COMP2017 COMP9017 Assignment 2
Due: 11:59PM Tuesday 28 March 2023 local Sydney time
This assignment is worth 5% + 30% of your final assessment
Task Description
Ah, yes, because what the world really needs right now is yet another virtual machine. But not just
any virtual machine, no! We need one with the highly coveted and incredibly useful feature of heap
banks. Because who needs to worry about memory allocation when you can just throw it in a heap, am
I right? So gather ’round, folks, and get ready to develop the vm_RISKXVII, because nothing says
"cutting edge" like a virtual machine named after a board game that this assignment has absolutely
nothing to do with. Now, let’s dive into the specs and get this party started!
In this assignment you will be implementing a simple virtual machine. Your program will take a
single command line argument being the path to the file containing your RISK-XVII assembly code.
Before attempting this assignment it would be a good idea to familiarise yourself with registers,
memory space, program counters, assembly and machine code. A strong understanding of these
concepts is essential to completing this assignment. Section 3.6 and 3.7 of the course textbook provide
specific detail to x86_64 architecture, however you can review these as a reference.
In order to complete this assignment at a technical level you should revise your understanding of
bitwise operations, file IO, pointers and arrays.
Some implementation details are purposefully left ambiguous; you have the freedom to decide on the
specifics yourself. Additionally this description does not define all possible behaviour that can be
exhibited by the system; some error cases are not documented. You are expected to gracefully report
and handle these errors yourself.
You are encouraged to ask questions on Ed1. Make sure your question post is of "Question" post type
and is under "Assignment" category→ "A2" subcategory. As with any assignment, make sure that
your work is your own2, and that you do not share your code or solutions with other students.
The Architecture
In this assignment you will implement a virtual machine for an 32-bit instruction-set. The memory
mapped virtual components of your machine are outlined below:
1https://edstem.org/au/courses/10466/discussion/
2Not GPT-3/4’s, ChatGPT’s or copilot’s, etc.
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COMP2017 COMP9017
0x0000 - 0x3ff: Instruction Memory - Contains 210 of bytes for text segment.
0x0400 - 0x7ff: Data Memory - Contains 210 of bytes for global variables, and function stack.
0x0800 - 0x8ff: Virtual Routines - Accesses to these address will cause special operations to be
called.
0xb700 +: Heap Banks - Hardware managed 128 x 64 bytes banks of dynamically allocate-able
memory.
Your machine also has a total of 32 registers, as well as a PC (program counter) that points to the
address of the current instruction in memory. Each of the general-purpose registers can store 4 bytes
(32 bits) of data that can be used directly as operands for instructions. All registers are general-
purpose except for the first one, which has an address of 0. This register is called the zero register,
as any read from it will return a value of 0. Writes to the zero register are ignored.
During execution you should not store any information about the state of the machine outside
of the virtual memory devices and the register bank.
Note: A register stores a single value using a fixed bit width. It the size of a register corresponding to
the processor word size, in this case 32 bits. Think of them as a primitive variable. Physical processor
hardware is constrained, and the number of registers is always fixed. There are registers which serve
specific purposes, and those which are general. Please identify these in the description and consider
them for your solution. You need not consider special purpose registers, such as floating point, in this
assignment.
RISK-XVII Instruction-Set Architecture
An Instructions-Set Architecture(ISA) specifies a set of instructions that can be accepted and executed
by the target machine. A program for the target machine is an ordered sequence of instructions.
Our virtual machine will operate on a home-brewed ‘RISK-XVII’ instruction set architecture. During
marking, you will be provided with binaries in this ISA to run on your virtual machine. RISK-
XVII is a reduced version of the well-known RV32I instruction set architecture, and your virtual
machine should be able to execute binary programs compiled for RV32I, as long as they do not
include instructions that were not specified by ‘RISK-XVII’.
