Control Path

<aside> ❗ Before the Lab:

Get EEP0 work
Download the 8x8 regfile and add it to the directory
Before Task 1 take a screenshot of RF1 on your eep1 sheet showing where each of its ports connects. This will help you remember this when rewiring the larger register file.

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Changing the register file

T1

<aside> ✅ Task 1.

On the eep1 sheet delete RF1 and its TR0-3 Viewers, and add a single new regfile8x8 component. Wire the new component as the old one. Using the Properties tab change the Bus Select components controlling the AD1, AD2, AD3 inputs so that they select the new positions and sizes of the a,b fields as in Figure 1.8.

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FIG 1.1: The new regfile8x8 vs the old one. Note that the TR0-TR3 viewers have relocated to inside the regfile8x8.

FIG 1.2 Inside the regfile8x8 – basically two regfiles in tandem. However, AD1, AD2 and AD3 are 3 bits in width compared to 2 bits to regfile’s previous AD1-AD3.

Let’s look at a single regfile. (not shown)

ADDR1-3 are 2 bits each. ADDR1 chooses DOUT1 (2 bits to choose 4 options) , and ADDR2 chooses DOUT2 (2 bits to choose 4 options). ADDR3 chooses which register gets written to, if any (2 bits to choose 4 options) It defaults to R0 obviously, but no write happens unless WEN3 allows it.

AD1 and AD2 has included an additional bit for the MUX to choose between the first or the second regfile (R0-R3 or R4-R7). That additional bit in AD1 handles MUX1 and AD2’s additional bit handles MUX2. The idea is still the same, AD1 chooses DOUT1 and AD2 chooses DOUT2.

What about AD3? AD3 can choose from (R0-R7) to write to.

AD1	What Happens	AD2	What Happens	AD3	What Happens
000	DOUT1 = Reg0	000	DOUT2 = Reg0	000	DIN3 writes to Reg0 only when WEN3 is 1
001	DOUT1 = Reg1	001	DOUT2 = Reg1	001	DIN3 writes to Reg1 only when WEN3 is 1
010	DOUT1 = Reg2	010	DOUT2 = Reg2	010	DIN3 writes to Reg2 only when WEN3 is 1
011	DOUT1 = Reg3	011	DOUT2 = Reg3	011	DIN3 writes to Reg3 only when WEN3 is 1
100	DOUT1 = Reg4	100	DOUT2 = Reg4	100	DIN3 writes to Reg4 only when WEN3 is 1
101	DOUT1 = Reg5	101	DOUT2 = Reg5	101	DIN3 writes to Reg5 only when WEN3 is 1
110	DOUT1 = Reg6	110	DOUT2 = Reg6	110	DIN3 writes to Reg6 only when WEN3 is 1
111	DOUT1 = Reg7	111	DOUT2 = Reg7	111	DIN3 writes to Reg7 only when WEN3 is 1

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FIG 1.8.

Changing Decode for the new Instruction Formats