Serial vs. Parallel Communication

• Memory systems are various types of data storage used by computers, categorized by speed, size, and purpose

Serial vs. Parallel Communication

• Serial communication sends data one bit at a time over a single channel
• Parallel communication sends multiple bits simultaneously over multiple channels
• Serial data transfer is slower as it transmits one bit at a time
• Parallel data transfer is faster, as multiple bits are sent simultaneously
• Serial wiring complexity requires fewer wires, typically one for data and one for ground
• Parallel wiring complexity requires more wires, with one for each bit plus ground and control
• Serial communication is suitable for longer distances, potentially up to several kilometers
• Parallel communication is suitable for short distances due to signal degradation
• Serial communication is more cost-effective, needing fewer, simpler designs
• Parallel communication is more expensive due to multiple wires and connectors
• Serial communication is less prone to signal interference and crosstalk over long distances
• Parallel communication is prone to signal degradation, particularly over long distances
• Examples of serial communication include RS-232, USB, Bluetooth, Ethernet, SPI, and I2C
• Examples of parallel communication include older computer printer ports and internal connections like RAM

Applications of Serial and Parallel Ports

• RS-232 is used in serial ports on computers, modems, and communication devices
• USB connects peripherals like keyboards, mice, external storage, cameras and printers
• USB ports are used on computers for charging and data transfer between devices
• Bluetooth is for wireless communication between devices
• Ethernet is used for networking and internet communication
• Parallel ports (IEEE 1284) are used in older printers and computer peripherals
• Integrated Drive Electronics (IDE) connects hard drives and optical drives in older systems

Primary Memory (Volatile Memory)

• Primary memory is directly accessible by the CPU, is fast, and loses data when power is off
• RAM temporarily stores data and instructions actively used or processed by the CPU
• Dynamic RAM (DRAM) is slower, less expensive, and requires constant refreshing to retain data
• Static RAM (SRAM) is faster, more expensive, and retains data as long as power is supplied
• RAM is volatile, typically used for active programs and data and has a larger capacity than cache memory but is slower
• Cache memory stores frequently accessed data for quick retrieval by the CPU, acting as a high-speed intermediary between the CPU and RAM
• L1 cache is integrated directly into the CPU chip, is the smallest, and the fastest
• L2 cache is located on the CPU chip or close to it, larger but slower than L1
• L3 cache is typically shared between cores, slower than L1 and L2, but much larger
• Cache memory is extremely fast, but limited in size
• Cache memory improves the CPU's performance by reducing access time to frequently used data

Secondary Memory (Non-Volatile Memory)

• Secondary memory is for long-term data storage, retains data without power, is slower but has a larger capacity than primary memory
• Hard Disk Drives (HDDs) store large amounts of data for long-term access and are mechanical with moving parts
• HDDs are slower than SSDs but still widely used for large-scale storage that is cost-effective
• Solid State Drives (SSDs) are a faster alternative to HDDs for storing data
• SSDs use flash memory with no moving parts and have faster read/write speeds
• SSDs are more expensive per gigabyte than HDDs, but have higher durability and lower power consumption
• Optical Discs (CD, DVD, Blu-ray) are used for media distribution and archiving data
• Data on optical discs is read/written using lasers
• Optical discs have slower access times compared to HDDs and SSDs
• Flash Memory (USB Drives, SD Cards) is used for portable data storage and transfer
• Flash memory is non-volatile and used in USB drives, memory cards, and external drives
• Flash memory is faster than optical discs but slower than SSDs

Tertiary and Off-Line Storage

• Tertiary and offline storage solutions are less frequently accessed, offering large storage capacities for archival purposes
• Magnetic tape is used for backup, archiving, and large-scale data storage
• Magnetic tape is relatively slow but cost-effective for long-term storage and used for backups and disaster recovery
• Cloud storage is off-site storage via the internet and provides scalable storage solutions on-demand
• Cloud storage is accessible from anywhere with an internet connection and is used for file storage, backups, and cloud computing applications.

