Chapter 7 Microsequencer Control Unit Design

7.1 Basic Microsequencer Design It stores its control signals in a lookup ROM, microcode memory. The lookup ROM asserts the control signals in the proper sequence to realize the instructions. Microsequencer operations Microinstruction: consists of several bit fields, which are micro-operation field and next address field.

Figure 7.1 Generic microsequencer organization

Microinstruction formats Figure 7.2 Select field: To determine the source of the address of the microinstruction ADDR field: To specify an absolute address for performing an absolute jump by the microsequencer. Micro-operations field: To generate control signals Horizontal microcode Vertical microcode

Figure 7.2

Horizontal microcode and Vertical microcode One bit in the micro-operations field of the microinstruction is assigned to each micro-operation. This can result in large microinstruction. Vertical microcode The micro-operations are grouped into fields. Vertical microinstructions require fewer bits than their equivalent horizontal microinstructions. The microsequencer must incorporate a decoder for each micro-operation signals.

7.2 Design and implementation of a very simple microsequencer Layout: Figure 7.3

Figure 7.3

Mapping logic The microsequencer will use the same mapping function(Refer to Figure 6.9. 1 IR[1..0] 0 This will produce addresses of 1000, 1010,1100,1110 for ADD1, AND1, JUMP1, and INC1, respectively. Figure 7.4 , Table 7.1, Table 7.2

Figure 7.4

Generating the micro-operations using horizontal microcode A microsequencer has two tasks To generate the correct micro-operations To follow the correct sequence of the states. The micro-operations and their mnemonics are shown in Table 7.3. The complete list of control signals is given in Table 7.6.

Generating the micro-operations using vertical microcode Figure 7.5

Figure 7.5

Guidelines for grouping Whenever two micro-operations occur during the same state, assign them to different fields. Include a NOP in each field if necessary(Table 7.7). Distribute the remaining micro-operations to make best use of the micro-operation field bits. Group together micro-operations that modify the same registers in the same fields. Table 7.8, Figure 7.6

Figure 7.6

Nonoinstructions Figure 7.A It encodes all micro-operations in a single field. The microcode memory outputs a value that points to a location in nano-memory.

Figure 7A

7.3 Design and implementation of a Relatively simple microsequencer Refer to Figure 6.12 Two state, JNPZ1 and JMPZ1 are created for the states -JMPZY1, JMPZN1, JPNZY1, JPNZN1(Figure 7.7)

Figure 7.7

Basic layout for the microsequencer Figure 7.8 The box “+1” is a hardware incrementer. Since the state diagram has 39 states, microsequencer needs a 6-bit address. Mapping function: IR[3..0]00. Table 7.11

SEL signal Unconditional jump and conditional jump (Refer to Table 7.12) BT: Branch logic (Table 7.14)

Figure 7.8

7.4 Reducing the number of microinstructions Microsubroutine Figure 7.9 & 7.10

Figure 7.9

Figure 7.10

Reducing the number of microinstructions(continued) Microcode jumps Figure 7.11

Figure 7.11

7.5 Microprogrammed control vs. Hardwired control Complexity of the instruction sets Ease of modification Clock speed

7.6 Pentium Microprocessor Figure 7.12

Figure 7.12