IBM 610 workstation computer 3387

Still, it used binary-coded decimal for memory addressing, which arguably made it more of a decimal machine than one that used binary addresses.

Binary is faster for calculations, but requires conversion to and from the decimal which is usually used on input and output. (Few people have been persuaded to read reports or prepare input data in bases other than 10.) The time required to do this conversion could be significant, especially on machines of the day. Thus, scientific applications, which typically do a lot of number-crunching but relatively little I-O, run faster in binary - while business applications, whose calculations are relatively simple but are repeated over thousands or even millions of records, run faster in decimal.

The 360's packed decimal fields, with a maximum capacity of 31 digits, were adequate for just about any application, even when you add a generous helping of guard digits.

It sounds like a truncation of integer fields, which is typical hardware behaviour. And yes, the easiest way to convert this behaviour to rounding is to add a 5 to the high-order guard digit (i.e. add .005 if you wanted going to round to a hundredth of a dollar). It was standard programming practice in buttembly language, where a prior multiplication would result in excess low-order digits. Add 5 to the highest-order digit you were going to drop; truncation would then yield a properly rounded result. Negative numbers are a bit tricky, though - you have to determine whether you're rounding toward the next highest negative integer or toward zero, so you'd know whether to add or subtract that .005 (or -.005). Working in packed decimal had the advantage that you could copy just the sign to a work area that had been initialized to .005, which meant that you could do the calculation without fussing with conditionals.

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