## Chapter 1 — Computing systems ### Lesson map Lesson 01 published. Map, example, EX001. ```mermaid flowchart TD N0["Learning outcomes"] N1["1.1 Many computers, few of them obvious"] N2["1.2 Embedded versus general-purpose"] N3["1.3 Availability first"] N4["Synthesis"] N0 --> N1 --> N2 --> N3 --> N4 N4 --> EX["Worked example"] EX --> Q["Exercise EX001"] ``` ### Logical drawing ```mermaid flowchart LR N0["Learning outcomes"] N1["1.1 Many computers, few of them obvious"] N2["1.2 Embedded versus general-purpose"] N3["1.3 Availability first"] N4["Synthesis"] N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX001"] ``` ```text [Learning outcomes] | v [1.1 Many computers, few of them obvious] | v [1.2 Embedded versus general-purpose] | v [1.3 Availability first] | v [Synthesis] | v [Worked example] --> [Exercise EX001] ``` ### Learning outcomes You will (1) name the computing devices in a cold-storage warehouse and what each one does; (2) separate embedded controllers from general-purpose computers; (3) explain why uptime expectations differ; and (4) start the asset notebook used through the whole course. ### 1.1 Many computers, few of them obvious The synthetic Northgate Cold Storage (NCS) warehouse keeps frozen food at −22 °C and chilled goods at 2–4 °C. Its obvious computers are the office PCs. Less obvious are two refrigeration controllers (PLC-R1 and PLC-R2) that run the ammonia compressors, an operator screen (HMI-C), a historian (HIST), forty wireless temperature loggers that report through a gateway (GW-LOG) to a cloud service, and IP cameras on the loading dock. A technologist's first job is to see all of them. Priya's first notebook row, for GW-LOG, already shows why: if it stops, cooling continues but the temperature record has a gap. ### 1.2 Embedded versus general-purpose A PLC runs one control program in a fixed loop and may stay powered for a decade. A Windows HMI runs many processes, receives updates and can be infected by ordinary malware. Loggers are microcontrollers with radio and a battery; they cannot run antivirus at all. Each class needs a different protection approach, which is why NIST SP 800-82r3 discusses OT device types separately. Priya's notebook records the class in its type column, and the three classes lead to three plans. PLC-R1 and PLC-R2 are protected by controlling who can reach them, because they cannot run security software. HMI-C is hardened like a Windows host within the vendor's certified build. The forty loggers depend entirely on the vendor's firmware and on the gateway in front of them. ### 1.3 Availability first If an office PC reboots, someone loses ten minutes. If PLC-R1 stops, compressor suction pressure drifts and the freezer warms by roughly a degree every 15–20 minutes; after a few hours stock worth hundreds of thousands may be condemned. That consequence shapes every later decision: when to patch, how to test, who must be present. At NCS this is why a firmware fix for PLC-R1 waits for a maintenance window with the refrigeration contractor present, and why Priya's later firewall change in Chapter 27 has a two-minute rehearsed rollback: a missing flow that stops the historian recording for an hour matters more than the change itself. ### Worked example (synthetic NCS) Priya Nair, a technologist trainee, builds her asset notebook: device, type (embedded or general-purpose), function, location, who changes it, what happens if it stops. Her row for GW-LOG reads: "embedded gateway; forwards logger readings to the cloud; comms room; vendor updates it remotely; if it stops, temperature records have a gap and food-safety evidence is lost, but cooling continues." > **Synthetic case — Northgate Cold Storage.** NCS, Priya Nair and all device names are invented for teaching. ### Errors and limits Do not assume "air-gapped" without checking for cellular modems or vendor laptops. Warming rates depend on door openings and load; use the plant's own data before quoting one. ### Exercises - **EX001 — Embedded or general-purpose?** Classify HMI-C, PLC-R2 and a temperature logger, giving one reason each. *Rubric:* 3 = general-purpose, embedded, embedded with reasons; 2 = classes right, weak reasons; 1 = one right; 0 = none. - **EX002 — Consequence of a stopped gateway.** State what NCS loses if GW-LOG fails for six hours and what it does not lose. *Rubric:* 3 = loses temperature records/alerts, cooling continues; 2 = one side; 1 = says freezers stop; 0 = none. - **EX003 — Notebook row for PLC-R1.** Write the six-column asset-notebook row for PLC-R1. *Rubric:* 3 = all six columns plausible; 2 = four or five; 1 = name only; 0 = none. ### Synthesis Seeing every computer in a facility — and knowing what each one's failure costs — is the foundation of OT security work. ### Continue studying — chapter 01 of 30 [← Course home](../../README.md) · [Course chapter index](../SUBJECTS.md) · [Chapter 02 (draft) →](#ch-02) ## Worked answers **EX001.** HMI-C — general-purpose: a Windows PC running many processes and updates. PLC-R2 — embedded: one control program in a fixed scan loop. Logger — embedded: a battery microcontroller with radio and no general operating system. **EX002.** NCS loses the cloud temperature record and any remote alerts for six hours, a gap in food-safety evidence; it does not lose cooling, because the compressors are controlled by PLC-R1/R2 independently of the gateway. **EX003.** PLC-R1 | embedded controller | runs ammonia compressors 1–2 and suction pressure control | machine room panel MP-1 | refrigeration contractor via engineering laptop | freezer warms and stock is at risk within hours.