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Introduction to Registers

Introduction to Registers. Being just logic, ALUs require all the inputs to be present at once. They have no memory. A. B. S. F. ALU. Basic Computation.

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Introduction to Registers

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  1. Introduction to Registers • Being just logic, ALUs require all the inputs to be present at once. • They have no memory. A B S F ALU

  2. Basic Computation • Computers perform complicated tasks only because the programmer has broken that task down into a sequence of simple primitive operations. • Moving information • Performing simple arithmetic and logical operations • To perform operations in sequence, the results of previous operations must be remembered somehow.

  3. Using Memory • Even the simplest computations require memory. • For example, take a pocket calculator. To add 4 and 3: • Press ‘4’ – calculator remembers the number • Press ‘+’ – calculator remembers ‘4’ and ‘add’ • Press ‘3’ – calculator remembers ‘4’, ‘add’ and ‘3’ • Press ‘=’ – calculator works out the answer and remembers it so it can be displayed • To achieve this kind of functionality, we need a circuit that can remember binary numbers – a register.

  4. Memory Circuits • Simple memory circuit can be built using a pair of NOR gates: • If the SET input is high, the output goes high • If the RESET input is high, the output goes low • If neither input is high, the output stays in its previous state

  5. 0 1 (0) (1) (0) 0 0 1 1 0 (0) (1) (1) (0) (0) (1) 1 0 How it Works NB. Output of NOR gate = 1 only if both inputs are 0 0 0 (1) (0) (1) 0

  6. Flip-Flops • The R-S flip-flop is an asynchronous flip-flop. Its output changes immediately in response to an input. • Synchronous flip-flops only respond when they are ‘clocked’ by a separate clock-in. • Synchronous devices are preferred in computer design to avoid problems with unpredictable propagation delays. • Simplest synchronous flip-flop is probably the D-type flip-flop. • It is also the basis of a single-bit memory register.

  7. D CLK Q The Flip-Flop. A Single Bit Register. CLK Q Q D

  8. D0 D1 CLK Q0 Q1 A Two-bit Register CLK Q Q Q0 CLK D0 D Q Q Q1 CLK D1 D

  9. CLK CLK D D Q Q 7 7 CLK CLK D D Q Q 6 6 CLK D D Q Q 5 5 CLK CLK 8 8 D D Q Q 4 4 D0-7 D0-7 Q0-7 Q0-7 CLK D D Q Q 3 3 CLK D D Q Q 2 2 CLK (shorthand form) D D Q Q 1 1 CLK D D Q Q 0 0 Multiple Bit Register • To remember a whole byte, just use eight flip-flops…

  10. The ‘Working Register’ • The most common use of a register in a micro-controller is in conjunction with an ALU. • To simplify the inputs, one of the operands of any ALU operation is stored in a register. • Depending on the device, this register is known as: • The Working Register (W) • The Accumulator (A)

  11. Using a Working Register

  12. Single input byte 8 Working CLK register D Q A B Control S ALU inputs C F OUT 8 Carry Result output Example Operation • To subtract 3 from 5… • Stage 1 • Store the number 3 in the working register • Ignore the ALU output • Stage 2 • Input the number 5 and the selection word, S, representing (A – B) • ALU output equals 2

  13. Are registers the same as memory ? • Yes • Computer memory (RAM) is, effectively, a big bank of registers. • In the PIC microcontrollers, the registers are the memory. • No • Memory is usually physically separate from the microprocessor. • Most microprocessors have only a few registers. • Registers can be read from and written to very quickly. • Register contents can be exchanged directly.

  14. CLK1 CLK 8 8 D0-7 Q0-7 Q0-7 CLK2 CLK 8 8 D0-7 Q0-7 Q0-7 Interconnecting Registers • How do we transfer information between two registers ? Like this ?

  15. Three Registers • OK, that sort of works, what about 3 registers ? Don’t try to make sense of this. It’s an utter mess ! CLK 8 8 D0-7 Q0-7 CLK CLK 8 8 8 8 D0-7 Q0-7 D0-7 Q0-7

  16. Three Registers Mk II CLK1 CLK 8 8 D0-7 Q0-7 CLK2 CLK 8 8 8 D0-7 Q0-7 CLK3 CLK 8 8 D0-7 Q0-7

  17. Interconnecting Registers • All registers are connected to a common set of 8 wires – a bus. • Each register needs a ‘switch’ to determine whether the bus is connected to: • The register output • Register is outputting to the bus • The register input • Register is inputting from the bus or… • … register is ignoring the bus • Using this basic scheme, any number of registers can be connected to the bus with no increase in complexity.

  18. Summary – ALUs and Registers • ALUs, despite their complexity, they are just combinational logic circuits with no memory. • Registers are fast access memory elements found inside microprocessors, micro-controllers etc. • Registers and their interconnections will be be discussed in more detail next time.

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