1.2.4 - Memory
1. Overview of Computer Memory
Computer memory operates at multiple levels, similar to human memory, with a trade-off between access speed and capacity. It can be categorized into short-term and long-term memory, analogous to human memory, but with significant differences. The terms primary (short-term) and secondary (long-term) storage are more commonly used to describe the different stages of computer memory.
2. Short-Term Memory (STM) and RAM
Registers: These are the lowest level of computer memory, located on the computer chip. They mainly influence the computer's speed and do not affect the user directly.
Random Access Memory (RAM): This is where most of the currently used data is stored. RAM is volatile, meaning its contents are lost when power is turned off. Typical access times are about 10 nanoseconds, and information can be accessed at approximately 100 Mbytes per second. Modern computers have 64–256 Mbytes of RAM.
Non-Volatile RAM: Some systems include a small amount of non-volatile RAM, which uses a battery to retain data even when the computer is off. This type of RAM is more expensive but useful for critical applications.
Example: PDAs (Personal Digital Assistants) often use non-volatile RAM or flash memory, ensuring data is retained even without continuous power.
3. Long-Term Memory (LTM) and Disks
Magnetic Disks: These include hard disks and floppy disks, which use magnetic coatings to store data. Hard disks have larger capacities (up to several gigabytes) and faster access times compared to floppy disks (300 kbytes to 1.4 Mbytes).
Optical Disks: These use laser light to read/write data, including CD-ROMs with a capacity of about 650 Mbytes. Optical disks are durable and suitable for storing large amounts of data, such as multimedia.
Example: CD-ROMs are ideal for software distribution and storing large multimedia files due to their capacity and durability.
4. Understanding Speed and Capacity
Documents like a book or the Bible can easily fit into main memory (2–4 Mbytes), but multimedia files like videos or high-resolution images require much larger storage capacities.
Interaction speed is influenced by memory and disk capacities. For example, switching between applications may cause delays due to the von Neumann bottleneck, which limits the data transfer rate between RAM and disk.
Example: A 10 Mbyte per second video stream would require significant storage and fast access speeds, highlighting the need for efficient data management in multimedia applications.
5. Storage Formats and Standards
Text Storage: ASCII is the standard code for text storage, representing characters with a 7-bit code. Extended ASCII and UNICODE are used for additional symbols and international characters.
Formatted Text: Rich Text Format (RTF) and XML are used for storing documents with formatting information, enabling compatibility across different software.
Bitmap Graphics: Various formats exist for storing images, considering factors like size, color depth, and compression.
Example: XML is widely used for web applications due to its flexibility and ability to store structured text.
6. Methods of Access
Key-Based Access: Databases often use key fields for indexing, similar to how a phone book uses names to find numbers. Multiple keys and indices can improve usability but increase storage requirements.
Free Text Retrieval: Systems like UNIX's "grep" or web search engines use free text search to find documents containing specific words or phrases.
Example: The Soundex system matches words that sound similar, useful for searching names with different spellings.
Summary
Understanding the differences between short-term and long-term memory, the capabilities of different storage media, and the importance of efficient data access methods are crucial for designing user-friendly computer systems. As technology advances, these considerations will continue to evolve, providing greater flexibility and performance in data management and storage.