RV64I ISS
Computer Architecture
Assignment — RISC-V RV64I ISS — Stage 1
Your task for this assignment is to develop an instruction set simulator (ISS) for the RV64I subset of the RISC-V
instruction set. An instruction set simulator is a program used by computer architects to simulate execution of a
computer’s instructions. It contains representations of the computer’s memory and the internal registers of the CPU.
It responds to commands that specify initialization and inspection of the memories and registers, and control
execution of instructions.
The RISC-V instruction set is described in The RISC-V Instruction Set Manual, Volume I: Unprivileged ISA, available
on the course web site. The RV64I subset is described in Chapter 5 of the Instruction Set Manual, building on the
RV32I subset described in Chapter 2. The instruction encoding is summarized in Chapter 24. For this assignment,
you should implement just the RV64I base integer instruction set, with the following exceptions:
• FENCE: This instruction should be decoded as a legal instruction, but perform no operation.
• ECALL, EBREAK: These instructions should be decoded as legal instructions, but a message should be
displayed indicating they are unimplemented.
For load and store instructions, you can assume that the effective address is properly aligned. Do not implement misaligned
loads or stores. For any fetched instruction word that does not represent an RV64I instruction, a message
should be displayed indicating the instruction is illegal. In Stage 2, you will implement exception handling, which
will be used for illegal instructions.
We have provided a skeleton program on the course web site for you to use as a starting point. The program is written
in C++, and islocated in the Assignment file folder (also linked in the Assignment module). You can download either
rv64sim.zip or rv64sim.tgz; the content is the same in each. The skeleton program implements processing of command-line
options and input commands. Your task isto implement classesto model the processor and memory. Header
files are provided showing the member functions required. You can add additional classes if you need to.
The only command-line option you need to implement at this stage is the -v option to enable verbose output. If the –
v option is specified on the command line, your program can display debugging information. If the option is omitted,
your program should display only the output required for each command. Your program must format that output
exactly as specified below, since the assessment process will compare you output with expected output.
The rv64sim program reads commands from the standard input stream, one command per line. The commands are:
Command Operation performed
xn Show the content of register xn in hex (n is register number, from 0 to 31). The value is
displayed as 16 hex digits with leading 0s.
xn = value Set register xn to value (value in hex).
pc Show content of PC register in hex. The value is displayed as 16 hex digits with leading
0s.
pc = address Set PC register to address (address in hex).
m address Show the content of memory doubleword at address (address in hex, rv64sim rounds it
down to nearest doubleword-aligned address). The value is displayed as 16 hex digits
with leading 0s.
m address = value Set memory doubleword at address to value (address in hex, rv64sim rounds it down to
nearest doubleword-aligned address; value in hex).
l “filename” Load memory from Intel hex format file named filename. If the file includes a
start address record, the PC is set to the start address.
. Execute one instruction.
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Command Operation performed
. n Execute n instructions.
b address Set an execution breakpoint at address. If the simulator is executing multiple
instructions (. n command), it stops when the PC reaches address without executing that
instruction. There is only one execution breakpoint; using the b command with a
different address removes any previously set breakpoint.
b Clear the breakpoint.
Each command may be followed by a comment, starting with the‘#’character and extending to the end of the line.
Blank lines are permitted, as are lines containing only a comment.
The initial value of all processor general purpose registers should be 0, and the initial value of the PC should also be
- The memory should appear to have all locations initialized to 0. Your program should count the number of
instructions executed. This will be reported on completion of execution.
You can test your ISS by using the“m”command to set memory locations to the encoded value of RISC-V
instructions, using the“pc”command to set the PC to the start of the code, then using the“.”command to execute
the code. Alternatively, you can use the RISC-V GNU Compiler Toolchain (C compiler, assembler, linker, binutils;
available at https://github.com/riscv/risc…) to generate hex files to load into memory. We will use
both of these processes when we assess your ISS.
Performance of an ISS program is important. Computer architects typically use them to develop code for embedded
system, so they must be able to execute 100s of thousands of instructions per second. You should design your ISS
with performance in mind. The skeleton program provided uses the native integer data types uint32_t and uint64_t to
represent instructions and data, rather than using a dynamically allocated class-typed object or string. When you
implement the memory, you should not attempt to represent it using a large array of words. Since addresses are 64
bits, that would imply an array of 264 bytes. Instead, consider a representation that allocates blocks of memory on
demand (that is, on the first read or write to an address within a block).
Please keep an eye on the Questions and Answers forum on the course web site. There will no doubt be questions of
clarification of requirements arising that we will answer there. We will also announce incremental releases of a test
suite that you should use to test your program.
You must develop your program and check it into a subdirectory named 20xx/s1/ca/rv64sim in your SVN repository
(xx represents the last 2 digits of the year). We will provide a web submission script that will check out this
subdirectory, make your ISS, and run it with several test cases. Compliance with this development process will
count toward the assessment of the assignment. The script will compare your output with our expected output using
the“diff -iw”command (differences ignoring case and white-space).
Your work for Stage 1 will be assessed in the web submission system based on the following criteria, with points
awarded out of 1500:
• Program builds and runs using web submission script — 100 points
• Correct execution of instruction, based on the number of test cases that pass — 1300 points
• Program efficiency, based on run time not exceeding a limit — 100 points
The points for this assignment will comprise 15% of your final assessment for the course.
For the submission deadline, please refer to the course page on MyUni.
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Postgraduate requirements
If you are enrolled in the postgraduate course (COMP SCI 7026), you should implement the following additional
requirements:
During simulated execution, you should count the number of simulated clock cycles, in addition to the number of
instructions executed. You should use the following cycle counts for various instruction types:
• Conditional branch instruction: 2 cycles if the branch is taken, or 1 cycle if the branch is not taken.
• Unconditional branch instruction: 2 cycles.
• Load instruction: 3 cycles.
• Store instruction: 2 cycles.
• All other instructions: 1 cycle.
On completion of a simulation, if the -c option is specified on the command line, your program will report the total
number of simulated clock cycles taken.