COMP30023 Computer Systems

How COMP30023 is assessed

What COMP30023 covers

COMP30023 runs across twelve teaching weeks in two halves: an operating-systems half (Weeks 1-5 — the OS role, processes and threads, IPC, scheduling, memory management, then cryptography and TLS) and a networking half (Weeks 6-12 — the OSI model, sockets, DNS, HTTP, UDP, TCP, flow and congestion control, addressing, routing and middleboxes).

It is assessed by two C programming projects (30%), weekly tutorial quizzes (10%), a mid-semester test (10%) and a final exam (50%), with a dual hurdle you must clear across the exam pair and across the coursework pair. This map walks each week's topic in teaching order toward that exam.

Weeks 1-5 are the operating-systems half: what an operating system is and the kernel/user-mode boundary, the process and thread abstractions, interprocess communication and mutual exclusion, CPU scheduling, memory management and paging, then a cryptography-and-secure-communication bridge (symmetric vs asymmetric encryption, hashes, digital signatures, certificates and TLS).

Weeks 6-12 are the networking half: layered protocol models and the OSI/TCP-IP stack, sockets and DNS and mail, HTTP and UDP, TCP and reliable transport, flow and congestion control and IP addressing, routing and control protocols, and middleboxes (NAT and firewalls).

The whole subject is taught from the programmer's perspective — the two C socket-programming projects and the exam both reward reasoning about mechanisms rather than memorising facts. Assessment (delivered through Canvas) is two programming projects (30%), weekly tutorial-participation quizzes (10%), a mid-semester test (10%) and an end-of-semester examination (50%).

The load-bearing rule is a dual hurdle: you must score at least 30/60 across the MST and final exam combined, AND at least 20/40 across the projects and tutorial quizzes combined, on top of clearing the overall pass — miss either hurdle and the result is capped. The final exam is on-campus and digital, 15 minutes reading plus 2 hours writing, closed-book except for one double-sided A4 note sheet.

Your COMP30023 result feeds the Weighted Average Mark (WAM) that later computing subjects build on. Confirm the exact weights, hurdles and permitted materials on the subject's Canvas page and the current subject outline.

Worked example · free

Average turnaround under FCFS vs SJF — why SJF wins

Key terms

Process vs program

A program is static code sitting on disk (a recipe); a process is a dynamic, running instance of it (the act of cooking), with its own program counter, registers, stack, heap and data. Several processes can run the same program at once.

Kernel vs user mode

A hardware mode bit in the program status word sets the privilege level. Kernel mode can run every instruction and touch all memory; user mode is restricted (no privileged/I/O instructions, only permitted memory). A system call is the controlled gateway that switches user → kernel to request a privileged service.

Dual hurdle

COMP30023's pass rule beyond the weighted total: you must score at least 30/60 across the MST + final exam combined AND at least 20/40 across the projects + tutorial quizzes combined. Failing either hurdle caps your result regardless of the overall mark.

Page table / address translation

A per-process table mapping each virtual page number to a physical frame. A virtual address splits into a page number (top bits) and an offset (bottom bits); physical address = frame number × frame size + offset. The offset is copied through unchanged.

Three-way handshake

TCP's connection setup: SYN (SEQ = x) → SYN-ACK (SEQ = y, ACK = x+1) → ACK (SEQ = x+1, ACK = y+1). It guarantees exactly one connection forms and both sides agree on initial sequence numbers; SYN and FIN each consume one byte of sequence space.

Public-key (asymmetric) encryption

Each principal holds a key pair: encrypt with the recipient's public key, decrypt with their private key (RSA, ElGamal). It removes the secret-key-exchange problem but is slow, so real systems use hybrid encryption — asymmetric to exchange a symmetric session key, then symmetric (AES) for the bulk data.

COMP30023 FAQ

Questions students ask in each COMP30023 chapter

What is the difference between a program and a process?

A program is a static list of instructions stored on disk — like a recipe. A process is a dynamic, running instance of that program — the act of following the recipe — with its own program counter, registers, stack, heap and data. Multiple processes can execute the same program simultaneously. This one-line contrast is a recurring MST/exam opener, so be able to state it crisply with the static-vs-dynamic framing. Chapter 01 · Operating Systems Overview

What is the difference between a process and a thread?

A process is a running program with its own address space; a thread is a unit of execution inside a process. Threads of the same process share the code, global data, heap and open files, and each has only its own program counter, registers and stack. So threads are cheaper to create and switch between and share data easily, but they are less isolated — one faulty thread can crash the whole process, whereas separate processes are protected from each other. Chapter 02 · Processes and Threads

What exactly is a race condition?

A race condition occurs when two or more processes or threads access a shared resource and the final result depends on the exact order in which their steps interleave. Because an operation like count = count + 1 is really load-add-store (not atomic), a context switch in the middle can cause a lost update. The defining phrase to use in the exam is 'the outcome depends on the timing/order of scheduling', which distinguishes it from deadlock, starvation and mutual-exclusion violations. Chapter 03 · Interprocess Communication and Synchronization

How do I compute average turnaround and response time?

Draw the Gantt chart for the policy first. Turnaround for each process is its completion time minus its arrival time; response time is when it first starts running minus its arrival time. Average each over all processes. The commonest slips are forgetting to subtract a non-zero arrival time, and reading completion times wrong from the chart — so always build the strip and read the numbers off it rather than doing arithmetic in your head. Chapter 04 · CPU Scheduling

How do I convert a virtual address to a physical address under paging?