There are in total 33 instructions defined in RISK-XVII, they can be classified into three groups by
their functionality:
1. Arithmetic and Logic Operations - e.g. add, sub, and, or, slt, sll
2. Memory Access Operations - e.g. sw, lw, lui
3. Program Flow Operations - e.g. jal, beq, blt
These instructions provide access to memory and perform operations on data stored in registers, as
well as branching to different locations within the program to support conditional execution. Some
instructions also contain data, i.e., an immediate number, within their encoding. This type of instruc-
tion is typically used to introduce hard-coded values such as constants or memory address offsets.
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The RISK-XVII instruction set is Turing complete and, therefore, can run any arbitrary program, just
like your PC!
Instructions in the RISK-XVII instruction set architecture are encoded into 4 bytes of data. Since each
instruction can access different parts of the system, seven types of encoding formats were designed
to best utilize the 32 bits of data to represent the operations specified by each instruction: R, I, S, SB,
U, UJ. The exact binary format of each encoding type can be found in the table below.:
Type Format
31 25 24 20 19 15 14 12 11 7 6 0
R func7 rs2 rs1 func3 rd opcode
I imm[11:0] rs1 func3 rd opcode
S imm[11:5] rs2 rs1 func3 imm[4:0] opcode
SB imm[12 | 10:5] rs2 rs1 func3 imm[4:1 | 11] opcode
U imm[32:12] rd opcode
UJ imm[20 | 10:1 | 11 | 19:12] rd opcode
Let’s take a look at some common fields in all types of encoding:
opcode - used in all encoding to differentiate the operation, and even the encoding type itself.
rd, rs1, rs2 - register specifiers. rs1 and rs2 specify registers to be used as the source
operand, while rd specifies the target register. Note that since there are 32 registers in total, all
register specifiers are 5 bits in length.
func3, func7 - these are additional opcodes that specify the operation in more detail. For
example, all arithmetic instructions may use the same opcode, but the actual operation, e.g.
add, logic shift, are defined by the value of func3.
imm - immediate numbers. These value can be scrambled within the instruction encoding. For
example, in SB, the 11st bit of the actual value was encoded at the 7th bit of the instruction,
while the 12rd bit was encoded at the 31th bit.
An RISK-XVII program can be illustrated as below:
[Instruction 1 (32 bits)]
[Instruction 2 (32 bits)]
[Instruction 3 (32 bits)]
[...]
[Instruction n (32 bits)]
RISK-XVII Instructions
We will now cover in detail all instructions defined in RISK-XVII. Pay close attention as your virtual
machine need to be able to decode and execute all of these to be eligible for a full mark! You are
encouraged to summarise them into a reference table before implementing your code.
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Let’s use M to denote the memory space. M[i] denotes access to the memory space using address
i. For example, to write an immediate value 2017 to the first word of data memory: M[0x400]
= 2017. Similarly, we use R to denote the register bank, e.g. R[rd] = R[rs1] + R[rs2]
denotes an operation that adds the value in rs1 and rs2, then store the result into rd.
Arithmetic and Logic Operations
1 add
Add - This instruction simply adds two numbers together.
Operation - R[rd] = R[rs1] + R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 000
– func7 = 0000000
2 addi
Add Immediate - Add a number from register with an immediate number.
Operation - R[rd] = R[rs1] + imm
Encoding:
– Type: I
– opcode = 0010011
– func3 = 000
3 sub
Subtract
Operation - R[rd] = R[rs1] - R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 000
– func7 = 0100000
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4 lui
Load Upper Immediate - Load the upper part of an immediate number into a register and set
the lower part to zeros.
Operation - R[rd] = {31:12 = imm | 11:0 = 0}
Encoding:
– Type: U
– opcode = 0110111
5 xor
XOR
Operation - R[rd] = R[rs1] ? R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 100
– func7 = 0000000
6 xori
XOR Immediate
Operation - R[rd] = R[rs1] ? imm
Encoding:
– Type: I
– opcode = 0010011
– func3 = 100
7 or
OR
Operation - R[rd] = R[rs1] | R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 110
– func7 = 0000000
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8 ori
OR Immediate
Operation - R[rd] = R[rs1] | imm
Encoding:
– Type: I
– opcode = 0010011
– func3 = 110
9 and
AND
Operation - R[rd] = R[rs1] & R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 111
– func7 = 0000000
10 andi
AND Immediate
Operation - R[rd] = R[rs1] & imm
Encoding:
– Type: I
– opcode = 0010011
– func3 = 111
11 sll
Shift Left
Operation - R[rd] = R[rs1] ? R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 001
– func7 = 0000000
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12 srl
Shift Right
Operation - R[rd] = R[rs1] ? R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 101
– func7 = 0000000
13 sra
Shift Right Arithmetic - the right most bit is moved to the left most after shifting.
