【存储器】半导体存储器

计算机组织与结构

       We should know that the use of semiconductor chips for main memory is almost universal.

Properties

1. They exhibit two stable (or semi-stable) states, which can be used to represent binaries 1 and 0.
2. They are capable of being written into (at least once), to set the state.
3. They are capable of being read to sense the state.

Types

       Here is a table

RAM

1. It’s full name is Random-Access Memory.
2. Characteristics :
   Reading/writing can be easy and rapid.
   However, it is Volatile.
3. Types :
   DRAM : Dynamic RAM
   SRAM : Static RAM

DRAM

       Store data as charge on capacitors, and the presence or absence of charge in a capacitor is interpreted as a binary 1 or 0.

Advantage : Smaller volume and higher level of intergration.

Disadvantage : Require periodic charge refreshing to maintain data storage(Why ? capacitors have a natural tendency to discharge).

This picture show the structure of DRAM :

SRAM

       Store data with traditional flip-flop logic-gate configurations.

Advantage : More stable(as long as power is supplied to it) and faster than DRAM.

Disadvantage : Larger volume and more expensive than DRAM.

This picture show the structure of SRAM :

DRAM&SRAM

       DRAM tend to be favored for large memory requirements.
       SRAM is used for cache and DRAM is used for main memory.

Advanced DRAM Organization

Problem :
       The traditional DRAM chip is constrained both by its internal architecture and by its interface to the processor’s memory bus.
Types :

1. Synchronous DRAM (SDRAM)
2. DDR SDRAM (DDR)

SDRAM

How does the traditional DRAM work :
       The processor presents addresses and control levels to the memory, indicating that a set of data at a particular location in memory should be either read from or written into the DRAM.

The traditional DRAM’s disadvantage :
       The DRAM performs various internal functions, such as activating the high capacitance of the row and column lines, sensing the data, and routing the data out through the output buffers, and the processor must simply wait through this delay, after a delay, the DRAM either writes or reads the data.

How the SDRAM solve this problem :
       SDRAM exchanges data with the processor synchronized to an external clock signal and running at the full speed of the processor/memory bus without imposing wait states. Since SDRAM moves data in time with system clock, CPU knows when data will be ready.
       The CPU and SDRAM get an appointment so that when CPU tell SDRAM he needs a data, and after numbers of clock, he will come here and get the data, through the time SDRAM read data, the CPU can do something else.

This picture show the structure of SRAM :

DDR SDRAM

It’s full name is Double-data-rate SDRAM.

As the modified version of SDRAM, it can get data rapidly.

How does DDR SDRAM works :
       Send data twice per clock cycle, once on the rising edge of the clock pulse and once on the falling edge.（时钟周期的上升沿和下降沿）

Comparison of DDR, DDR2 and DDR3 :

1. Increase operational frequency.
2. Increase the prefetch buffer.

This picture show the structure of SRAM :

ROM

1. It be seen as the Read-only memory, which doesn’t mean that ROM can’t be written into, the ROM can be written into just for one time.
2. Characteristics :
   Nonvolatile : no power source is required to maintain the bit.
   Possible to read a ROM but not possible to write new data.
   Write Mechanism : Masks.
   Lower costs when intp production.
3. Applications :
   Microprogramming, library subroutines, system programs, function tables

Programmable ROM(PROM)

As the Advanced ROM Organization, it can solve the problems that ROM has.

Problem :
Data insertion incurs a relatively large fixed cost
No room for error: If one bit is wrong, the whole batch of ROMs must be thrown out.

Characteristics :
Write Mechanism : Electrically(Special equipment is required for writing).
Nonvolatile.

Compared to ROM :
PROMs provide flexibility and convenience.
The ROM remains attractive for high-volume production runs.

Read-mostly memory

1. Characterics :
   Nonvolatile storage.
   Read operations are far more frequent than write operations.
2. Types :
   Erasable PROM(EPRM)
   Electrically Erasable PROM(EEPRM)
   Flash Memory

Erasable PROM(EPRM)

Characteristics :
Erasable : UV light(chip-level, 20 minutes).
Write Mechanism : Electrically(Special equipment is required for writing).
Nonvolatile.
EPROM is more expensive than PROM, but it can be updated multiple times.

Electrically Erasable PROM(EEPRM)

Characteristics :
Erasable : Electrically(chip-level).
Write Mechanism : Electrically(Special equipment is required for writing).
Nonvolatile.
EEPROM is more expensive than EPROM and also is less dense.

Flash Memory

Characteristics :
Erasable : Electrically(block-level).
Write Mechanism : Electrically(Special equipment is required for writing).
Nonvolatile.
Intermediate between EPROM and EEPROM in both cost and functionality.

Summary

This picture :