11.1 Memory types and hierarchies in embedded systems
3 min read•august 7, 2024
Memory types and hierarchies are crucial in embedded systems. They determine how data is stored and accessed, impacting performance and . From fast, volatile RAM to non-volatile storage like , each type serves a specific purpose in the system's architecture.
The memory hierarchy organizes these types based on speed, capacity, and cost. Caches provide quick access to frequently used data, while offers larger capacity for long-term storage. Understanding this hierarchy helps optimize embedded system design for efficiency and functionality.
Volatile Memory
Random Access Memory (RAM)
RAM provides temporary storage for data and instructions currently in use by the CPU
loses its contents when power is removed
Faster than like hard drives or flash storage
Two main types of RAM: Static RAM () and Dynamic RAM ()
Static RAM (SRAM) and Dynamic RAM (DRAM)
SRAM uses flip-flops to store each bit, retaining data as long as power is supplied
SRAM is faster but more expensive and consumes more power than DRAM
DRAM stores each bit using a transistor and capacitor, requiring periodic refreshing to maintain data
DRAM is slower but cheaper and more power-efficient than SRAM (commonly used in )
Cache Memory
Small, fast memory located close to the CPU to store frequently accessed data and instructions
Reduces the average time to access data from main memory
Hierarchy of cache levels: L1 (fastest, smallest), L2, and L3 (largest, slowest)
Cache hit: requested data found in cache, reducing
Cache miss: requested data not found in cache, requiring access to slower main memory
Non-volatile Memory
Read-Only Memory (ROM)
Non-volatile memory that retains data even when power is removed
Stores firmware, boot instructions, and other critical data
Contents are permanently written during manufacture or with special equipment
Examples include mask ROM, PROM (Programmable ROM), and EPROM (Erasable Programmable ROM)
Flash Memory and EEPROM
Flash memory is a type of non-volatile memory that can be electrically erased and reprogrammed
Stores data in an array of memory cells, with each cell holding one or more bits
Used in solid-state drives (SSDs), USB flash drives, and embedded systems for storage and firmware
EEPROM (Electrically Erasable Programmable ROM) allows byte-level erasure and reprogramming
EEPROM is slower and more expensive than flash memory but offers more flexibility for frequent updates
Secondary Storage
Non-volatile storage devices used for long-term data storage and retrieval
Examples include hard disk drives (HDDs), solid-state drives (SSDs), and magnetic tapes
Slower access times compared to main memory but offer larger storage capacities
Used for storing files, documents, media, and other persistent data
Memory Hierarchy
Hierarchy Overview
Memory hierarchy organizes storage devices based on their speed, capacity, and cost
Faster memory (cache, main memory) is located closer to the CPU, while slower memory (secondary storage) is farther away
Trade-off between speed and capacity: faster memory is more expensive and has lower capacity
Goal is to optimize overall system performance by minimizing access time and maximizing hit rates
Cache Memory and Main Memory
is the fastest and most expensive, with the smallest capacity
Main memory (RAM) is slower than cache but faster than secondary storage
CPU first checks cache for required data, then main memory, and finally secondary storage
Effective use of cache can significantly improve system performance by reducing memory access time
Secondary Storage
Largest capacity but slowest access times compared to cache and main memory
Used for long-term storage and persistence of data
Data from secondary storage is loaded into main memory when needed by the CPU
Virtual memory techniques use secondary storage to extend the apparent size of main memory
and swapping algorithms manage the transfer of data between main memory and secondary storage
Key Terms to Review (22)
Access Time: Access time is the duration it takes for a system to retrieve data from a memory location and return it for processing. This crucial metric impacts the overall performance and efficiency of embedded systems, where quick data retrieval is essential for real-time operations. The shorter the access time, the faster the system can respond to commands or inputs, directly influencing application responsiveness and user experience.
Address Bus: An address bus is a collection of wires or signals used to specify a memory address in the context of a microcontroller or computer system. This bus allows the processor to communicate with memory by indicating which memory location it wants to read from or write to. The size of the address bus determines how much memory can be accessed, with a larger address bus allowing for a greater range of memory addresses.
Cache memory: Cache memory is a small, high-speed storage area located close to the CPU that temporarily holds frequently accessed data and instructions, allowing for quicker retrieval and improved performance. By reducing the time it takes to access data from the main memory, cache memory plays a vital role in enhancing system efficiency and speed, which is especially important in embedded systems where resource optimization is crucial.
Cost-performance trade-off: The cost-performance trade-off refers to the balance between the expense of a system and its effectiveness in performing tasks. In embedded systems, this concept is crucial as designers must often choose between high-performance components that are expensive and lower-cost alternatives that may not meet performance requirements. Understanding this trade-off helps in making informed decisions regarding hardware and software design to achieve optimal system functionality within budget constraints.
