Processing Unit Design - Fundamentals of Computer Organization and Architecture - page 102

Figure 5.13 Microinstruction for Add R 1 , R 2 , R 0

Example 4 In this example, we will use vertical microinstructions, in which

decoders will be needed. We will use a three-bus datapath as shown in Figure 5.5.
Assume that there are 16 general-purpose registers and that the ALU supports eight
functions. The following tables show the encoding for ALU functions, registers
connected to out-bus 1 (Source 1), registers connected to out-bus 2 (Source 2),
and registers connected to in-bus (Destination).

Purpose Number of bits Explanations

ALU 4 bits 8 functions þ none

Source 1 5 bits 16 general-purpose registers þ 4 special-purpose

registers þ none

Source 2 5 bits 16 general-purpose registers þ none

Destination 5 bits 16 general-purpose registers þ 4 special-purpose

registers þ none

Encoding ALU function

0000 None (ALU will connect out-bus1 to in-bus)

0001 Add

0010 Subtract

0011 Multiple

0100 Divide

0101 AND

0110 OR

0111 Shift left

1000 Shift right

Encoding Source 1 Destination Encoding Source 2

00000 R 0 R 0 00000 R 0

00001 R 1 R 1 00001 R 1

00010 R 2 R 2 00010 R 2

00011 R 3 R 3 00011 R 3

00100 R 4 R 4 00100 R 4

00101 R 5 R 5 00101 R 5

00110 R 6 R 6 00110 R 6

00111 R 7 R 7 00111 R 7

01000 R 8 R 8 01000 R 8

01001 R 9 R 9 01001 R 9

01010 R 10 R 10 01010 R 10