in binary, and the clock could count time internally just fine. Now you could use 3 registers store the hour, minutes etc. Conceptually, you need to store the hour (0-23), minutes (0-59) and seconds (0-59) in some registers, and display the content of the first two at the very least. Think of something like a simple digital clock. No processor and not more than a couple bits of memory needed. A monitoring device, such as a printer, could similarly be interfaced using this method with only circuit controls. This method was easy to pull directly off of various Nixie or numerical readouts and sent to the output port. Why do all this? There was no processor in any of this, and only a couple bits of memory outside of the numerical counters. Quite a bit more efficient than the 405CR. On the output connector three digits are encoded on 4-wire BCD, with a single wire for the leading "1", for a total of 13 lines. Somewhat later, consider equipment such as the Hewlett Packard 3403C True RMS Voltmeter. This used what was known as "Ten-line code", where each digit had its own output wire, so the three digits needed 30 lines. A short time after the IBM 650, consider the Hewlett Packard 405CR Digital Voltmeter. It's possible to have errors and rounding in binary that are different from what happens with decimal if binary numbers are represented fractionally. This was a fully-decimal computer, using BCD coding: One of the key concerns at the beginning was how to deal with precise fractional and decimal values with a binary computer. The most familiar example of a BCD computer is the IBM 650. Binary A through F are generally undefined. Short answer: BCD encodes 0 through 9 just like binary 0 through 9. For these applications, some small processors feature BCD arithmetic modes, which assist when writing routines that manipulate BCD quantities. Often, smaller code results when representing numbers internally in BCD format, since a conversion from or to binary representation can be expensive on such limited processors. The same argument applies when hardware of this type uses an embedded microcontroller or other small processor. Most pocket calculators do all their calculations in BCD. Therefore, in cases where the calculations are relatively simple, working throughout with BCD can lead to a simpler overall system than converting to and from binary. If the numeric quantity were stored and manipulated as pure binary, interfacing to such a display would require complex circuitry. This matches much more closely the physical reality of display hardware-a designer might choose to use a series of separate identical seven-segment displays to build a metering circuit, for example. By employing BCD, the manipulation of numerical data for display can be greatly simplified by treating each digit as a separate single sub-circuit. BCD is very common in electronic systems where a numeric value is to be displayed, especially in systems consisting solely of digital logic, and not containing a microprocessor.
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