Hardware: How RAM works.

What is the most common type of memory?

Random access memory (RAM).

Dynamic random access memory (DRAM)

DRAM works by sending a charge through the appropriate column to activate the transistor at each bit in the column.

storage cells

The ________ on a memory chip are laid out in a rectangular array of rows and columns.

Why is RAM considered random access?

RAM is considered random access because it can access any memory cell directly regardless of location in memory.

What does Serial access memory (SRAM) does?

It is a type of memory that stores data in a sequential order.

Sequential Memory

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.

How are memory cells created?

By combining a transistor and a capacitor.

What is a capacitor?

It holds the bits of information.

It acts like a bucket that can store electrons.

To store a 1, the bucket is filled up with electrons.

To store a 0, the bucket is emptied.

What does a transistor do?

It acts as a switch that lets the control circuitry on the memory chip to read the transistor.

How does DRAM works?

The CPU and the memory controller has to come along and recharge all of the capacitors holding 1’s before it gets discharged.

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

DRAM has to be _____ ________ all of the time or it. _____ all it is holding.

DRAM has to be dynamically refreshed all of the time or it forgets what it is holding.

What is a negative side of DRAM refresh operations?

It takes a long time and it slows down memory.

______ _______ are attached to a silicon wafer in an array of rows and columns.

Memory cells are attached to a silicon wafer in an array of rows and columns.

What is memory made of?

Memory is made of bits arranged in two-dimensional grids.

The intersection of the _____ and __ constitutes the address of the memory cell.

The intersection of the wordline and bitline constitutes the address of the memory cell.

_____ works by sending a charge through the appropriate column to activate the transistor at each bit in the column.

DRAM works by sending a charge through the appropriate column to activate the transistor at each bit in the column.

Reading and Writing

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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.

How long does it take to read and recharge each cell in DRAM?

Takes 70 nanoseconds to completely read and recharge each cell.

How does 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.

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*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.

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### 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.

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### 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.

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### 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.