Initial Memory Reads
IA32 processors always begin operation in Real Mode. All memory addresses are formed by adding the 20-bit segment base address specified in a 16-bit segment register (“Memory Addressing” on page 71 explains how a 20-bit address is specified in a 16-bit segment register) to the 16-bit offset that is specified in:
the 16-bit IP (Instruction Pointer) register when the processor is forming the memory address for a code fetch.
the 16-bit SP (Stack Pointer) register when the processor is forming the memory address for a write into or a read from stack memory.
the instruction when the processor is forming the memory address for a data access.
Since the offset is a 16-bit value, all segments are restricted to a length of 64KB.
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