Editing Magnetic-core memory (section)

===Physical characteristics===

====Speed====
The performance of early core memories can be characterized in today's terms as being very roughly comparable to a clock rate of 1&nbsp;[[MHz]] (equivalent to early 1980s home computers, like the [[Apple II]] and [[Commodore 64]]). Early core memory systems had cycle times of about 6&nbsp;[[μs]], which had fallen to 1.2&nbsp;μs by the early 1970s, and by the mid-70s it was down to 600&nbsp;[[nanosecond|ns]] (0.6&nbsp;μs). Some designs had substantially higher performance: the [[CDC 6600]] had a memory cycle time of 1.0&nbsp;μs in 1964, using cores that required a half-select current of 200 mA.<ref>{{cite book |title=Control Data 6600 Training Manual |section=Section 4 |date=June 1965 |publisher=Control Data Corporation |id=Document number 60147400}}</ref> Everything possible was done in order to decrease access times and increase data rates (bandwidth). To mitigate the often slow read times of core memory, read and write operations were often paralellized, with one word's worth of single-bit memory arrays set to work together so that a whole word's worth of memory could be read in a single memory access cycle.

====Reliability====
Core memory is [[non-volatile storage]]—it can retain its contents indefinitely without power. It is also relatively unaffected by [[electromagnetic pulse|EMP]] and radiation. These were important advantages for some applications like first-generation industrial [[programmable controllers]], military installations and vehicles like [[fighter aircraft]], as well as [[spacecraft]], and led to core being used for a number of years after availability of [[semiconductor]] MOS memory (see also [[MOSFET]]). For example, the [[Space Shuttle]] [[IBM System/4 Pi|IBM AP-101B]] flight computers used core memory, which preserved the contents of memory even through the ''[[Space Shuttle Challenger|Challenger]]''{{'}}s disintegration and subsequent plunge into the sea in 1986.<ref>{{cite web |url=http://www.magnet.fsu.edu/education/tutorials/museum/magneticcorememory.html |title=Magnetic Core Memory |publisher=National High Magnetic Field Laboratory: Museum of Electricity and Magnetism |location=US |archive-url=https://web.archive.org/web/20100610140932/http://www.magnet.fsu.edu/education/tutorials/museum/magneticcorememory.html |archive-date=10 June 2010 |url-status=dead}}</ref>

====Temperature sensitivity====
Another characteristic of early core was that the coercive force was very temperature-sensitive; the proper half-select current at one temperature is not the proper half-select current at another temperature. So a memory controller would include a temperature sensor (typically a [[thermistor]]) to adjust the current levels correctly for temperature changes. An example of this is the core memory used by [[Digital Equipment Corporation]] for their [[PDP-1]] computer; this strategy continued through all of the follow-on core memory systems built by [[Digital Equipment Corporation|DEC]] for their [[Programmed Data Processor|PDP]] line of air-cooled computers.

Another method of handling the temperature sensitivity was to enclose the magnetic core "stack" in a temperature-controlled oven. Examples of this are the heated-air core memory of the [[IBM 1620]] (which could take up to 30 minutes to reach [[operating temperature]], about {{convert|106|F}} and the heated-oil-bath core memory of the [[IBM 7090]], early [[IBM 7094]]s, and [[IBM 7030]]. Core was heated instead of cooled because the primary requirement was a ''consistent'' temperature, and it was easier (and cheaper) to maintain a constant temperature well above room temperature than one at or below it.

====Diagnosing====
Diagnosing hardware problems in core memory required time-consuming diagnostic programs to be run. While a quick test checked if every bit could contain a one and a zero, these diagnostics tested the core memory with worst-case patterns and had to run for several hours. As most computers had just a single core-memory board, these diagnostics also moved themselves around in memory, making it possible to test every bit. An advanced test was called a "[[Schmoo plot|Shmoo test]]" in which the half-select currents were modified along with the time at which the sense line was tested ("strobed"). The data plot of this test seemed to resemble a cartoon character called "[[Shmoo]]," and the name stuck. In many occasions, errors could be resolved by gently [[Percussive maintenance|tapping]] the printed circuit board with the core array on a table. This slightly changed the positions of the cores along the wires running through them, and could fix the problem. The procedure was seldom needed, as core memory proved to be very reliable compared to other computer components of the day.

<gallery widths="200px" heights="180px">
File:8 bytes vs. 8Gbytes.jpg|This [[microSDHC]] card holds 8 billion bytes (8 GB). It rests on a section of magnetic-core memory that uses 64 cores to hold eight bytes. The microSDHC card holds over one billion times more bytes in much less physical space.
File:Magnetic-core memory, 18x24 bits.jpg|Magnetic-core memory, 18×24 bits, with a [[US quarter]] for scale
File:Magnetic-core memory close-up.JPG|Magnetic-core memory close-up
File:Magnetic-core memory, at angle.jpg|At an angle
</gallery>