Operation - R[rd] = R[rs1] ? R[rs2]
Encoding:
– Type: R
– opcode = 0110011
– func3 = 101
– func7 = 0100000
Memory Access Operations
14 lb
Load Byte - Load a 8-bit value from memory into a register, and sign extend the value.
Operation - R[rd] = sext(M[R[rs1] + imm])
Encoding:
– Type: I
– opcode = 0000011
– func3 = 000
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15 lh
Load Half Word - Load a 16-bit value from memory into a register, and sign extend the value.
Operation - R[rd] = sext(M[R[rs1] + imm])
Encoding:
– Type: I
– opcode = 0000011
– func3 = 001
16 lw
Load Half Word - Load a 32-bit value from memory into a register
Operation - R[rd] = M[R[rs1] + imm]
Encoding:
– Type: I
– opcode = 0000011
– func3 = 010
17 lbu
Load Byte Unsigned - Load a 8-bit value from memory into a register
Operation - R[rd] = M[R[rs1] + imm]
Encoding:
– Type: I
– opcode = 0000011
– func3 = 100
18 lhu
? Load Half Word Unsigned - Load a 16-bit value from memory into a register
? Operation - R[rd] = M[R[rs1] + imm]
? Encoding:
– Type: I
– opcode = 0000011
– func3 = 101
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19 sb
? Store Byte - Store a 8-bit value to memory from a register.
? Operation - M[R[rs1] + imm] = R[rs2]
? Encoding:
– Type: S
– opcode = 0100011
– func3 = 000
20 sh
Store Byte - Store a 8-bit value to memory from a register.
Operation - M[R[rs1] + imm] = R[rs2]
Encoding:
– Type: S
– opcode = 0100011
– func3 = 001
21 sw
Store Byte - Store a 8-bit value to memory from a register.
Operation - M[R[rs1] + imm] = R[rs2]
Encoding:
– Type: S
– opcode = 0100011
– func3 = 010
Program Flow Operations
22 slt
Set if Smaller - Set rd to 1 if the value in rs1 is smaller than rs2, 0 otherwise.
Operation - R[rd] = (R[rs1] < R[rs2]) ? 1 : 0
Encoding:
– Type: R
– opcode = 0110011
– func3 = 010
– func7 = 0000000
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23 slti
Set if Smaller Immediate - Set rd to 1 if the value in rs1 is smaller than imm, 0 otherwise.
Operation - R[rd] = (R[rs1] < imm) ? 1 : 0
Encoding:
– Type: I
– opcode = 0010011
– func3 = 010
– func7 = 0000000
24 sltu
Set if Smaller - Set rd to 1 if the value in rs1 is smaller than rs2, 0 otherwise. Treat numbers
as unsigned.
Operation - R[rd] = (R[rs1] < R[rs2]) ? 1 : 0
Encoding:
– Type: R
– opcode = 0110011
– func3 = 011
– func7 = 0000000
25 sltiu
Set if Smaller Immediate - Set rd to 1 if the value in rs1 is smaller than imm, 0 otherwise.
Treat numbers as unsigned.
Operation - R[rd] = (R[rs1] < imm) ? 1 : 0
Encoding:
– Type: I
– opcode = 0010011
– func3 = 011
– func7 = 0000000
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26 beq
Branch if Equal.
Operation - if(R[rs1] == R[rs2]) then PC = PC + (imm ? 1)
Encoding:
– Type: SB
– opcode = 1100011
– func3 = 000
27 bne
Branch if Not Equal.
