Types of Write Ability in Computer Memory

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Questions and Answers

What type of memory is characterized as having the highest write ability, allowing the processor to write to memory simply and quickly?

RAM

What type of memory requires special equipment, known as a 'programmer', to write to it?

EPROM, OTP ROM

What is the characteristic of nonvolatile memory?

Holds bits after power is no longer supplied

What type of memory is characterized as having the highest storage permanence, essentially never losing its bits?

Mask-programmed ROM Signup and view all the answers

What is the main difference between ROM and Mask-programmed ROM?

Mask-programmed ROM is programmed during fabrication, while ROM is programmed externally Signup and view all the answers

What type of memory is used for final design of high-volume systems?

Mask-programmed ROM Signup and view all the answers

How is OTP ROM programmed?

Using a ROM programmer device that blows fuses to create connections Signup and view all the answers

What is the main advantage of EPROM over OTP ROM?

EPROM can be erased and reprogrammed thousands of times Signup and view all the answers

How does EPROM differ from other types of memory in terms of storage permanence?

EPROM has reduced storage permanence, susceptible to radiation and electric noise Signup and view all the answers

What is the typical usage of EPROM?

During design development Signup and view all the answers

What is the duration of EPROM programming?

5-30 minutes Signup and view all the answers

What type of memory is often used as main memory in processor-based embedded systems due to its high capacity and low cost?

DRAM (Dynamic Random Access Memory) Signup and view all the answers

What type of DRAM is capable of very high throughput, with data latched on both rising and falling edges of the clock?

Rambus DRAM (RDRAM) Signup and view all the answers

What is the main difference between the design goals of conventional logic (IC) designers and DRAM designers?

Minimizing parasitic capacitance vs. creating capacitor cells to retain stored information Signup and view all the answers

What type of memory is non-volatile and can be electrically erased and reprogrammed?

Flash memory Signup and view all the answers

What is the purpose of refresh circuitry in DRAM?

To periodically refresh the memory content Signup and view all the answers

What is PSRAM?

Pseudo-static RAM, a type of DRAM with a built-in memory refresh controller Signup and view all the answers

Why is it difficult to integrate DRAM on the same chip as a processor?

Different chip making processes between DRAM and conventional logic Signup and view all the answers

What is eDRAM?

Embedded DRAM, a type of DRAM integrated on the same die Signup and view all the answers

What is the main limitation of using SRAM in embedded systems?

Limited capacity and high cost Signup and view all the answers

What is the advantage of SDRAM/ESDRAM compared to traditional DRAM?

Synchronous operation, allowing for higher speeds Signup and view all the answers

What is the primary step in a cache operation when a request for main memory access is made?

Check cache for a copy Signup and view all the answers

What happens when a cache hit occurs?

The copy is in cache, allowing for quick access Signup and view all the answers

What are the three main design choices for cache?

Cache mapping, replacement policies, and write techniques Signup and view all the answers

What are the primary duties of a Memory Management Unit (MMU)?

Handles DRAM refresh, bus interface and arbitration, and translates logic memory addresses Signup and view all the answers

What is the key difference between SRAM and DRAM?

Storage permanence Signup and view all the answers

What is the primary advantage of flash memory?

Non-volatile storage Signup and view all the answers

What type of memory is slower to access than SRAM?

DRAM Signup and view all the answers

What feature of NVRAM allows it to hold data after external power is removed?

Non-volatility Signup and view all the answers

What is the purpose of the battery in battery-backed RAM?

To provide power to the SRAM when external power is removed Signup and view all the answers

What is the advantage of NVRAM with EEPROM or flash over battery-backed RAM?

It stores the complete RAM contents on EEPROM or flash before power is turned off Signup and view all the answers

What is the main difference between FRAM and DRAM?

FRAM uses a ferroelectric layer instead of a dielectric layer Signup and view all the answers

What is the data retention period of FRAM?

Higher than 10 years Signup and view all the answers

What is the limitation of FRAM in terms of storage capacity?

Lower storage densities than flash devices Signup and view all the answers

Why does FRAM need a refresh after every read?

Due to its destructive read process Signup and view all the answers

What is the main difference between EPROM and EEPROM?

EPROM is one-time programmable, while EEPROM can be programmed and erased electronically. Signup and view all the answers

What is the advantage of NAND or NOR-flash memory over EEPROM?

It allows for fast erase of large blocks of memory, making it suitable for storing large data items in nonvolatile memory. Signup and view all the answers

What is the main characteristic of RAM?

It is volatile memory that loses its contents when the power supply is removed. Signup and view all the answers

What is the main difference between SRAM and DRAM?

SRAM uses flip-flops to store bits, while DRAM uses a capacitor and transistor to store bits, requiring periodic refreshes. Signup and view all the answers

What is the purpose of the 'busy' pin in EEPROM?

