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Real Mode Memory Real Mode Memory AddressingAddressingReal mode, also called real address

mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space (giving exactly 1 MB of addressable memory) and unlimited direct software access to all memory, I/O addresses and peripheral hardware. Real mode provides no support for memory protection, multitasking, or code privilege levels

Real Mode Memory Real Mode Memory AddressingAddressingSegment and offsetsSegment address located in one of

the segment registers defines the beginning address of any 64K memory segments.

Offset address selects any location within the 64K memory segments.

Offset sometimes referred to as displacement.

Ending address is found by adding FFFFH with the beginning address.

Default Segment and Offset Default Segment and Offset RegistersRegistersCS:IP Instruction AddressSS: SP or BP(base pointer) Stack

AddressDS: BX, DI Data address.ES :DI String destination address.

Protected Mode Memory Protected Mode Memory AddressingAddressingIn computing, protected mode,

also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPU). It allows system software to utilize features such as virtual memory, paging, safe multi-tasking, and other features designed to increase an operating system's control over application software.

Protected Mode Memory Protected Mode Memory AddressingAddressingDescriptor describes the location,

length and access rights of the segment of memory.

Segment register contains a selector that selects a descriptor from a descriptor table.

There are two descriptor tables used with segment registers.

Global Descriptor(System descriptor): Contain segment definitions that apply to all programs.

Local Descriptors(application descriptor): Unique to an application.

Base address: Indicates the starting location of the memory segment.

Segment Limit: Contains the last offset address found in a segment.

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