FIT1047 Unit Cheatsheet

Context: FIT1047_MOC · the WHOLE unit in one re-read — systems (W1–5) → networks (W6–9) → security (W10–12). Hand-execution skills first; links only for depth.

Quick Revision

• 🎯 Objective: every assessed skill is a hand procedure ➔ convert, simplify, trace, subnet, match goal→mechanism.
• ⚡ Key Constraint: the unit’s marks live in EXACT constants — SkipCond $000/400/800$, K-map order $00,01,11,10$, channels $1/6/11$, $\times 8$ bits↔bytes, $\frac{n(n-1)}{2}$ keys.

1️⃣ Data Representation (W1)

• Positional value ➔ digit × base${}^{\text{position}}$; dec→bin by repeated ÷2 (remainders bottom-up); bin↔hex by 4-bit groups from the RIGHT.
• $n$ bits ➔ $2^n$ values, max $2^n-1$ (zero takes a slot); always subscript the base ($101_2\ne 101_{16}$).
• Two’s complement negate ➔ flip all bits, add 1 (involution — do it twice to decode). Range asymmetric: $-2^{n-1}\dots 2^{n-1}-1$.
• Overflow rule ➔ same-sign operands, opposite-sign result ⟹ overflow. The discarded carry is NOT the test ($(-1)+(-2)$ discards a carry, correct).
• IEEE 754 double ➔ 1 sign + 11-bit exponent (excess-$K$ bias, not 2C) + 52-bit fraction of normalised $1.xxx\times 2^e$; $0.1_{10}$ has no exact form ⟹ never == floats.
• ASCII ➔ 7-bit, 128 chars; Unicode assigns codepoints, UTF-8/16 decide bytes (variable-length — byte count ≠ char count).

2️⃣ Boolean Logic → Circuits (W2–3)

• Laws ➔ commutative · associative · idempotent · complement · identity · null · absorption · distributive (TWO directions, unlike numbers) · De Morgan ($\overline{XY}=\overline{X}+\overline{Y}$ — bar extent matters: $\overline{X}Y\ne \overline{XY}$).
• SOP extraction ➔ one AND-product per $F=1$ row, complement the 0-variables in that row, OR products. Correct but non-minimal ➔ K-map next.
• K-map procedure ➔ Gray-code columns $00,01,11,10$ → circle maximal power-of-2 groups of 1s (wrap-around legal) → per group drop every variable that varies inside it. Two 2-groups where a 4-group fits = marked down.
• Combinational blocks ➔ half adder = XOR (sum) + AND (carry); full adder = + carry-IN; ripple = chained full adders. Decoder activates 1-of-$2^n$; MUX selects; ALU computes ALL ops in parallel, MUX picks by op-code.
• Sequential ➔ SR latch: $S=R=0$ hold (the hold row IS the memory), $S=R=1$ forbidden; D flip-flop = clocked; $n$ flip-flops = register.

3️⃣ Architecture & MARIE (W3–4)

• Von Neumann ➔ CPU (ALU + registers + control) + memory + I/O; stored program = code and data share one addressed memory — same word is instruction OR number by how execution reaches it.
• Fetch (ALWAYS 4 steps) ➔ $MAR\leftarrow PC$ · $MBR\leftarrow M[MAR]$ · $IR\leftarrow MBR$ · $PC\leftarrow PC+1$ (PC increments in fetch — Jump then overwrites it). Decode 5–6 vary per instruction. MBR is the only door to memory.
• Clock ≠ speed ➔ GHz counts cycles; one instruction = several cycles (Add ≈ 7).

MARIE essentials	Payload
word format	4-bit opcode + 12-bit address, 16-bit word; AC is the only working register
SkipCond constants	$000:AC<0$ · $400:AC=0$ · $800:AC>0$ — skips exactly ONE instruction (pair with Jump)
indirect	LoadI/StoreI/AddI = $M[M[X]]$ pointer deref
subroutine	JnS X stores return PC at X itself, jumps to $X+1$ ⟹ subroutine starts HEX 0; return = JumpI X; no stack ⟹ no recursion
directives	DEC = value · Adr = address (pointer) · never in the execution path
pointer walk	Load ptr / Add One / Store ptr — 3 instructions, forget Store ⟹ infinite loop on element 0

4️⃣ Memory, I/O, Boot, OS (W4–5)

• Hierarchy ➔ registers → caches → RAM ($100\times$ slower than registers) → disk → network; cache pulls hot data toward CPU, swap pushes cold data to disk — opposite arrows.
• Addressability ➔ $n$ address bits reach $2^n$ locations; capacity needs BOTH facts (MARIE: $2^{12}$ words × 2 bytes = 8 KiB).
• CPU↔device ➔ memory-mapped registers (cost: address space + buggy-code hazard) · polling · interrupts checked between instructions, before each fetch · DMA (CPU free to compute, NOT free to use the shared bus).
• Boot chain ➔ power-good → CPU reset → ROM code → POST (before OS search — beep codes because video may not work) → hardware init → boot sector → boot code → kernel → drivers → GUI. ROM bootstraps; OS runs from RAM.
• OS state cycle (hand-traceable) ➔ Running →(timer) Ready →(scheduled) Running →(I/O wait) Blocked →(I/O done) Ready. Blocked waits on I/O, Ready waits on CPU only. User programs must system-call for I/O; preemption needs timer interrupts.
• Virtual memory ➔ MMU checks EVERY access: foreign address ⟹ fatal interrupt; on-disk page ⟹ page fault (normal, OS swaps in). One mechanism, two jobs: protection + overcommit.

