When a program runs, it typically uses two types of memory to store its instructions and data: main memory and secondary storage. Let’s take a closer look at each of these memory types and understand their roles in program execution.
Main memory, also known as primary memory or random-access memory (RAM), is the primary storage area used by programs during runtime. It is volatile, meaning that its contents are lost when the power is turned off. Main memory is directly accessible by the CPU, making it an essential component for executing programs.
Program instructions and data that are currently being used by the CPU are stored in main memory. Instructions include the code that tells the CPU what operations to perform, while data includes variables, arrays, and other information manipulated by the program.
In main memory, instructions and data are organized into blocks, with each block assigned a unique address. The CPU uses these addresses to fetch and store data from/to main memory. The size of a block varies depending on the computer architecture but is typically 4 bytes or 8 bytes.
To optimize performance, modern computers employ a technique called caching. Caches are small but fast memories located between the CPU and main memory.
They store frequently accessed instructions and data for quick retrieval. Caching reduces the time it takes for the CPU to access main memory, thereby speeding up program execution.
Secondary storage refers to non-volatile storage devices such as hard disk drives (HDDs) and solid-state drives (SSDs). Unlike main memory, secondary storage retains its contents even when power is turned off.
Programs are typically stored on secondary storage devices, such as the hard drive, when they are not actively running. When a program is executed, its instructions and data are loaded from secondary storage into main memory for processing by the CPU.
Secondary storage has a much larger capacity than main memory but is slower to access. Therefore, programs often load only a portion of their instructions and data into main memory at a time. This process is known as swapping and allows programs to efficiently manage their memory usage.
In addition to main memory and secondary storage, modern operating systems also utilize a concept called virtual memory. Virtual memory extends the addressable space beyond the physical limits of main memory by using secondary storage as an overflow area.
When there is not enough space in main memory to hold all active programs and data, the operating system swaps out less frequently used portions of main memory onto the disk. This technique allows multiple programs to run simultaneously without requiring an excessive amount of physical RAM.
In summary, when a program runs, it primarily uses main memory to store its instructions and data. Main memory provides fast access for the CPU and is volatile. Secondary storage serves as a long-term storage solution for programs when they are not actively running.
The combination of these two types of memory, along with caching and virtual memory techniques, ensures efficient program execution while effectively managing limited resources.