Operation - if(R[rs1] != R[rs2]) then PC = PC + (imm ? 1)
Encoding:
– Type: SB
– opcode = 1100011
– func3 = 001
28 blt
Branch if Less Than/
Operation - if(R[rs1] < R[rs2]) then PC = PC + (imm ? 1)
Encoding:
– Type: SB
– opcode = 1100011
– func3 = 100
Virtual Routines
Virtual routines are operations mapped to specific memory addresses such that a memory access
operation at that address will have different effects. This can be used to allow programs running in
the virtual machine to communicate with the outside world through input/output (I/O) operations.
As part of your task to implement necessary I/O functions for your virtual machine, you are required
to develop the following routines:
1 0x0800 - Console Write Character
A memory store command to this address will cause the virtual machine to print the value being stored
as a single ASCII encoded character to stdout.
2 0x0804 - Console Write Signed Integer
A memory store command to this address will cause the virtual machine to print the value being stored
as a single 32-bit signed integer in decimal format to stdout.
3 0x0808 - Console Write Unsigned Integer
A memory store command to this address will cause the virtual machine to print the value being stored
as a single 32-bit unsigned integer in lower case hexadecimal format to stdout.
4 0x0812 - Console Read Character
A memory load command to this address will cause the virtual machine to scan input from stdin
and treat the input as an ASCII-encoded character for the memory load result.
5 0x0816 - Console Read Signed Integer
A memory load command to this address will cause the virtual machine to scan input from stdin
and parse the input as a signed integer for the memory load result.
6 0x0820 - Dump PC
A memory store command to this address will cause the virtual machine to print the value of PC in
lower case hexadecimal format to stdout.
7 0x0824 - Dump Register Banks
A memory store command to this address will force the virtual machine to perform an Register Dump.
See Error Handling.
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8 0x0830, 0x0834 - Heap Banks
These addresses are used by a hardware-enabled memory allocation system.
9 0x0850 and above - Reserved
Our program will not call these addresses. You are free to use them for your own test cases and
debugging purposes.
Heap Banks
One of the biggest selling points of our RISK-XVII-based virtual machine is the hardware (virtual)
enabled memory allocation support in addition to the two built-in static memory banks. Programs
running inside the virtual machine can request more memory by interfacing with the allocation sub-
system through virtual routines.
Your virtual machine program should manage memory allocation requests and ensure that the own-
ership of each block is always unique to malloc requests unless it is not used (free). You need to
manage a total of 128 memory banks, each with 64 bytes. Each memory bank is a memory device
that can be accessed as a linear array of bytes. To handle allocation requests larger than the size of a
single bank, multiple consecutive banks need to be searched and allocated for the request. An error is
returned if it is not possible to fulfill such a memory request. The mapped address of the initial byte
of the first bank is 0xb700, and there are a total of 8192 bytes of memory that can be dynamically
allocated.
Specification The below interfaces are defined for the virtual machine program:
1 0x0830 - malloc
A memory store command to this address will request a chunk of memory with the size of the value
being stored to be allocated. The pointer of the allocated memory (starting address) will be stored in
R[28]. If the memory cannot be allocated, R[28] should be set to zero.
2 0x0834 - free
A memory store command to this address will free a chunk of memory starting at the value being
stored. If the address provided was not allocated, an illegal operation error should be raised.
Example Let’s consider a scenario where all 128 banks are not allocated yet (free), and we need to
handle a malloc request with size 270. To fulfill the request, we need to find five free banks that are
located consecutively, e.g., 64 + 64 + 64 + 64 + 14. We will set the first five blocks as used and return
the address at the beginning of the first block: R[28] = &Block[0].
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Now suppose another request just came in with size 12. We need only a single block to fulfill the
request, and after a search the first one-consecutive block is the sixth. We will mark the sixth block
as used and return the address to complete the allocation: R[28] = Block[5].
Hint: You are encouraged to use a linked list internally to store and maintain a record of the current
allocation.
