===========
ALU Control [TOC]
===========

In the (for today) final step, a circuit can now be created so that depending
on the Op-code and the operands `X`, `Y`, `Z`, and `XY`, the following commands
for controlling the ALU are supported:

+-----------+---------------------------------------------------------------+
|   `00`    | Add the contents of registers `%X` and `%Y` and write the	    |
|	    | result to `%Z`. Update the status flags accordingly.	    |
+-----------+---------------------------------------------------------------+
|   `01`    | Subtract the content of register `%Y` from `%X` and write the |
|	    | result to `%Z`. Update the status flags accordingly.	    |
+-----------+---------------------------------------------------------------+
|   `10`    | Write the bit pattern `XY` to register `%Z`.		    |
+-----------+---------------------------------------------------------------+
|   `11`    | Not currently in use (currently identical to `Op` = `10`).    |
+-----------+---------------------------------------------------------------+

For an Op-code of `00` (Addition) or `01` (Subtraction), data_in is connected to
the output s of the ALU. The result `s` will thus be written to the destination
register `%Z` at the next rising edge of the clock signal. For an Op-code of
`10` or Op-code of `11`, data_in is connected to `XY`. At the next rising edge
of the clock signal, the bit pattern XY will be written to the destination
register `%Z`.

The status flags are only updated after an addition or subtraction.

---- IMAGE (width=600) -----------
session04/s04_8.png
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