Virtual Memory

• Virtual memory uses secondary storage (HDD/SSD) as an extension of RAM
• The OS divides memory into fixed-size pages and swaps them between RAM and storage
• Data from RAM not in use swaps to the hard drive to free space
• Virtual memory enhances multitasking and allows more programs to run simultaneously as well as supporting more running processes
• Accessing data from a disk is slower than from RAM

Memory Hierarchy

• Registers are inside the CPU and used for immediate processing
• Cache Memory (L1, L2, L3) holds fast, small memory close to the CPU for frequently accessed data
• Main Memory (RAM) is larger and slower than cache, but faster than secondary memory
• Secondary Storage includes HDDs, SSDs, and optical drives used for long-term data storage
• Tertiary and Off-line Storage includes magnetic tapes and cloud storage used for backup and archival purposes
• A memory hierarchy organizes different types of memory in a system, ordered by speed, cost, and size

Types of Memory

• Primary memory is directly accessible by the CPU, volatile, and used for running programs and storing active data and is very fast, but limited in size
• RAM and cache memory are examples of primary memory
• Secondary memory is non-volatile, used for long-term data storage, is slower than primary memory, but larger in capacity
• HDDs, SSDs, optical discs (CD/DVD), and USB flash drives are examples of secondary memory
• RAM is volatile, loses data when the power is off, stores actively used data and can be read from and written to
• DRAM is slower and requires constant refreshing and SRAM is faster, does not require refreshing, but is more expensive
• ROM is non-volatile, meaning data is retained when the power is off
• Data is typically written to ROM during manufacturing and cannot be modified easily
• ROM is used for storing firmware
• PROM can be written to once
• EPROM can be erased and reprogrammed with special equipment.
• EEPROM can be electrically erased and reprogrammed

Cache Memory and its Importance

• Cache memory is small and high-speed located between the CPU and RAM
• Cache memory holds frequently used data or instructions that are likely to be reused
• L1 cache is integrated into the CPU and is very fast but small; L2 cache may be on-chip or close to the CPU, is larger than L1 but slower
• L3 cache is shared among multiple CPU cores and is larger and slower than L1 and L2
• Cache memory speeds up the CPU by reducing the time it takes to access data from the main memory
• Frequently accessed data are retrieved faster, improving overall performance
• Without cache memory, the CPU would frequently have to access slower main memory

Memory Hierarchy

• The memory hierarchy is a tiered structure with the hierarchy that balances cost, speed, and capacity
• Registers are fastest form of memory. They are located inside the CPU, very small in size and used to store data that is immediately needed by the CPU for operations
• Cache is high-speed memory located between the CPU and RAM
• It’s small compared to RAM but larger than registers
• It holds frequently used instructions and data reducing access time to RAM
• Cache (L1 and L2) is very fast but slower than registers
• Main memory (RAM) stores programs and data when they are in use by the system, is larger than cache but smaller than secondary storage
• Main Memory stores data and program that are actively in use and is faster than secondary storage
• Level 4 is non-volatile Storage that retains data even when the computer is turned off

Memory Management Overview

• Paging is a memory management scheme where each program is divided into logical segments of varying sizes. It divides physical memory into fixed-size blocks called "pages" and divides logical memory into blocks of the same size, called "page frames"
• Paging avoids fragmentation by allocating memory in fixed-sized blocks
• Programs can be loaded into any available memory frame
• Paging virtual memory enables programs to use more memory than is physically available
• The operating system maintains a page table, which maps virtual pages to physical frames
• Memory management addresses are protected by using the segment or page tables to map virtual addresses to physical addresses
• The OS can restrict access to certain areas of memory by setting permissions (read, write, execute)

Segmentation

• Segmentation matches the logical structure when dividing its structure, the OS divides memory in logical manner
• Segments can grow independently of each other, especially the heap and stack
• External fragmentation occurs because segments may not fit perfectly in memory
• Managing different-sized segments can lead to inefficient use of memory
• Each segment has a base (starting address) and a limit (size)
• The isolation and protection of memory allows programs to exceed the physical memory size and it improves security