Error Handling
Register Dump
When an register dump was requested, the virtual machine should print the value of all registers,
including PC, in lower case hexadecimal format to stdout. The output should be one register per line
in the following format:
PC = 0x00000001;
R[0] = 0xffffffff;
R[1] = 0xffffffff;
R[...] = 0xffffffff;
R[31] = 0xffffffff
Not Implemented
If an unknown instruction was detected, your virtual machine program should output Instruction
Not Implemented: and the hexadecimal value of the encoded instruction to stdout, fol-
lowed by a Register Dump and terminate. For example, when if 0xffffffff were found in the
instruction memory, your program should output:
Instruction Not Implemented: 0xffffffff
Illegal Operation
When an illegal operation was raised, your virtual machine program should output Illegal Operation:
and the hexadecimal value of the encoded instruction to stdout, followed by a Register Dump and
terminate.
Any memory accesses outside of defined boundaries will cause this error to be raised.
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Starting the Virtual Machine
A binary file will be provided as an image of the instruction and data memory at the beginning of the
program. This file can be found by opening the file path supplied as the first command line argument.
The below C code outlines the format of the binary input file as a struct:
#define INST_MEM_SIZE 1024
#define DATA_MEM_SIZE 1024
struct blob {
char[INST_MEM_SIZE] inst_mem;
char[DATA_MEM_SIZE] data_mem;
}
All registers, including PC, will be initialised to zero at the initialisation of the virtual machine.
During each cycle, the virtual machine should fetch and execution the instruction pointed by
PC, and increase PC by 4.
Example 1 - Printing "H"
The following RISK-XVII assembly program will print the first letter of "Hello, World" to stdout:
00000000 <_start>:
0: 7ff00113 addi sp,x0,2047
4: 004000ef jal ra,8
00000008
:
8: 000017b7 lui a5,0x1
c: 04800713 addi a4,x0,72
10: 80e78023 sb a4,-2048(a5)
14: 00000513 addi a0,x0,0
18: 00008067 ret
In the program above, ret is a mnemonic for the instruction jalr where rs1 = 1. The start
function loads the address of C stack to R[2], aka sp, the stack pointer using the addi instruction.
This is necessary to setup a runtime for the machine to execute C program. Using addi with the zero
register as the source operand is a common method to load immediate value into a register.
The stack address was initialised to the bottom of data memory which indicates that the stack is empty,
as stack grows upward. sb instruction writes to 0x0800 which will call the Console Write Character
virtual routine to print of the character.
The second column indicates encoded instruction in hexadecimal format. Below illustrates the mem-
ory image input file for this program:
[1301F07F]
[EF004000]
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[B7170000]
[13078004]
[2380E780]
[13050000]
[67800000]
[1017 bytes of padding for instruction memory]
[1024 bytes of padding for data memory]
The input memory image binary file will always be 2048 bytes large.
Below is the equivalent C code to the RISK-XVII assembly program above:
char volatile *const ConsoleWriteChar = (char *)0x0800;
int main() {
*ConsoleWriteChar = 'H';
return 0;
}
Example 2 - Adding two numbers
The following C program running in the virtual machine will scan for two signed integer in stdin,
then print the sum of these numbers to stdout:
int volatile *const ConsoleWriteSInt = (int *)0x0804;
inline int scan_char() {
int result;
int addr = 0x0816;
asm volatile("lw %[res], 0(%[adr])"
: [res]"=r"(result)
: [adr]"r"(addr));
return result;
}
int main() {
int a = scan_char();
int b = scan_char();
*ConsoleWriteSInt = a + b;
return 0;
}
And the equivalent RISK-XVII assembly program:
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00000000 <_start>:
0: 7ff00113 li sp,2047
4: 004000ef jal ra,8
00000008
:
8: 00001737 lui a4,0x1
c: 81670793 addi a5,a4,-2026
10: 0007a683 lw a3,0(a5)
14: 0007a783 lw a5,0(a5)
18: 00d787b3 add a5,a5,a3
1c: 80f72223 sw a5,-2044(a4)
20: 00000513 li a0,0
24: 00008067 ret
Compilation and Execution
Your virtual machine program will be compiled by running the default rule of a make file. Upon
compiling your program should produce a single vm_riskxvii binary. Your binary should accept
a single argument in the form of the path to a ‘RISK-XVII‘ memory image binary file to execute.
make
./vm_riskxvii
Please make sure the above commands will compile and run your program. An example Makefile
has been provided in the Scaffold, but you’re encouraged to customize it to your needs. Additionally,
consider implementing the project using multiple C source files and utilizing header files.