DRAM: Dynamic Random Access Memory (DRAM) is a type of volatile memory that stores each bit of data in a separate capacitor within an integrated circuit. Due to its design, DRAM requires constant refreshing to maintain the data, making it distinct from other types of memory like SRAM. It is widely used in embedded systems for temporary data storage due to its balance between density and speed.
Endurance: In the context of embedded systems, endurance refers to the ability of a memory type to withstand repeated read and write cycles without significant degradation in performance or reliability. This characteristic is crucial when considering the longevity and stability of memory components used in applications where frequent data updates occur, impacting the choice of memory technology for specific tasks.
Flash Memory: Flash memory is a type of non-volatile storage that retains data even when the power is turned off, making it essential for embedded systems. It allows for the quick reading and writing of data, which is crucial for applications requiring fast access times, such as firmware updates and data logging. Flash memory is commonly used in various devices, including microcontrollers, to store both program code and data.
I2C: I2C, or Inter-Integrated Circuit, is a communication protocol used to connect low-speed devices like sensors, microcontrollers, and peripherals in embedded systems. It allows multiple devices to communicate with one or more master controllers using only two wires, simplifying the connections in complex systems while supporting multiple data rates.
Latency: Latency refers to the time delay between a request for data and the delivery of that data. It is a critical metric in embedded systems as it affects system responsiveness and performance, especially in real-time applications where timely processing of information is crucial.
Main memory: Main memory, often referred to as RAM (Random Access Memory), is a type of volatile memory used in computers and embedded systems to store data and programs that are currently in use. It acts as a bridge between the CPU and secondary storage, allowing quick access to active data. The speed and size of main memory significantly impact the overall performance and efficiency of an embedded system.
Memory Controller: A memory controller is a crucial component in embedded systems that manages the flow of data to and from the memory. It acts as a bridge between the processor and various types of memory, ensuring that data is stored and retrieved efficiently. The memory controller plays a significant role in optimizing performance and power consumption by controlling read and write operations and interfacing with different memory types within the memory hierarchy.
Memory mapping: Memory mapping is the process of assigning a specific range of memory addresses to various hardware components, peripherals, or software functions in an embedded system. This organization helps optimize the management of memory resources and facilitates communication between the CPU and different memory types, ensuring efficient data access and retrieval.
Non-volatile memory: Non-volatile memory is a type of computer memory that can retain stored information even when not powered. This characteristic makes it essential for storing firmware, configurations, and important data in embedded systems where power loss might occur. It contrasts with volatile memory, which loses its data when the power is turned off, and plays a key role in both the structure and functioning of embedded devices.
Paging: Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory and allows the computer to retrieve processes from the secondary storage in blocks, or pages. This technique helps in efficiently managing memory by breaking it into fixed-size units, thus facilitating a virtual memory system that allows multiple processes to run simultaneously while optimizing the use of physical memory.
Power Consumption: Power consumption refers to the amount of electrical energy used by a system or component during its operation. In embedded systems, power consumption is a critical factor influencing design choices, performance, and functionality, as it affects battery life in portable devices, thermal management, and overall system efficiency.
Read/write cycles: Read/write cycles refer to the process of accessing and modifying data in memory. Each cycle consists of a read operation, where data is retrieved from memory, followed by a write operation, where data is stored back into memory. This concept is crucial for understanding how different memory types interact with data and how their performance impacts embedded systems.
Secondary storage: Secondary storage refers to non-volatile storage devices that retain data even when the power is turned off. This type of storage is essential in embedded systems for saving larger amounts of data long-term, offering advantages over primary storage such as RAM by providing more capacity and permanence.
Size-power trade-off: The size-power trade-off refers to the balance between the physical size of an embedded system and its power consumption. In embedded systems design, this concept is critical as larger components typically consume more power, while smaller components may be limited in performance or functionality. Understanding this trade-off is essential for optimizing both the efficiency and effectiveness of embedded systems.
SPI: SPI, or Serial Peripheral Interface, is a synchronous serial communication protocol used for short-distance communication between microcontrollers and various peripheral devices. It allows for high-speed data transfer and supports multiple devices connected to the same bus, making it an essential protocol for embedded systems that require efficient and reliable data exchange.
SRAM: Static Random Access Memory (SRAM) is a type of semiconductor memory that uses bistable latching circuitry to store each bit. Unlike dynamic RAM (DRAM), SRAM does not need to be refreshed periodically, making it faster and more reliable for certain applications, especially in embedded systems where speed and stability are crucial.
Throughput: Throughput is the measure of how many units of information or tasks are successfully processed in a given amount of time. It's essential in evaluating the efficiency of systems, as it directly influences performance and resource utilization across various functions.
Volatile memory: Volatile memory is a type of computer memory that requires power to maintain the stored information, meaning it loses all data when the power is turned off. This kind of memory is crucial in embedded systems as it is often used for temporary data storage, such as in RAM, where fast read and write access is essential for efficient system performance. Volatile memory helps in maintaining the quick operation of devices by providing fast access to data that is frequently updated or modified.