The laboratory that freezes for seconds at a time
A secondary school computer laboratory has 24 desktop machines used for word processing and, increasingly, for editing photographs for the school magazine. Each machine has 4 GB of RAM, a 500 GB hard disk spinning at 5 400 rpm, a dual-core processor and a 32-bit operating system. Staff report that once ten to fifteen browser tabs are open beside a 40 MB photograph, the machines become almost unusable: the disk activity light stays on continuously, the pointer freezes for several seconds at a time, and a warning about low virtual memory appears. At those exact moments the processor is only about 30 per cent busy. The laboratory has money to make exactly one change to every machine this term, and the head teacher wants to know whether to buy more RAM, solid-state drives or faster processors. Separately, two machines show no power light at all when switched on, and the supplier's quote for whichever upgrade is chosen covers all 24 machines.
1. Using the symptoms described, identify the bottleneck and recommend which single upgrade should be bought. Justify your choice with evidence from the scenario, and state one problem the upgrade will not solve.
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The evidence points at main memory, not the processor. The disk light is on continuously while the processor is idle at 30 per cent, and the operating system is warning about low virtual memory: RAM is full, so pages are being written to and read back from the hard disk (thrashing). A 5 400 rpm disk needs several milliseconds to move its heads, and each freeze of several seconds is thousands of those slow accesses. A faster processor would leave it just as starved of RAM.
Given one change only, the solid-state drive is the better buy. The real cause is too little RAM, but the machines run a 32-bit operating system, so a RAM upgrade would be largely invisible (see the address-space limit). An SSD removes the seek delay, so paging becomes perhaps a hundred times faster in random access terms and the visible freezes largely disappear, while also speeding up start-up and file loading. Adding RAM without changing the operating system wastes money.
The upgrade will not solve the two dead machines: no power light means a power-supply or mains fault, not a performance problem.
2. A teacher suggests fitting 8 GB of RAM instead. Explain, with a calculation, why the machines would not benefit fully, and what else would have to change for the extra memory to be used.
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A 32-bit processor generates addresses that are 32 bits wide, so it can distinguish 2^32 = 4 294 967 296 separate byte addresses, that is 4 GiB in total. That 4 GiB is the whole address space, not the RAM allowance: part of it must be mapped to the graphics card, BIOS, and other device memory, so a machine with 4 GB fitted typically shows only about 3.2 to 3.5 GiB usable. Fitting 8 GB would leave roughly 4 GB that the system physically cannot address; the modules would be paid for and never read.
To use more memory the school would need all of: a 64-bit operating system, a processor that supports 64-bit operation, a motherboard and chipset that accept the larger modules, and 64-bit drivers and applications. That is a reinstall and a compatibility exercise, not a five-minute swap, and it is outside the one-change budget. It is also worth checking the board's maximum RAM per slot before ordering, because older laboratory machines often cap at 4 GB or 8 GB in total regardless of the operating system.
3. What should be done about the two machines that show no power light before the order is placed, and why does this matter for the budget and for safety?
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They should be diagnosed first and removed from the upgrade list until their status is known. No power light at all is not a slow-machine symptom; it is a power-path fault. The technician should work from the outside in: confirm the wall socket is live using another device, check the cable, check the power supply's rear switch and voltage selector, then test with a known-good power supply unit. If the machine then starts, the fault was the PSU or cable; if not, the fault is likely the motherboard or the front-panel switch, and the machine may be beyond economic repair at its age.
This matters for the budget because paying to fit an SSD or RAM into a machine that cannot start converts a working upgrade budget into a repair bill, and the head teacher may end up with 22 working machines rather than 24 while having paid for 24 upgrades. It matters for safety because the PSU holds charge after unplugging: it must not be opened by staff or pupils, and the machine should be unplugged and left a few minutes before a swap is attempted by a competent person.