Introduction to Registers

1
Introduction to Registers

• Being just logic, ALUs require all the inputs to
  be present at once.
• They have no memory.

A
B
F
S
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
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
The Flip-Flop. A Single Bit Register.
CLK
Q Q
D
8
A Two-bit Register
Q Q
Q0
CLK
CLK
D
D0
Q Q
Q1
CLK
D
D1
9
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
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
Interconnecting Registers

• How do we transfer information between two
  registers ? Like this ?

15
Three Registers

• OK, that sort of works, what about 3 registers ?

Dont try to make sense of this. Its 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.