and area. When running simulations, a timing diagram can be produced and with some CAD tools it is also possible to back annotate the schematics with simulation values to aid in debugging.
Machine language test programs can be hand assembled, assembled using the SPIM assembler, or even produced using the C compiler. This machine language file is then read into the VHDL model's memory with a read command or hardcoded into a VHDL synthesized ROM used to simulate instruction memory. On current generation PCs, the entire synthesis process for the smaller model requires just a few minutes of computer time. Approximately 3,700 gates are used to implement the basic MIPS VHDL model. After student modifications that add new features, the improved pipelined MIPS processor uses approximately 6,000 gates.
In Computer Engineering 3510, Computer Architecture II, students study pipelining techniques, caches, virtual memory, and I/O hardware. In five laboratory assignments, students modify the synthesizable VHDL model to create an improved MIPS RISC processor with pipelining. In the first laboratory assignment, students pipeline the existing MIPS VHDL model. The VHDL model contains separate modules for each of the five MIPS pipeline stages making the student pipelining process much easier. In this assignment, students add the pipeline registers to each stage or module and modify the control unit. A top-level VHDL structural model is used to connect the five stages together. This structure generates two levels of hierarchy in the design. Breaking up the model into different pipeline stages makes the model much easier to understand and to synthesize. The textbook contains detailed block diagrams of the pipelined MIPS processor. Instructions are traced one clock cycle at a time as they move through the pipeline stages and the value of all busses and control signals are shown in a detailed set of diagrams. Short machine language test programs identical to those presented in the course textbook are run in the simulator to verify correct operation. Using the simulation output, a timing diagram with bus values indicated in hexadecimal, students can check the computer's operation against the textbook's diagrams to verify correct operation. On the PC platform, the VHDL gate level timing simulator can execute the MIPS test programs in a few seconds.
Automatic pipeline stalling using a hazard detection unit is added to the pipelined MIPS VHDL model in the second laboratory assignment. The textbook modifies the control unit to detect a data hazard between instructions in the pipeline and to stall or wait a clock cycle. An example data hazard would be an instruction in the pipeline that writes a register that is then read by the instruction that immediately follows it. The pipelined register write occurs after the read operation for the next instruction and the incorrect register data value would be used. This problem is solved by stalling and later by forwarding. These problems are carefully avoided by the MIPS machine language test programs until they are discussed in the textbook and corrected by adding additional hardware.
Forwarding is added between the pipeline stages to reduce the number of stalls in the third laboratory assignment. As outlined in the textbook, two forwarding multiplexers are added to the ALU inputs along with a more complex control unit that selects the appropriate input when it detects a data hazard. In the case of a register data hazard, the new value of the register is supplied to the ALU by the forwarding multiplexer before it is written back to the register file.
Branch flushing is added to the pipelined MIPS VHDL model in the fourth assignment. When a branch is taken several instructions after the branch are already in the pipeline and they must be automatically cleared out of the pipeline or flushed. The control unit and pipeline registers are modified to detect a branch flush operation and to clear the appropriate pipeline registers to avoid executing unwanted instructions already in the pipeline.
Additional features such as a cache, exception handling hardware, or a different branch instruction scheme are added to the pipelined MIPS VHDL mode in the final laboratory assignment. On the MIPS processor exceptions, such as overflow, save the current PC in a special register and jump to a trap address. In each laboratory assignment, complete synthesis of the design to gate level, followed by a full gate level timing simulation is used to execute short machine language test programs.
Many students encounter timing problems present in their VHDL models that cause errors in the VHDL gate level timing simulation. In every case, it was a real timing problem traced back to VHDL coding errors. They were able to detect and diagnose the problem using the simulation, modify the VHDL model, resynthesize, and eliminate the timing problems. Problems included uninitialized values in flip-flops, combinatorial loops, gating signals with the clock, and logic hazards on critical write control lines. These real world problems familiar to most experienced digital designers are not detected by most RTL or VHDL behavioral simulations.
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