5.5: Implementing the CU

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It is now possible to specify how to set the control wires from the CU. First the ALU opt is the value of bits 8-11 of the ALU, so these are split off and sent to control the ALU.

The top 4 bits, bits 12-15, are used to set the other control wires, and from the previous discussion can be set according to the following table².

Table 5-1: Operations and control wires
Operation	Code	WriteAc	ALUSrc	ClrAc	MemWr	Beqz
Immediate Operation³	0x1	1	1	0	0	0
Memory Operation⁴	0x2	1	0	0	0	0
clac	0x3	1	x	1	0	0
stor	0x4	0	x	x	1	0
beqz	0x5	0	x	x	0	1

To implement this table, a decoder is implemented to split out the individual operations. These operations are then combined to produce the correct output behavior. The CU is shown in the figure below.

Screen Shot 2020-07-03 at 12.45.05 PM.png

² An “x” in the table means a don’t care condition, e.g. the value can be either 0 or 1 as it does not affect the working of the CPU. As a convention, all x values should be coded as 0.
³ addi, subi, etc.
⁴ add, sub, etc.