Add information about riscv compliance tests
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title = {GTKWave},
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url = {http://gtkwave.sourceforge.net},
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}
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@online{riscv-compliance,
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author = {Jeremy Bennett, Lee Moore},
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title = {RISC-V Compliance Task Group},
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url = {https://github.com/riscv/riscv-compliance},
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}
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@ -143,4 +143,20 @@ The exact timing differs between models, so all periods can be customized using
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% TODO
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\section{Testing}
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\subsection{RISC-V Compliance Tests}
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The RISC-V Compliance Test Suite\cite{riscv-compliance} can be used to empirically confirm the correct functionality of a RISC-V processor. It consists of a series of programs that perform some operations related to a specific feature, then write some result data to a memory region. This memory region is then compared to a ``golden signature'', which was produced by a processor implementation that is known to be correct.
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The initial implementation of the compliance tests uncovered several bugs in the processor core:
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\begin{itemize}
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\item The bitshift instructions (SLL, SRL, SRA, etc.) must, according to the RISC-V standard, only use the lower 5 bits of the second operand as a shift offset. The implementation used all 31 bits instead, causing a test failure.
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\item Reading a signed value of a size less than 32 bits from memory would not perform proper sign extension. For example, reading a byte value of 0xFF (-1) would result in an expanded machine word of 0x0000_00FF (255) instead of 0xFFFF_FFFF.
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\item The \icode{SLTIU} (Set less than immediate; unsigned) instruction compares a given register with a constant provided as part of the instruction (the immediate). While the comparison is unsigned, the 12-bit immediate must be sign-extended as if it were a signed integer. The implementation wrongly assumed that the sign-extension should be unsigned as well.
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\item The Instruction Set Manual specifies exceptions that must be raised when a misaligned memory access occurs. These exceptions were not yet implemented, but since the compliance tests check for them, the functionality was added to make the tests pass.
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\end{itemize}
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Since these tests are easily automated, they were added to the GitLab Continuous Integration (CI) configuration. Whenever a new git commit is pushed to GitLab, the tests are run automatically, and any failures are reported to the responsible committer via email.
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\end{document}
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