Computer Memory: Purpose, Location & Storage

• Memory is characterized by total storage capacity (in bits), organization, memory access method/policy, access/operation type, operating speed, permanence, and cost.

Memory Purpose

• Stores binary codes for program instructions.
• Stores binary data needed to execute instructions.

Memory as a Location

• Location where information is kept when not in current use.
• A collection of storage devices - each holding one bit.
• Storage devices are grouped into 8s and 8 of location can be accessed together.
• Typically measured by the number of bytes it can hold.

Memory as Storage

• Programs are stored in memory when entered into a computer.
• The microprocessor brings instructions from memory one at a time to execute them.
• The microprocessor reads (brings in) and/or writes (stores) results into memory when done.

Memory Types/Classifications

• Random Access Memory (RAM) loses information when power is off, making it volatile memory.
• Permanent Memory (NVM) retains information, ranging from 10 years to the component's life, or with energy source autonomy.
• Static Random-Access Memory (SRAM) sustains its logical state to store data.
• Dynamic Random-Access Memory (DRAM) stores electrical charges in stored-charge memory.

Memory Cell Functions

• Select: Obtains the desired location.
• DataIn: Inputs data to the selected location.
• DataOut: Grabs data from the selected location.
• Read/Write: Function to read/write to selected location.

Read/Write Timing Diagrams

• A clock signal is a periodic logic signal that rates or synchronizes the operation of a circuit/system such as a microprocessor, a synchronous memory, or an I/O controller.
• An independent clock source (oscillator) can be associated with electronics to generate signals for subsets of the system (buses, memory, processors).
• Timing diagrams detail the evolution of signals as a function of time.
• Cycle begins on the falling edge of the clock (time marker 1).
• The address becomes valid/usable at time marker 2.
• The rising edge of the clock (time marker 3) allows temporal quantification of data write time.
• Data read will be valid after an access time (time marker 5).
• The falling edge of the clock marks the end of a cycle by reading data by the master or writing in the slave (time marker 6).
• Access time from Address (tAA) is the time required for the RAM array to decode the address and provide value to the data bus.
• Write time (tW) is the time data must be held valid to decode the address and store value in memory cells.
• tA (Access time) & tC (Cycle time): Difference between access time and cycle time involving a bit line precharge operation.
• tDHR (Data hold from RAS) is the time to hold data from RAS

Registers

• Registers are composed of a set of Flip-Flops that stores a bit.
• The parallel register is the memory element of the microprocessor with fast memory and word capacity.
• The qualifier "fast" means the register operates at the speed of the component.
• Registers can be accessed in write and/or read and are accessible to the programmer/user.

Common Register Types

• Accumulator is a special register that implicitly receives the result of an operation and can also be used by the programmer
• Auxiliary or generalized registers increase execution speed.
• The program counter (PC) holds the address of the next instruction to be executed by the microprocessor.
• It's also called Instruction Pointer (IP).
• During the fetch cycle, the CPU searches main memory for the instruction code to be executed and stores it in the instruction register (IR).
• Status and control registers are status indicators (condition codes) or status flags.
• These provide information on the current state of the processor:
  • Zero (ZF)
  • Sign (SF) or Negative Flag (NF)
  • Logical parity (PF)
  • Carry (CF) and half-carry flag
  • Auxiliary Flag (AF) or digit carry (DC indicators)
  • Arithmetic overflow (OF)
• Indirection register allows indirect addressing.
• It contains an address to access a memory location and implement the pointer.
• Stack pointer register (SP) points to the memory location to store information or the last location of a backup.
• Processors with supervisor and user modes have two separate pointers for the system and user.
• Special registers depend on implementations and may include feature-specific registers.
• These include registers for virtual memory management or the segmentation registers and descriptor table registers of the x86 computer family.
• The registers are used to control the behavior of the processor for self-test and debugging in newer generations.