Tag Archives: computer

Disk drive sizes

In the International System of Units there are standard prefixes, based on powers of ten, used to indicate multiplication or division: kilo- to indicate multiplication by one thousand (103), mega- to indicate multiplication by one million (106), giga- to indicate multiplication by one billion (109), and so on.

But computer scientists don’t like powers of ten. The most basic unit of digital storage, the bit, is represented either as a one or a zero (with eight bits making a byte) and thus computer scientists are much happier working in binary, with powers of two rather than powers of ten. Standard binary prefixes do exist: kibi- for 210, mebi- for 220, gibi- for 230, etc.

SI Unit Size /B Binary Unit Size /B
kilobyte (kB) 1000 kibibyte (KiB) 1024
megabyte (MB) 1 000 000 mebibyte (MiB) 1 048 576
gigabyte (GB) 1 000 000 000 gibibyte (GiB) 1 073 741 824

The problem is that barely anyone uses the standard binary prefixes. During the “kilobyte era”, because 1000 and 1024 aren’t much different (2.4%) the difference was mostly ignored. But as file and hard disk drive (HDD) sizes have increased the difference between them has become more noticeable.

HDD manufacturers have stuck with SI (10x) sizes whilst operating systems calculate sizes in binary, but incorrectly use SI prefixes. A 256 gigabyte hard drive (i.e. one containing 256 billion bytes) will be reported by an operating system as being only 238 GB in size, a 6.9% difference. As HDDs becomes ever larger the problem will get worse: at the terabyte level the difference is 9.1% and at the petabyte level it is 11.2%.

Persuading operating systems to alter the way they report file sizes, thereby confusing users in the process, is unlikely to be a successful approach. A far better approach would be to persuade HDD manufacturers to change their marketing so that users purchasing a HDD receive the size they are expecting.*

* Though obviously, as a physicist, it causes me great mental anguish to abuse SI units in this fashion!

Curiosity’s RAD750 radiation-hardened computer

You can’t use just any computer on a Mars rover.

Two British Aerospace RAD750 single board computers, as used aboard the Curiosity rover.

Mars has no magnetic field and its atmosphere is very thin, about 0.6 kilopascals compared with Earth’s 101 kilopascals. This means that the surface of Mars is bathed in cosmic ray radiation, about 500 millisieverts per year according to an instrument aboard the Curiosity rover. This is about one thousand times the dose on Earth.

The charged particles that make up cosmic ray radiation smashing into electrons in electronic circuitry can knock them loose and cause noise and current spikes. This can turn a binary 0 into a binary 1 or vice versa (a “bit flip”) and thus any computer hardware travelling outside Earth’s protective bubble must be “hardened” to protect it from radiation.

There are a number of ways that hardware can be hardened:

  • By the use of physical shielding, such as lead or tungsten, designed to stop energetic particles from reaching components.
  • By replacing the semiconductor wafers on which chips are built with insulators such as sapphire (aluminium oxide). It is orders of magnitude harder to knock electrons loose from an insulator than from a semiconductor.
  • By replacing the Dynamic RAM (DRAM) used in regular computers with the bulkier and more expensive Static RAM (SRAM) that is less susceptible to bit flips.
  • By the use of error-correcting code in the computer’s code that checks for the damage (e.g. bit flips) caused by energetic particles.

The RAD750 single board computer manufactured by British Aerospace is a favourite of spacecraft designers, even at a cost of $200 000 per unit. The RAD750 has been used on board Curiosity, Juno, the Solar Dynamics Observatory, the Wide-field Infrared Survey Explorer, the Lunar Reconnaissance Orbiter, the Kepler habitable exoplanet observatory, the Fermi gamma-ray space telescope and the Mars Reconnaissance Orbiter.

Colour temperature and f.lux

All objects emit electromagnetic radiation, and the type and amount of radiation emitted depends on the object’s temperature. The hotter the object, the higher the energy of the emitted EM radiation: a cold object will emit radio waves and as temperature increases, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

The surface of the Sun is about 6000K which means that it produces light right across the spectrum, peaking in the green. It is this green coloured light that humans (and other land mammals with colour vision) are most sensitive to – you have twice as many green-sensitive cones as red- and blue-sensitive ones.

A standard incandescent filament lightbulb uses a titanium filament at a temperature of 1500K. This is significantly colder than the Sun which means less higher-energy green and blue light is emitted, leading to an overall yellow colour. Flourescent lighbulbs do not work in the same way so their colour temperature is adjusted by altering the mix of phosphors inside the bulb.

Left to right: simulated 6500K, 2000K, 2650K and 3000K compact fluorescent bulbs

I spend a lot of time in front of a computer screen; something that is not good for the eyes. I have a program called f.lux installed on my laptop that adjusts the colour temperature of my monitor automatically throughout the day; during the daytime the colour temperature is 6500K, after sunset it drops slowly to 3400K. This helps to reduce eyestrain and maintain circadian rhythms.