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Duncan wrote: |
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>The history of the term "enable" in computer usage, I believe traces to |
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>its use for computer hardware. Consider the old serial protocols as used |
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>in telephone modems and the like. Being binary logic, each signal line |
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>had two states, 0/off, typically at ground-reference voltage, and 1/on, |
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>typically at 5 volt reference relative to ground. The following |
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>description should be generally accurate, altho I never mapped the |
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>specifics for any particular protocol or pin-out into |
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>permanent brain-memory, only absorbing enough to understand the principles |
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>of how it worked. |
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> |
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> |
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OK, time to date myself. In hardware (yes, even pre-computer hardware), |
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one of |
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the lowest design elements is a gate (could be transistors, could be |
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relays, etc). |
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With a gate you can open or close it. In digital logic, the gate can be |
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in two states, |
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all the way open or all the way close. When open, then things are |
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allowed to pass |
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thru. When closed they are not. The logic signal for controlling the |
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gate is normally |
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referred to as the enable. If the gate is enabled, then things can flow |
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thru it. |
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|
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Notice that the term is used to describe a logic function. It is |
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possible to have |
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active-high or active-low enables. So the value necessary to enable the |
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gate is |
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implementation dependent. For example, the gate could be enabled on: 1, |
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0, +5V, |
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0V, +12V, -12V, +20psi, -10C, whatever. |
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|
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Back to software, normally a zero is used to represent deasserted, a |
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non-zero represents |
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asserted. Then we like to semantically assign boolean states such as |
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on/off, true/false |
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to asserted/deasserted. It is best not to think of booleans as having |
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numerical values, |
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but the base 2 numbering system's set of digits is {0,1}, so some |
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software will use |
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base 10's 0 and 1 digits for booleans (I have seen 0xffff used for true |
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in a C compiler). |
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|
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Now getting back to the original discussion. It's clearer to state: |
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"the enable is asserted" |
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than "the enable is true" or "the enable is high" or "the enable is on" |
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or "the enable is 1". |
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But common mis-usage has them all equivalent. |
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|
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Have fun, |
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Roy |
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-- |
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gentoo-amd64@g.o mailing list |
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