1 7-7 Register-Cell Design A single-bit cell of an iterative combinational circuit connected to a flip-flop that provides the output forms a two-state sequential circuit called a register cell. Example 7-1 Register-cell design A register A and is to implement the following register transfers with an input B :

2 Example 7-1 Assumption  Only one of AND, EXOR, and OR is equal to 1  For all AND, EXOR, and OR equal to 0, the content of A remains unchanged

3 Example 7-1 One solution  LOAD=AND+EXOR+OR  From Table 7-11, we can rewrite the solution as

4 Example 7-1 Simplify the equation  Share the control variables to all register cells since they are the same for each cell  Simplification from 2nd solution in the previous slide

5 Example 7-1  Simplification from the 1st solution in slide page 3

6 Example 7-1 Use the simplification in slide page 4 can save about 40% (for 16 cells) gate cost and hence time delay compared to those by using the simplification in slide page 5. Why?

7 Example 7-2 A register A is to implement the following register transfers with an input B : Assumption: Only one of SHL, EXOR, and ADD is equal to 1 For all SHL, EXOR, and ADD equal to 0, the content of A remains unchanged

8 Example 7-2 Solution  LOAD=AND+EXOR+OR  Another solution (combine ADD and SHL (share C i )) 

9 Example 7-2 Simplification (from 2nd solution in previous slide) (C i =0 for EXOR)

10

Multiplexer and bus-based transfers for multiple register Dedicated multiplexer 2n AND gate cost and n OR gate cost per multiplexer Total of 9n gate cost

12 Single bus 3n AND gate cost and n OR gate cost Total of 4n gate cost

13 Single bus Note: The 3rd case in the table is possible for dedicated multiplexer architecture

14 Three-state bus

Serial transfer and Microoperations Information in a system is transferred or manipulated one bit at a time

16 Serial transfer

17 Serial Addition

18 We are now neglecting the following two sections 7-10 Two design examples 7-11 HDL

Microprogrammed Control A control unit with its binary control values stored as words in memory Each word in the control memory contains a microinstruction A microinstruction specifies one or more microoperations for a system A sequence of microinstructions constitutes a microprogram

20 Two registers Control address register (CAR): a register specifies the address of the microinstruction Control data register (CDR): a register holds the microinstruction currently being executed by the datapath and the control unit

21 Next-address generator When a microinstruction is executed, the next- address generator produces the next address The address of next instruction to be executed may be next one or located somewhere else in the control memory A function of control word is to determine the address of the next microinstruction to be executed Sometimes it is called sequencer

22 Microprogrammed control unit organization

23 CISC A simple instruction set computer (SISC) as introduced above can’t fit the complex applications for today’s computer. A complex instruction set computer (CISC) has emerged (Chap. 11)