Tests will be compiled and run using two make rules; make tests and make run_tests.
make tests
make run_tests
These rules should build any tests you need, then execute each test and report back on your correct-
ness.
Failing to adhere to these conventions will prevent your markers from running your code and tests. In
this circumstance you will be awarded a mark of 0 for this assignment.
Marking Criteria
The following is the marking breakdown, each point contributes a portion to the total 5% + 30% of
the assignment. You will receive a result of zero if your program fails to compile.
For full marks the total size on disk of your program’s binary should not exceed 20kB.
Marks are allocated on the basis of:
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Automated Test Cases - 5 - Passing automatic test cases, a number of tests will not be released
or run until after your final submission.
Viva - 30 - You will need to answer questions from a COMP2017 teaching staff member re-
garding your implementation. You will be required to attend a zoom session with COMP2017
teaching staff member after the code submission deadline. A reasonable attempt will need to
be made, otherwise you will receive zero for the assessment.
In this session, you will be asked to explain:
– How your program counter is affected by the opcodes executed.
– How your code organises and manages the stack memory for function calls.
– What are the edge cases you considered for when your program returns 1.
– Answer further questions.
– Your code will also be assessed on C coding style conventions (see Ed resources). Clean
code will attract the best grade.
Correctness is based on:
vm_riskxvii - return value of the program.
vm_riskxvii - printed standard output of the binary program execution (all virtual routines).
Additionally marks will be deducted on the basis of:
Compilation - If your submission does not compile you will receive an automatic mark of zero
for this assessment.
Style - Poor code readability will result in the deduction of marks. Your code and test cases
should be neatly divided between header and source files in appropriate directories, should be
commented, contain meaningful variable names, useful indentation, white space and functions
should be used appropriately. Please refer to this course’s style guide for more details.
Tests - A lack of tests, or a lack of thorough testing will result in the deduction of marks. Please
provide your test cases along with appropriate scripts to build, run and report the results of your
tests. As a number of tests will not be released until after your final submission you are strongly
encouraged to test all aspects of your program.
Graceful Error Handling - The description above contains a number of undefined behaviours.
Your program should gracefully catch each of these error, report, and exit. Should your program
crash marks will be deducted. Your error messages should be meaningful in addition to the
specified error handling format.
Memory Leaks - Code that leaks memory will receive a mark of 0.
Warning: Any attempts to deceive or disrupt the marking system will result in an immediate zero for
the entire assignment. Negative marks can be assigned if you do not follow the assignment description
or if your code is unnecessarily or deliberately obfuscated.
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Academic declaration
By submitting this assignment you declare the following:
I declare that I have read and understood the University of Sydney Student Plagiarism: Coursework Policy and Procedure, and except where specifically
acknowledged, the work contained in this assignment/project is my own work, and has not been copied from other sources or been previously submitted
for award or assessment.
I understand that failure to comply with the Student Plagiarism: Coursework Policy and Procedure can lead to severe penalties as outlined under
Chapter 8 of the University of Sydney By-Law 1999 (as amended). These penalties may be imposed in cases where any significant portion of my
submitted work has been copied without proper acknowledgment from other sources, including published works, the Internet, existing programs, the
work of other students, or work previously submitted for other awards or assessments.
I realise that I may be asked to identify those portions of the work contributed by me and required to demonstrate my knowledge of the relevant material
by answering oral questions or by undertaking supplementary work, either written or in the laboratory, in order to arrive at the final assessment mark.
I acknowledge that the School of Computer Science, in assessing this assignment, may reproduce it entirely, may provide a copy to another member of
faculty, and/or communicate a copy of this assignment to a plagiarism checking service or in-house computer program, and that a copy of the assignment
may be maintained by the service or the School of Computer Science for the purpose of future plagiarism checking.
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