|here is a fascinating old model from hitachi, possibly from
1972 or so.
the red and black case is rather stylish resembling mr spock's
tricorder and the model would have been the height of suavity for sherlock
at first, it seems like another four function model with a memory but
in fact there are many hidden features. the display shows 8 digits but the
machine itself calculates to 12 and indeed you can enter and work with 12
digit numbers. the special "<>" display exchange
key facilitates the banking of the lower 12 digits and 4 upper. for
example, pressing 1234567890 shows 34567890, whence the <>
key toggles the display of 12. the display banking feature was also
used by the casio memory personal mini to
reduce the cost of the then expensive vacuum fluorescent displays. which
could also be the reason here, although this model looks like it was not a
budget design so perhaps 8 was most you could get at that time. a spare
leftmost digit isnt wasted for a minus sign. instead there is a custom
digits on the rightmost which can display a dot (overflow) M for
memory and a bar for minus. unlike the personal mini, a 12 digit result
could be chained on.
general operation is like a desk calculator with the += and
-= postfix operation style. for example 2-3 is entered 2 += 3 -=
and 7/-3 is entered 7 / 3 -=.
the three switches control the decimal rounding and tally enablement,
and constant operation and the rightmost on/off switch. decimal rounding
is interesting, on power up the default is no digits after the point. ie
round to integer. to select a decimal place, switch to ds, press
exactly one number key and switch back from ds either to the middle
position or to the summation position (also enables the tally). the
maximum is 7 and the 8 and 9 keys are interpreted as 7 for this purpose.