It indicates to the processor that the EEPROM is still writing and is not ready for the next operation. Signup and view all the answers

What is the typical refresh rate for DRAM?

15.625 microsec. Signup and view all the answers

What is the advantage of EEPROM over EPROM?

EEPROM is more convenient and can be in-system programmable with built-in circuit to provide higher than normal voltage. Signup and view all the answers

What is the main application of NAND or NOR-flash memory?

It is used in embedded systems storing large data items in nonvolatile memory, such as digital cameras and cell phones. Signup and view all the answers

How many transistors are required to implement a memory cell in SRAM?

6 transistors. Signup and view all the answers

What is the storage permanence of EEPROM and EPROM?

About 10 years. Signup and view all the answers

Study Notes

Ranges of Write Ability

High-end: Processor writes to memory quickly and easily, e.g., RAM.
Middle range: Processor writes to memory, but slower, e.g., EEPROM.
Lower range: Special equipment, a "programmer", is required to write to memory, e.g., EPROM, OTP ROM.
Low-end: Bits are stored only during fabrication, e.g., Mask-programmed ROM.

In-System Programmable Memory

Can be written to by a processor in the embedded system using the memory.
Memories in high-end and middle range of write ability.

Ranges of Storage Permanence

High-end: Essentially never loses bits, e.g., mask-programmed ROM.
Middle range: Holds bits for days, months, or years after memory's power source is turned off, e.g., NVRAM.
Lower range: Holds bits as long as power is supplied to memory, e.g., SRAM.
Low-end: Begins to lose bits almost immediately after being written, e.g., DRAM.

Nonvolatile Memory

Holds bits after power is no longer supplied.
High-end and middle range of storage permanence.

ROM: "Read-Only" Memory

Non-volatile memory.
Can be read from but not written to by a processor in an embedded system.
Traditionally written to, "programmed", before being inserted into the embedded system.

Mask-Programmed ROM

Connections are "programmed" at fabrication using a set of masks.
Lowest write ability, only once.
Highest storage permanence, bits never change unless damaged.
Typically used for final design of high-volume systems.

OTP ROM: One-Time Programmable ROM

Connections are "programmed" after manufacture by the user.
Very low write ability, typically written only once and requires a ROM programmer device.
Very high storage permanence, bits don't change unless reconnected to the programmer and more fuses are blown.
Commonly used in final products, cheaper and harder to inadvertently modify.

EPROM: Erasable Programmable ROM

Programmable component is a MOS transistor.
Better write ability, can be erased and reprogrammed thousands of times.
Reduced storage permanence, program lasts about 10 years but is susceptible to radiation and electric noise.
Typically used during design development.

EEPROM: Electrically Erasable Programmable ROM

Programmed and erased electronically.
Better write ability, can be in-system programmable with built-in circuit to provide higher than normal voltage.
Similar storage permanence to EPROM, about 10 years.
Far more convenient than EPROMs, but more expensive.

NAND or NOR-Flash Memory

Extension of EEPROM.
Same floating gate principle and write ability.
Same storage permanence.
Fast erase, large blocks of memory erased at once.
Used with embedded systems storing large data items in nonvolatile memory.

RAM: "Random-Access" Memory

Typically volatile memory.
Read and written to easily by the embedded system during execution.
Bits are not held without a power supply.

Basic Types of RAM

SRAM: Static RAM: Memory cell uses a flip-flop to store a bit.
DRAM: Dynamic RAM: Memory cell uses one MOS transistor and a capacitor to store a bit.

NVRAM: Non-volatile RAM

Holds data after external power is removed.
Battery-backed RAM, SRAM with a permanently connected battery.
Writes as fast as reads, no limit on the number of writes.
SRAM with EEPROM or flash, stores complete RAM contents on EEPROM or flash before power is turned off.

DRAM Variations

NVRAM: Non-volatile RAM: Ferroelectric RAM (FRAM), similar in construction to DRAM but using a ferroelectric layer.
FRAM: Lower power usage, faster write performance, and a much greater maximum read/write endurance.

DRAM Integration Problem

SRAM is easily integrated on the same chip as the processor.
DRAM is more difficult, due to different chip-making processes between DRAM and conventional logic.

Flash Memory

Non-volatile computer memory that can be electrically erased and reprogrammed.

Cache

Cache operation: request for main memory access, check cache for a copy, cache hit or cache miss.

Memory Management Unit (MMU)

Duties of MMU: handles DRAM refresh, bus interface, and arbitration, takes care of memory sharing among multiple processors, and translates logic memory addresses to physical memory addresses of DRAM.
Modern CPUs often come with MMU built-in.

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Description

Learn about the different ranges of write ability in computer memory, including high-end, middle-range, lower-range, and low-end types, and their characteristics.