Split the virtual address into a page number and an offset using the page size: page number = address ÷ page size, offset = address mod page size. Look up the page number in the page table to get the frame number. Then physical address = frame number × page size + offset. The offset is always copied through unchanged; only the page number is translated. If the page size is a power of two, the offset is just the low log2(page size) bits. Chapter 05 · Memory Management

What is the difference between symmetric and asymmetric encryption?

Symmetric encryption uses a single shared secret key for both encryption and decryption; it is fast and good for bulk data, but the parties must first exchange the key securely, and it scales as N(N−1)/2 keys for N users. Asymmetric (public-key) encryption gives each party a public/private key pair — you encrypt with the recipient's public key and only their private key decrypts — which removes the key-exchange problem and scales as 2N keys, but it is far slower. Real systems use hybrid encryption: asymmetric to exchange a symmetric session key, then symmetric for the payload. Chapter 06 · Cryptography and Secure Communication

What is the difference between a service and a protocol?

A service is the interface a layer provides to the layer above — the set of operations (primitives) that layer can request, defining what the layer does. A protocol is the agreement between peer entities at the same layer on both sides of the connection about the format and meaning of the messages they exchange, defining how it is done. Layering isolates the two: you can change a layer's protocol without changing the service it offers, and the layer above is unaffected. Chapter 07 · Network Protocols and the OSI Model

What is the correct order of socket calls for a TCP server and client?

Server: socket() → bind() → listen() → accept() → read()/write() → close(). Client: socket() → connect() → write()/read() → close(). The server calls bind (to claim a port), listen (to set up the accept queue) and accept (to wait for and pick up a connection); accept is the call that blocks until a client arrives. The client just creates a socket and connects. Getting listen and accept in the right order, and knowing accept blocks, are frequent single-mark points. Chapter 08 · Sockets, DNS and Mail

How many TCP connections does a web page need?

It depends on HTTP version and how many distinct hosts serve the objects. With persistent HTTP/1.1, the minimum is one connection per host, reused for all that host's objects — so a page whose objects come from two servers needs at least two connections. With non-persistent HTTP/1.0 you need a separate connection per object. Browsers may also open several parallel connections per host to speed loading, but the exam usually asks for the minimum, which counts hosts, not objects. Chapter 09 · HTTP and UDP

How does the TCP three-way handshake work and why three messages?

Host 1 sends SYN with its initial sequence number x; Host 2 replies SYN-ACK with its own initial sequence number y and ACK = x+1; Host 1 sends ACK = y+1. Three messages are needed so both sides confirm each other's initial sequence numbers even when setup packets are lost, duplicated or delayed. If Host 2's SYN-ACK is lost, Host 1 times out and resends the same SYN; Host 2 matches it to its half-open entry and resends the same SYN-ACK — so exactly one connection forms with agreed sequence numbers. Chapter 10 · TCP and Reliable Transport

What ACK does a TCP receiver send after a lost segment?

It sends a cumulative ACK equal to the first missing byte — one past the last byte received with no gap before it. If an earlier segment is lost but later ones arrive, those later segments are buffered but cannot advance the ACK past the hole, so the receiver keeps returning the same ACK number (duplicate ACKs). Once the sender sees three duplicate ACKs it fast-retransmits the missing segment without waiting for a timeout. The trap is thinking an out-of-order arrival advances the ACK — it does not. Chapter 11 · Flow Control, Congestion Control and Addressing

What is the difference between routing and forwarding?

Forwarding is the local, per-packet data-plane action: a router reads a packet's destination address, looks it up in its forwarding table, and sends the packet out the corresponding interface. Routing is the control-plane process of building those tables — a routing algorithm at each router plus a protocol that gathers the topology information the algorithm needs. In short, routing decides the paths; forwarding follows them. The exam often asks you to place each in the data plane or control plane. Chapter 12 · Network-Layer Routing and Control

How does NAT translate addresses?

Outbound, the NAT box replaces a packet's private source IP with its single public IP and replaces the source port with an index into its translation table, storing the original private IP and port in that entry and recomputing the IP and TCP/UDP checksums. Inbound, it reads the destination port of the reply, looks it up in the table to recover the original private IP and port, rewrites the destination fields back and recomputes checksums, then forwards to the internal host. Because it maps ports as well as addresses it is really NAPT. Chapter 13 · Middleboxes, NAT, Firewalls and Exam Revision

How to study for the exam

Treat COMP30023 as two linked skill sets — operating systems and networking — and rehearse both weekly rather than cramming through SWOTVAC, because the dual hurdle means you cannot afford to neglect either the exam pair or the coursework pair.

For the OS half, redraw the recurring diagrams from memory (the process state machine, single- vs multi-threaded memory layout, a paged address translation with the TLB) and work one quantitative item end to end each week: a scheduling Gantt for FCFS/SJF/RR, a virtual-to-physical address calculation, an effective-access-time weighting.

For the networking half, drill the hand-calculations until they are automatic: the TCP three-way-handshake SEQ/ACK arithmetic, a sliding-window cumulative-ACK read-off, CIDR membership and longest-prefix matching, and a Dijkstra shortest-path table.

Because the exam is short-answer in boxes and rewards 'explain why / compare X and Y / what would go wrong', practise stating mechanisms crisply — the man-in-the-middle attack and how certificates stop it, why a TLB is not strictly necessary but speeds translation, why NAT breaks end-to-end connectivity but aids security.

Keep the two C projects moving early so they never collide with exam revision, since they feed the 20/40 coursework hurdle. When a step won't click, ask Sia to explain that single step a different way and set you a fresh practice question in the same style; it teaches the method and checks your reasoning, and it never substitutes for your own graded work.

Confirm the exam date, room, weighting and permitted materials on Canvas and the University of Melbourne exam timetable.

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