Designing a Control Unit

Designing a Control Unit

AU: Dec.-14,18

• Here, we restrict ourselves to implement load word (lw), store word (sw), branch equal (beq), and the arithmetic-logical instructions add, sub, AND, OR, and set on less than. We will also the design to include a jump instruction (j).

The ALU Control

• The MIPS ALU defines the six following combinations of four control inputs:

• Depending on the instruction class, the ALU will need to perform one of these first five functions. (NOR function is needed for other parts of the MIPS instruction set. It is not included in the subset we are implementing.)

• In case of load word and store word instructions, we use the ALU to compute the memory address by addition.

• In case of the R-type instructions, the ALU needs to perform one of the five actions - AND, OR, subtract, add, or set on less than.

• In case of branch equal, the ALU must perform a subtraction.

• We can control the operation of ALU by the 4-bits ALU control input and 2-bits ALUOP. The 2-bits ALUOP is interpreted as shown in Table 7.4.1.

• Table 7.4.2 shows how to set the ALU control inputs based on the 2-bits ALUOP control and the 6-bits function code.

• Here, multiple levels of decoding technique is used.

Advantages of using multiple levels of decoding

• It reduces the size of the main control unit.

• Use of several smaller control units may also potentially increase the speed of the control unit.

• Table 7.4.3 shows how the 4-bits ALU control is set depending on these two input fields: 6-bits function fields and 2-bits ALUOp field.

• Once the truth table has been constructed, it can be optimized and can be implemented using logic gates.

Designing the Main Control Unit

• Before looking at the rest of the control design, it is useful to review the formats

of the three instruction classes: The R-type, branch and load-store instructions. Fig. 7.4.1 shows these formats.

• Format for R-format instructions: Opcode is 0. These instructions have three register operands: rs, rt, and rd. Fields rs and rt are sources, and rd is the destination. The funct (Function) field is an ALU function discussed in the previous section. The shamt field is used only for shifts.

Format for load and store instructions: Load (opcode ) or store (opcode ). The register rs is the base register that is added to the 16-bit address field to form the memory address. For loads, rt is the destination register for the loaded value. For stores, rt is the source register whose value should be stored into memory.

Format for branch equal: Opcode is 4. The registers rs and rt are the source registers that are compared for equality. The 16-bits address field is sign-extended, shifted, and added to the PC + 4 to compute the branch target address.

Important observations about this instruction format

• Bits 31: 26 in the instruction format is op field and gives opcode (operation code). We will refer to this field as Op[5: 0].

• Bits 25:21 and 20:16 in the instruction format always specify the rs and rt fields, respectively.

• Bits 25: 21 always give the base register (rs) for load and store instructions.

• Bits 15: 0 give the 16-bits offset for branch equal, load, and store.

• The destination register is in one of two places. For a load it is in bit positions 20: 16 (rt), while for an R-type instruction it is in bit positions 15: 11 (rd). Thus, we will need to add a multiplexer to select which field of the instruction is used.

• From the above information, we can add the instruction labels and extra multiplexer (for the Write register number input of the register file) to the simple datapath. Fig. 7.4.2 shows these additions plus the ALU control block, the write signals for state elements, the read signal for the data memory, and the control signals for the multiplexers. Since all the multiplexers have two inputs, they each require a single control line.

• Fig. 7.4.2 shows seven single-bit control lines (RegDst, RegW, ALUSrc, MemW, MemR, PCSrc and MemtoReg) plus the 2-bits ALUOP control signal.

• Table 7.4.4 describes the function of single-bit control lines.

• These nine control signals (seven single-bit control lines and the 2-bits ALUOP control signals) can be set according six input signals to the control unit, which are the opcode bits 31 to 26. Fig. 7.4.3 shows the datapath with the control unit and the control signals. [Refer Fig. 7.4.3 on next page]

• As shown in the Fig. 7.4.3, the input to the control unit is the 6-bits opcode field from the instruction.

• The outputs of the control unit consist of three 1-bit signals that are used to control multiplexors (RegDst, ALUSrc, and MemtoReg), three signals for controlling reads and writes in the register file and data memory (RegWrite, MemRead, and MemWrite), a 1-bit signal used in determining whether to possibly branch (Branch), and a 2-bits control signal for the ALU (ALUOP).

• An AND gate is used to combine the branch control signal and the Zero output from the ALU; the AND gate output controls the selection of the next PC.

• Table 7.4.5 defines whether each control signal should be 0, 1, or don't care (X) for each of the opcode values.

• For all R-format instructions (add, sub, AND, OR, and slt) the source register fields are rs and rt, and the destination register field is rd; this defines how the signals ALUSrc and RegDst are set (See first row of Table 7.4.5).

• R-type instruction also writes a register (Reg W=1), but neither reads nor writes data memory.

• For all R-format instructions, the PC should be unconditionally replaced with PC 4. Thus the Branch control signal is 0; otherwise, the PC is replaced by the branch target if the Zero output of the ALU is also high.

• The ALUOP field for R-type instructions is set to 10 to indicate that the ALU control should be generated from the funct field.

• The second and third rows of Table 7.4.5 give the control signal settings for lw and sw. These ALU Src and ALUOP fields are set to perform the address calculation.

• The MemRead and MemWrite are set to perform the memory access. Finally, RegDst and Reg W are set for a load to cause the result to be stored into the rt register.

• The branch instruction sends the rs and rt registers to the ALU. The ALUOP field for branch is set for a subtract ALU control 01), which is used to test for equality.

