outline

Levels of Abstraction

• Still hardware
• At a component (subsystem) level
   - transistors -- > gates -- >
   - circuits -- > subsystems
• Abstraction hides detail
   - important concept in many areas of CS

John Von Neumann

• Was a well-known mathematician
• Wrote the draft report on the design of the EDVAC (the first truly modern computer to be conceived)
• The report outlined the design of the modern computer -
   - central processing unit based on Boolean algebra
   - used binary arithmetic
   - random-access memory that contained both data and programs

Architecture

• The Von Neumann Architecture is a model for designing and building computers based on 3 characteristics:
   - a computer constructed of 4 major subsystems
      - memory, input/output, ALU, and control unit
   - stored program concept
   - sequential execution of instruction

Visual of the Architecture

ALU

• The "Brain"
• Separate, high-speed-access registers
   - storage for operands
• Obtain operands from registers, store the result in a register by activating correct BUS

ALU Operation

• Add, subtract, multiply divide and compare
• Circuits for each operation
   - we have already seen the add circuit
• Each operation is done
• Select the result you want
   - Multiplexor

3-register ALU

Multi-register ALU

Control Unit (CU)

• Fetch Memory (instruction)
• Decode
• Execute --> (ALU)
• Store

Memory

• Random-access memory - RAM
   - array of cells, each with an address
   - cell is the minimum unit of access
   - access time is address-dependent
• ROM – Read Only Memory
   - a section of RAM set aside for system software

Memory Structure

• Cell size or width (W)
   - generally 8 bits, called a BYTE
   - may need multiple cells to store large numbers
     or long program instructions
• Cell content vs. cell address

Memory Registers

• Special (large) storage cells
• 1 – Memory address register – MAR
   - holds cell address
   - N bits wide
• 2 – Memory data register - MDR
   - holds data values
   - multiple of W

Structure of RAM

Address Space

• Instructions refer to cell addresses
   - stored in binary form
• Suppose N bits for the address
   - can represent number in the range of 0 to 11 … 1 = 2^N - 1
   - determines memory size, 2^N
   - "address space"

Basic Memory Ops

• 1 – Fetch (address) – "Read"
   - load address into MAR
   - decode address
   - copy contents of cell at that address into MDR
   - nondestructive operation

Basic Memory Ops – 2

• 2 – Store (address, value) – "Write"
   - load address into MAR
   - load value into MDR
   - decode address
   - store contents of MDR into the cell with that address
   - destructive operation

Decode Address

• Convert a string of N bits (on N bits (on N input lines)) to a signal on a single output line
• This is a decoder
• To scale up, use a 2-dimensional arrangement

Memory/Decoding Logic

• 
• Organization of Memory and Decoding Logic

2-Dimensional Memory Org

RAM Organization

I/O

• Human-machine interface
   - keyboard, screen, printer
• Machine-machine interface
   - mass storage, secondary storage
   - floppy disks, hard disks, CDs, tape
• Most nonstandard subsystems