Computer register
- 1. REGISTERS 1
- 2. Outline ▪ Registers ▪ Benefits of Registers ▪ Basic Register ▪ Shift Register ▪ Bidirectional Shift Register with Parallel Load 2
- 3. Registers ■ Registers are common sequential devices. – They’re a good example of sequential analysis and design. – They are also frequently used in building larger sequential circuits. ■ Registers hold larger quantities of data than individual flip-flops. – Registers are central to the design of modern processors. – There are many different kinds of registers 3
- 4. Benefits of registers ■ Flip-flops are limited because they can store only one bit. – Two flip-flops are used for two-bit counters. – Most computers work with integers and single-precision floating-point numbers that are 32-bits long. ■ A register is an extension of a flip-flop that can store multiple bits. ■ Registers are commonly used as temporary storage in a processor. – They are faster and more convenient than main memory. – More registers can help speed up complex calculations. 4
- 5. A basic register ■ Basic registers are easy to build. We can store multiple bits just by putting a bunch of flip-flops together! ■ A 4-bit register is shown on the right, and its internal implementation is below. – This register uses D flip-flops – it’s easy to store data without worrying about flip-flop input equations. – All the flip-flops share a common CLK and CLR signal. 5
- 6. Figure: A 4-bit basic Register 6
- 7. Shift register ■ A shift register “shifts” its output once every clock cycle. ■ SI is an input that supplies a new bit to shift “into” the register. ■ For example, if on some positive clock edge we have: SI = 1 Q0-Q3 = 0110 then the next state will be: Q0-Q3 = 1011 ■ The current Q3 (0 in this example) will be lost on the next cycle. Q0(t+1) = SI Q1(t+1) = Q0(t) Q2(t+1) = Q1(t) Q3(t+1) = Q2(t) 7
- 8. Bidirectional shift register with parallel load A bidirectional shift register is one in which the data can be shifted either left or right. It can be implemented by using gate logic that enables the transfer of a data bit from one stage to the next stage to the right or to the left, depending on the level of a control line. 8
- 9. Bidirectional shift register with parallel load (cont.) S1 S0 Register Operation 0 0 No change (Hold) 0 1 Shift Left (down) 1 0 Shift Right (up) 1 1 Parallel Load Parallel load DOWN UP HOLD 9
- 10. Figure: Bidirectional Shift Register with Parallel Load 10