Von Neumann (Princeton Architecture)

Design

Design:

• John von Neumann (1945)
• Stored-program computer concept
  • Instruction and data stored in the same memory

Components:

• Processing Unit
  • Arithmetic Logic Unit (ALU) and Registers
• Control Unit
  • Instruction Register (IR) and Program Counter (PC)
• Memory Unit
  • Data and Instructions
• External Mass Storage
• Input and Output Mechanisms

Registers:

• PC: Program Counter
  • Address of next instruction to be executed
• IR: Instruction Register
  • Current instruction during processing
• AC: Accumulator
  • Stores intermediate arithmetic and logic results
• MAR: Memory Address Register
  • Stores address of memory to read into the MDR.
    • (To send data to the MDR you need to send a read signal)
• MDR: Memory Data Register
  • Where read data is stored.
• GP: General Purpose Registers

Note: MAR and MDR can be referred to as the BIU (Bus Interface Unit)

System on a Chip (SOC): When the CPU and memory unit are on the same silicon.

Memory: - Address and content

• Large array of locations where binary information is stored and fetched

Buses: Means by which data is transmitted from one part to another

Note: Registers used in the fetch-decode-execute cycle.

1. Fetch

• PC
• IR

2. Decode

• IR

3. Execute

• Every single register gets used!
• REGS
• IR
• PC
• MDR
• MAR
  • (If there is a jump command)

  # 1. Fetch Cycle
  MAR <- PC # Move program counter into Memory Address Register
  MR # Signal read through address bus, then data bus sends data to MDR
  IR <- MDR # (We need to move instruction to IR for CU to work on it)

Example: C = A + B

Suppose we have three locations in memory: A, B, and C.

Q: How can we do C = A + B?

A: Place A in the MAR, then send the read signal through the address bus.

• The data will be placed on the data bus, where the data will show up on the MDR.

In individual steps:

MAR <- @A
MR # memory read signal
MDR <- A
AC <- MDR
MAR <- @B
MR
MDR <- B
AC <- MDR
MAR <- C
MDR <- AC
MW

In assembly:

LD A // The first four lines of the above
ADD B
ST C // Store contents of AC in C

Example: A = A + B

LD A # Put A in the AC
ADD B # Add B to the contents of the AC
ST A # Store contents of AC in A

Example: Micro-stepping through AC <- AC + M1

# Fetch
MAR <- PC
MR
MDR < MMEM.MDR
IR <- MDR
# Decode
MAR <- @MI
MR
MDR <- MMEM.MDR
# Execute
ALU.lhs <- AC
ALU.rhs <- MDR
ALU.add
AC <- ALU.out

Harvard Architecture

Biggest Difference: Instruction and data memory are separate (two memory units).

• Used for digital signal processing where the bandwidth and speed are needed.