Types of computer memories

On a broad level, there are two types of computer memories — read only and read or write memory.

1. Read Only Memory (ROM) classifications

In its most simple realization, ROM consists of a pn diode and a tiny metal fuse at the cross-sections of word and bit lines of the memory matrix.

1. Mask Programmed ROM

Information stored during the manufacturing process

2. Fuse Programmed ROM or PROM (Programmable ROM)

Programmed in the field by selectively burning fuses with electrical pulses

2. Read or Write Memory classifications

1. Non Volatile

Can be further classified into,

1. Flash (Most popular)

2. EPROM (Electrically Programmable ROM)

3. EEPROM (Electrically Erasable PROM)

4. Magnetoresistive RAM

EEPROM and flash memories are variants of EPROM cell. EPROM are usually built using floating gate transistor memory cell. A floating gate transistor cell has two gates stacked on top of each other and separated by an additional layer of insulating oxide. The top gate is connected to the gate electrode and called the control gate. The floating gate is not connected to anything. EPROM is not electrically erasable.

Flash and EEPROM are electrically erasable.

2. Volatile

Can be further classified to,

1. RAM: Can be further classified to,

1. SRAM — 6- Transistor SRAM cell

2. DRAM — 1- Transistor DRAM cell, Trench DRAM cell, Advanced 1-Transistor DRAM cell

SDRAM is used in main memory (S implies that it is synchronous — read and write happens sychronously with the clock cycle and not whenever values are written into). SDRAM operates on the rising edge on the clock.

In contrast to SDRAM, DDR SDRAM operates on the rising and falling edge of the clock. Hence this speeds up the data burst transfer period by a factor of two.

In contrast to DDR SDRAM, DDR3 SDRAM operates at higher clock frequencies and hence further improvements of access bandwidth.

Multiport DRAM have multiple read and/or write ports.

Embedded DRAM — DRAM on one chip carrier (die).

3. Registers — CPU registers are integrated elements

2. Non Random Access: Can be further classified to,

1. LIFO

2. FIFO

3. CCD (Charge Coupled Devices)

3. CAM (Content Adressable Memory)

3. Performance parameters and memory terminologies

1. Access bandwidth
2. Latency
3. Cycle time
4. Asynchronous memory
5. Synchronous memory : All operations occur synchronous to a clock cycle

4. Memory architecture

1. Decoders
2. Array structure
3. Hierarchical

5. How to use memory in system design

Fig 1. Memory in system design

A state of the art memory hierarchy can be described as having CPU at the top and mass storage devices at the bottom and different memory technologies in between.

CPU caches are integrated with processor core and hence operate at the same cycle times as the CPU data path pipelines and have a size of several k to M bits.

Data access from On-chip or Off-chip SRAMs have additional latency because the CPU request has to traverse through the CPU bus.

SDRAM is used for large quantities of data.

6. An example of memory in system design

Figure 2 shows the block diagram of a general purpose microprocessor SoC.

The bottom left rectangle represents an embedded PowerPC processor core.

The gray area is on-chip.

Fig 2. Example of memory in system design