The Wang 700 ALU


Now some words about the Wang 700 calculator.
To be more exact: about the schematic of the PCB
logiblock ALU 5918.

(Schematics are available at oldcalculatormuseum.com)

If you happen to have interest to dig into the schematic,
this block diagram may be useful because things are not that
obvious to find in the schematic, which makes heavy use of
7400 and 7410 NAND gates... plus a few 7451.

(Omittet the control signals for better readibility.)

What happens inside the boxes "9s complement" and
"BCD correction" is a case for the article
BCD arithmetic, part 2.

The rest looks pretty obvious... except for one thing.
If you would try to build a logic unit according to the
school books, implementing AND, OR, XOR probably
would look like this:

But the Wang 700 uses a different approach:

(Ignore the data line from the SHR box.)

Note the blue marked control signals AND, XOR.
AND=1, XOR=0 gives you A AND B.
AND=0, XOR=1 gives you A XOR B.
AND=1, XOR=1 gives you A OR B.

And here the reason, why:


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(c) Dieter Mueller 2012