• It is important to note that the Mem to Reg field is irrelevant when the Reg W signal is 0: since the register is not being written, the value of the data on the register data write port is not used. Thus, the entry Mem to Reg in the last two rows of the Table 7.4.5 is replaced with X for don't care. Don't cares can also be added to RegDst when Reg W is 0.

Operation of the Datapath

Datapath for an R-type Instruction

• Fig. 7.4.4 shows the operation of the datapath for an R-type instruction. Here as an example we have considered instruction: add $t1, $t2, $t3. The asserted control signals and active datapath elements are highlighted. Although everything occurs in one clock cycle, the execution of the instruction can be divided into sequential steps according to flow of information as given below:

1. Fetch the instruction and increment the PC.

2.Read data from two registers, $t2 and $t3 and compute the setting of the control lines using main control unit.

3. Generate the ALU function using the function code for addition operation (bits 50, which is the funct field, of the instruction) and perform addition on the data read from the register file.

4. Write the result from the ALU into the register file using bits 15: 11 of the instruction that select the destination register ($t1).

Note The control lines, datapath units, and connections that are active are highlighted.

• Fig. 7.4.5 shows the operation of the datapath for load word instruction. Here as an example we have considered instruction: Iw $t1, offset($t2). The execution of the load instruction can be divided into sequential steps according to flow of information as given below:

1. Fetch the instruction and increment the PC.

2. Read data from register $t2 of register file.

3. Compute the sum of the value read from the register file and the sign-extended, lower 16 bits of the instruction (offset) using ALU.

4. Use the sum from the ALU as the address for the data memory.

5. Write the data from the memory unit into the register file; the register destination is given by bits 20: 16 of the instruction ($t1).

Note The control lines, datapath units, and connections that are active are highlighted.

• A load and store instructions operate very similarly. The main differences are that the memory control indicate a write rather than a read, the value of the second

register is used for the data to store, and the operation of writing the data memory value to the register file is not necessary.

Datapath for Branch - On - Equal Instruction

• Fig. 7.4.6 shows the operation of the datapath for branch-on-equal instruction. Here as an example we have considered instruction: beq $t1, $t2, offset. It operates much like an R-format instruction, but the ALU output is used to determine whether the PC is written with PC + 4 or the branch target address. The execution of the branch-on-equal instruction can be divided into sequential steps according to flow of information as given below :

1. Fetch the instruction and increment the PC.

2. Read data from two registers, $t2 and $t3.

3. Generate the ALU function using the function code for subtract operation (bits 5 0, which is the funct field, of the instruction) and perform subtraction on the data read from the register file. Add the value of PC + 4 to the sign-extended, lower 16 bits of the instruction (offset) shifted left by two to get the branch target address.

4.Use the Zero result from the ALU to decide which adder result to store into the PC.

Note The control lines, datapath units and connections that are active are highlighted.

Finalizing Control

• The control function can be precisely defined using the contents of Table 7.4.6. The outputs are the control lines, and the input is the 6-bits opcode field, Op [5: 0], i.e. bits 31 26 of the instruction. Thus, we can create a truth table for each of the outputs based on the binary encoding of the opcodes as shown in the Table 7.4.6.

Implementing Jumps

• The jump instruction is not conditional. Fig 7.4.7 shows the format of jump instruction.

• Like a branch, the low-order 2 bits of a jump address are always 00₂. The next lower 26 bits of this 32-bits address come from the 26-bits immediate (address) field in the instruction. The upper 4 bits of the address that should replace the PC come from the PC of the jump instruction plus 4. Thus, we can implement a jump by storing into the PC the concatenation of

• The upper 4 bits of the current PC + 4

• The 26-bits immediate field of the jump instruction

• The bits 00_two

•Fig. 7.4.8 shows the datapath operation for jump instruction. Here, additional control is added to Fig. 7.4.3 for jump instruction. An additional multiplexer is used to select the source for the new PC value, which is either the incremented PC (PC + 4), the branch target PC, or the jump target PC. One additional control signal is added for the additional multiplexer. This control signal, called Jump, is asserted only when the instruction is a jump-that is, when the opcode is 2.

Reasons for not using Single - Cycle Implementation

1. It is inefficient. Because the longest possible path in the processor determines the clock cycle. Remember that the clock cycle must have the same length for every instruction in single-cycle design.

2. The overall performance of a single-cycle implementation is likely to be poor, since the clock cycle is too long.

3. The penalty for using the single-cycle design with a fixed clock cycle is significant. Single-cycle designs for floating-point unit or an instruction set with more complex instructions do not work well at all.

4. It do not improve the worst-case cycle time. Thus it violates the great idea of making the common case fast.

Review Questions

1. State the advantages of multiple levels of decoding.

2. Give instruction formats for R type, load or store and branch instructions.

3. Draw the simple combine datapath with all necessary multiplexers and control lines.

4. State the functions of following control lines.

a) PC Src b) ALU Src c) RegDstd) Mem R

5. Draw and explain the simple datapath with the control unit and explain the execution of ALU instructions.  AU: Dec.-18, Marks 13

6. With the help of a neat diagram explain the operation of datapath for R type instruction.

7. With the help of a neat diagram explain the operation of datapath for load instruction.

8. Give the sequential steps required for execution of R-type instruction.

9. Give the sequential steps required for execution of load instruction.

10. Explain the operation of datapath for branch instruction.

11. Give the sequential steps required for execution of branch operation.

12. Draw the format of jump instruction.

13. Draw and explain the datapath operation for jump instruction.

14. State the reasons for not using single cycle implementation.

15. Explain data path control in detail.  AU: Dec.-14, Marks 8