16-bit Processors and Segmentation (1978)
The IA-32 architecture family was preceded by 16-bit processors, the 8086 and 8088. The 8086
has 16-bit registers and a 16-bit external data bus, with 20-bit addressing giving a 1-MByte
address space. The 8088 is similar to the 8086 except it has an 8-bit external data bus.
The 8086/8088 introduced segmentation to the IA-32 architecture. With segmentation, a 16-bit
segment register contains a pointer to a memory segment of up to 64 KBytes. Using four
segment registers at a time, 8086/8088 processors are able to address up to 256 KBytes without
switching between segments. The 20-bit addresses that can be formed using a segment register
and an additional 16-bit pointer provide a total address range of 1 MByte.
The Intel386™ Processor (1985)
The Intel386 processor was the first 32-bit processor in the IA-32 architecture family. It introduced
32-bit registers for use both to hold operands and for addressing. The lower half of each
32-bit Intel386 register retains the properties of the 16-bit registers of earlier generations,
permitting backward compatibility. The processor also provides a virtual-8086 mode that allows
for even greater efficiency when executing programs created for 8086/8088 processors.
In addition, the Intel386 processor has support for:
• A 32-bit address bus that supports up to 4-GBytes of physical memory
• A segmented-memory model and a flat memory model
• Paging, with a fixed 4-KByte page size providing a method for virtual memory
• Support for parallel stages
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