Interview Question - What are the various modes in Intel processor ?

Embedded C interview Question

What are the various modes in Intel processor ?

There are three types of modes in Intel processor. They are,

  • Real Mode
  • Virtual Mode
  • Protected Mode
In real mode, adding an address offset to the value of a segment Register shifted left four bits generates addresses. As the segment register and address offset are 16 bits long this results in a 20-bit address. This is the origin of the one-megabyte (2^20) limit in real mode.

Virtual Mode: An operating mode provided by the Intel 80386 and later processors to allow real mode programs to run under operating systems, which use protected mode. In this sub-mode of protected mode, an operating environment is created which mimics the address calculation in real mode.

In protected mode,
the segment registers contain an index into a table of segment descriptors. Each segment descriptor contains the start address of the segment, to which the offset is added to generate the address. In addition, the segment descriptor contains memory protection information. This includes an offset limit and bits for write and read permission. This allows the processor to prevent memory accesses to certain data. The operating system can use this to protect different processes' memory from each other, hence the name "protected mode".

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