Hardware: How RAM works.

How RAM works
  • Best known form of computer memory
  • RAM is considered random access because it can access any memory cell directly regardless of location in memory.
  • RAM is non-sequential.
Sequential Memory

Serial access memory (SAM): Type of a computer memory that stores data in a serial order. Data is accessed bit by bit in a sequential order.

If the data is not in the current location, each memory cell is checked until the data is found.

Computer short term memory: Memory chip is an integrated circuit made of millions of transistors and capacitors.

The most common form of memory is also known as dynamic random access.

Dynamic Random Access Memory (DRAM)

In DRAM a transistor and a capacitor are combined to create a memory cell.

The capacitor holds the bit of information in 0s and 1s.

The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor.

A capacitor is like a small bucked that can store electrons.

To store a 1, the bucked is filled with electrons.

To stored a 0, it is emptied.

For a DRAM to work, the CPU and the memory controller has to come along and recharge all of the capacitors holding 1s before it discharges.

To do this, the memory controller reads the memory and writes it right back.

DRAM Refresh Operations
  • Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it’s holding.
  • Bad thing about this refreshing is that it takes time and it slows down memory.
Memory Cells and DRAM
  • Memory is made of bits arranged in two-dimensional grids.
  • Memory cells are attached to a silicon wafer in an array of rows and columns.
  • The intersection of the wordline and bitline constitutes the address of the memory cell.
  • DRAM works by sending a charge through the appropriate column to activate the transistor at each bit in the column.
Reading and Writing
  • When writing, the row contains the state that the capacitor should take on.
  • When reading, the sense-amplifier defines the level of charge in the capacitor.
  • A counter tracks the refresh sequence based on which rows have been accessed in what order.
DRAM Rewrite Times
  • No time to do this so it is expressed as nanoseconds
  • Takes 70 nanoseconds to completely read and recharge each cell.
How DRAM Refresh Works
  • While the memory is operating, each memory cell must be refresh repeatedly.
  • It uses specialized cycles called refresh cycles.
  • the storage cells on a memory chip are laid out in a rectangular array of rows and columns.
  • The read process in DRAM is destructive. It removes the charge in a memory cell in an entire row.
  • There is a row of specialized latches on the chip called sense amplifiers. One for each column of memory cells, to temporarily hold the data.
  • During a normal read operation, the sense amplifiers latch and read the data.
  • The normal read electronics on the chip can refresh an entire row of memory in parallel, speeding up the refresh process.
  • The refresh cycle and the read cycle are similar, but refresh is faster.

  It is faster because: 1. For a refresh, only a row access is needed. 2. Data read from the cells doesn’t need to be fed into the output buffers.

Memory Cell Support Infrastructure

Functions of Circuits

  • Identifying each row and column.
  • Keeping track of the refresh sequence.
  • Reading and restoring the signal from a cell.
  • Telling a cell whether it should take a charge or not.
Static RAM (SRAM)
  • In SRAM, a form of flip-flop holds each bit of memory.
  • A flip-flop for a cell takes four or six transistors along with some wiring but never has to be refreshed.
  • This makes SRAM faster than DRAM.
  • Takes more space on a chip than a DRAM.
  • Get less memory per chip.
  • SRAM is fast and expensive.
  • SRAM is used to create the CPU’s speed-sensitive cache.
Physical Form

Memory chip in desktop computers used a pin configuration called dual inline package (DIP).

PCB Form Factor

Place the memory along with all of the support components, on a separate printed circuit board, then plugged into a special connector on the motherboard.

RAM Modules
  • Memory chips are available as part of a card called a module.
  • The difference between new pins and DIP is that the modern chip are surface-mounded to the PCB.