5️⃣ Networks (W6–9)

• Five layers ➔ Application → Transport → Network → Data Link → Physical; protocols run horizontally (peer↔peer), interfaces vertically; PDUs: segment / packet / frame per layer; routers unwrap only to layer 3.

Layer	Identity	Protocols / payload
Application	URL	HTTP (request line/headers ⟶ status/headers/body; each <img> = new GET) · SMTP→SMTP→POP/IMAP; MIME base64s binary; envelope (MAIL FROM) ≠ header (From:) — the spoofing gap
Transport	port	TCP: 3-way open, 4-way close (full-duplex ⟹ FIN each way), ARQ on seq = bytes sent / ack = bytes received; UDP: connectionless — correct choice for live media
Network	IP	routing table → next hop or default gateway; each router plans ONE hop. Distance vector = share tables, fewest hops (RIP, 15-hop cap) vs link state = share link quality, fastest path (OSPF). Interior: RIP/OSPF · exterior: BGP only
Data Link	MAC (LAN-scoped)	Ethernet CSMA/CD (sense · shared medium · jam + random wait) vs WiFi CSMA/CA (can’t detect radio collisions — hidden node); crossing a router swaps frames/MACs, never IPs
Physical	—	digital encodings (NRZ, Manchester) vs modulation (freq/amp/phase — more bits per symbol without new Hz)

• Address arithmetic ➔ $/26$ = first 26 bits network+subnet, $32-26=6$ host bits; same-subnet test = prefix compare. Transfer time: rates in bits/s, sizes in Bytes — ×8.
• Resolution: TWO steps ➔ DNS name→IP once (iterative = client walks, recursive = server walks — same answer, different walker); ARP IP→MAC per LAN, broadcast by necessity.
• Devices ➔ switch = layer-2 inside a LAN, learns table from sender MACs, floods when unknown (empty table ⟹ behaves as hub); router = joins networks. Hub looks star, behaves bus.
• WiFi ➔ 2.4 GHz: 22 MHz-wide channels 5 MHz apart ⟹ only 1, 6, 11 clean; 5 GHz faster but attenuates ⟹ MORE APs. BSS → ESS.
• Organisation scale ➔ LAN (owned) → backbone (owned fibre; access/distribution/core) → MAN/WAN (leased): dedicated circuit (guaranteed, dear) · carrier packet-switched (shared) · VPN (cheapest, zero guarantees — “private” is cryptographic only).
• Internet structure ➔ ASes exchange via border routers running BGP; peering (free, mutual, similar size) vs transit (paid, small→large). No one owns the Internet. Load balancing spreads within a data centre; CDN also moves content near users; only GET is cacheable.

6️⃣ Security (W8, W10–12)

Goal	Mechanism (match EXACTLY)
confidentiality	encryption — AES (symmetric, fast; key distribution problem, $\frac{n(n-1)}{2}$ keys)
integrity in transit	MAC (keyed) — a bare hash is recomputable by the attacker; plain RSA is malleable too
authenticity / identity	certificates: CA signs (public key + identity); trust moves to the CA; valid cert ≠ safe site
non-repudiation	asymmetric only — a shared key can never prove WHICH side signed
key agreement	Diffie-Hellman — beats eavesdroppers, not MITM (no authentication ⟹ needs certificates); $g$ must be a primitive root
the full channel	TLS = DH + certificates + AES; usually only the server authenticated; VPN secures tunnel endpoints only — beyond the gateway traffic is clear

• Passwords ➔ never cleartext; plain hash loses to rainbow tables ⟹ per-user salted hash (salt unique, not secret); login = re-hash and compare, never decrypt.
• AuthN ≠ authZ ➔ who you are (login, MFA — ≥2 of know/have/are; same-device “MFA” isn’t) vs what you may do (Access Control — raw ACLs don’t scale ⟹ tickets/Kerberos; SSO = concentrated risk).
• WiFi security ➔ open + captive portal is STILL open (login is application-layer; frames capturable); WEP broken by its static shared key; WPA2 = per-packet keys + AES; PSK strength = passphrase strength.
• Attacks ➔ virus needs a host, worm self-propagates; malware spreads without user interaction (Angler); anti-virus catches known only. Injection/XSS/buffer overflow share one root: missing input sanitisation + excess privilege.
• Defence layers ➔ firewall filters (stateless, perimeter-only, blind inside tunnels) → IDS detects/alerts → IPS also blocks → NGF proxies TLS (inspects apps but breaks end-to-end — single point holding plaintext). Perfect security impossible ⟹ manage residual risk (NIST RMF); controls = tech + people/process.
• IoT/privacy ➔ Mirai needed only default credentials; DNS (Dyn) was the target, IoT the weapon. Cookie banners = compliance, not protection; data minimisation serves privacy AND breach impact.

⚠️ Top Cross-Unit Traps

• 💡 Byte ≠ word ➔ byte fixed at 8 bits; word is architecture-defined.
• 💡 $2^n$ values but max $2^n-1$ ➔ zero occupies a slot.
• 💡 PC increments during fetch ➔ trace it at step 4, not at execute.
• 💡 Reliability lives in TCP ➔ data link only discards bad frames; retransmission is transport’s job.
• 💡 Goal ≠ mechanism ➔ encryption gives confidentiality, NOT integrity — the single most-tested security idea.
• 💡 Hash function ≠ Hash Table hash ➔ crypto wants irreversibility; bucketing wants cheap uniformity.