Why does operating system go to RAM after it leaves the storage device?

Answer 1

CPUs can only operate on data that is stored in a "register" (a special type of memory inside the CPU), the "cache" (memory next to or just inside the CPU), and RAM (referred to as system memory), and it can also read or write data to "ports" which are connected to various buses all over the system. Each time a port is read from or written to, the source or destination must be either a register or system memory (which may also come from the cache).

All other data in the system is inaccessible to the CPU until it is loaded into one of these three areas (the registers are the fastest type of memory, and system memory is the slowest type of CPU-addressable memory). A hard drive is not directly accessible to the CPU; it has to request the data from the hard drive, and place that data into RAM. Traditionally, this data came in through a data port, and was placed in memory by the CPU. Modern systems use a DMA controller to place the files into memory, allowing the CPU to perform other tasks while it waits for the files to load.

Since the CPU can only work with data stored in system memory, or within its internal memory, the operating system must reside in RAM in order to be useful to the CPU. A typical boot sequence becomes: the BIOS initiates a predetermined call to a hard drive or removable media to locate bootable code (a "boot loader"), which in turn prepares some memory, initializes hardware, etc, then loads the core of the OS into memory, and finally that core loads the rest of the operating system files that it needs to manage the computer.

Storage devices are there only to store data while the computer is powered off. They are not designed to provide immediate access to data, and the CPU is not designed to treat storage devices as immediately accessible data.