Lecture: Computer Registers and Common Bus System

Overview

To design a basic computer, a minimum number of essential registers is required. This lecture covers the processor registers necessary for such a system, their roles, sizes, and key signals associated with them.

Processor Registers

1. Program Counter (PC)

   • Size: 12 bits
   • Function: Holds the address of the next instruction to be executed.

2. Address Register (AR)

   • Size: 12 bits
   • Function: Holds the address of a memory location; points to memory.

3. Instruction Register (IR)

   • Size: 16 bits
   • Function: Holds the instruction code.

4. Temporal Register (TR)

   • Size: 16 bits
   • Function: Holds temporary data generated during certain micro-operations; used in call/return of subroutines.

5. Data Register (DR)

   • Size: 16 bits
   • Function: Holds memory operands/data fetched from memory.

6. Accumulator (AC)

   • Size: 16 bits
   • Function: Stores intermediate results of arithmetic/logic operations.

7. Output Register (OUTR)

   • Size: 8 bits
   • Function: Holds the output character to be sent to the output device.

8. Input Register (INPR)

   • Size: 8 bits
   • Function: Holds input character from an input device; operates asynchronously.

Memory

• Size: 4096 words, each of 16 bits

Signals for Registers & Memory

1. Memory

   • Signals: Read, Write

2. Registers

   • Signals: Load, Increment (INR), Clear
   • Registers with Load, Increment, Clear: AR, PC, DR, AC
   • Registers with Load only: IR, OUTR
   • Special Case: INPR does not need any of these signals as it is connected directly to the input device and operates asynchronously.

Common Bus System

• A bus system used to connect memory and registers through specific arrangements and signals.
• Key Registers/Memory Connections:
  • Registers/Memory are connected via a bus and provided with clock signals for synchronization.
  • Outputs from AR directly connected to memory.
  • Inputs to registers (except AC and INPR) come from the bus.

Arithmetic and Logic Unit (ALU)

• Inputs:
  • From DR
  • From AC
  • From INPR
• Outputs:
  • To AC

Extra (E) Bit

• Purpose: Stores carry or overflow bits generated during arithmetic operations; used as an overflow flag.

Multiplexing via Bus Select Lines

• Multiplexing Signals: S2, S1, S0
  • Based on these lines, data from specific registers can be transferred to the bus.

Example: Transferring Data from Memory to Accumulator

1. Load Memory Data to Bus:
   • Set select lines (S2, S1, S0) to 111 to transfer memory data to the bus.
2. Load Bus Data into Data Register (DR):
   • Enable load signal for DR.
3. Transfer Data from DR to Accumulator (AC):
   • Set select lines to 011 to transfer data from DR to the bus.
   • Data from DR also moves to the ALU, which then outputs to AC upon enabling the load signal for AC.

• Note: This method involves intermediate steps for data transfer due to the absence of a direct path from memory to AC.

Conclusion

Understanding the arrangement and functionality of registers and their connection via a common bus system is crucial for designing a basic computer.