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Usually none. Long ago BIOS(which isn't really part of an OS...) was stored on ROM. These days you've usually got your OS installed on Hard Drive or a SSD drive.
Most computers nowadays do not store any portion of the operating system in ROM. Among those that did and still do, only the kernel and basic system libraries are placed in ROM.
ROM
Rom is a non-volatile memory. A computer system needs it because it stores the data permanently. If there is no ROM the data can be stored only until the system is on. Once the system is off, all the data stored in RAM(volatile memory) will be lost. So, ROM is used to store the data for a desired period of time.
BASIC INPUT OUTPUT SYSTEM. it is stored on the ROM.
ROM - the read-only memory
ROM
Bios rom
post routine is a part of BIOS.it's stored on d ROM memory.it's called as ROM BIOS.it's an automatic test of ur system wen u power on ur PC.It's the first routine/job done by ur PC when u on it.
read-only memory
circut board - the read-only memory (ROM)
ROM