|
|
|
@ -143,26 +143,4 @@ The exact timing differs between models, so all periods can be customized using
|
|
|
|
|
|
|
|
|
|
% TODO
|
|
|
|
|
|
|
|
|
|
\subsection{External Bus}
|
|
|
|
|
|
|
|
|
|
Bridging the internal SoC bus with the external peripheral bus requires a few steps. For one, the external data bus is bidirectional, so tri-state outputs must be used on the FPGA. In addition, the internal bus arbitrates components using addresses alone, while the external bus uses chip enable signals and overlapping address spaces.
|
|
|
|
|
|
|
|
|
|
Due to a mistake in the adapter board layout, the nibbles of the address and data buses are reversed (MSB to LSB are pins 7 to 0 on the FPGA, but 3 to 0 followed by 7 to 4 on the board). Thanks to the completely arbitrary mapping of FPGA pins, this can be mitigated without using any additional resources.
|
|
|
|
|
|
|
|
|
|
\section{Testing}
|
|
|
|
|
|
|
|
|
|
\subsection{RISC-V Compliance Tests}
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
The initial implementation of the compliance tests uncovered several bugs in the processor core:
|
|
|
|
|
|
|
|
|
|
\begin{itemize}
|
|
|
|
|
\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.
|
|
|
|
|
\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.
|
|
|
|
|
\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.
|
|
|
|
|
\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.
|
|
|
|
|
\end{itemize}
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
\end{document}
|
|
|
|
|