The 80286 is called the second generation of microprocessor, it is more advanced to the 80186. This is the first Intel microprocessor offering multitasking and virtual memory. It is a 16-bit processor capable of addressing up to 16 MB (as it had an address but of 24-bits) of RAM and could also work with virtual memory (1GB). . It had a prefetch queue of 6 instructions. The 286 is the first “real” processor. It introduced the concept of protected mode and real mode. To ensure proper operation, we must protect the operating system and all other programs and the shared resources. The approach taken by many operating systems provide hardware support that allows us to differentiate among various modes of operation. A bit called the mode bit is added to the hardware of the computer to indicate the current mode. With the mode bit, we are able to distinguish between a task that is executed on behalf of the operating system and one that is executed on behalf of the user. The dual mode of operation provides us with the means for protecting the operating system from errant users. This is accomplished by designing some of the machine instructions that may cause harm as “privileged instructions”, these instructions are executed only in monitor mode. The 286 had an extra register called the Machine Status Register (MSW) whose lower nibble (containing D3 D2 D1 D0) defined the mode of operation and moreover it uses a four level memory protection which is an extension of the user/supervisory (protected/real) mode concept. It also had on-chip Memory Management Unit (MMU). This is also the first Intel processor that could run all of the software written for its predecessor. It has 134,000 transistors and could run at 6 to 12.5MHz

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Towards the end of 1995 the Pentium Pro was announced. This Pentium introduced a new socket (Socket 8), utilizing 387 pins. The Pro series included ability to run multiple instructions in one cycle, could execute instructions out of order, and had dynamic branch prediction, as well as speculative execution. Also included was an
impressive cache arrangement. For programmers, the Pro looks like a classic CISC CPU, while internally the CPU is very RISC oriented in design.

This 3.3 Volt CPU (3.1V at 150 MHz) was designed with a 32-bit operating system (OS) such as Windows NT in mind. While the Pro had Level 1 cache in the CPU, its real forte was the integrated Level 2 cache which allowed upwards of 1MB of cache to reside inside the CPU packaging to run at processor speed. This really improved performance in SMP based system boards. The Pro chip was the first chip to be offered in the AT or the ATX format.

The ATX format was preferred, as the Pro consumed more than 25 W of power, which generated a fair amount of heat. There were several major improvements of Pentium pro over Pentium, for example it had a superscalar architecture (microprocessor architecture containing more than one execution unit), 12-stage super pipeline, internal micro-ops similar to RISC like instructions and internal thermal protection. This microprocessor could be clocked to 200.00 MHz and consisted of 5.5 million transistors.


Pentium PRO Microprocessor

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The 80386 is the first popular 32-bit microprocessor. IA-32 first appeared with the 80386 processor, but the architecture was by no means completely new. IA-32’s 8-bit predecessor first appeared in the Datapoint 2200 programmable terminal, released in 1971.

It wasn’t just an evolutionary product in Intel’s growing family of microprocessors; it was revolutionary. It is a 32-bit chip that contained 275,000 transistors, could process five million instructions per second, and could run all popular operating systems, including Windows. It is also “multitasking,” meaning it could run multiple programs at the same time.

It has a pre-fetch queue length of 16 bytes. It has extensive memory management capabilities. It incorporates a sophisticated technique known as paging, in addition to the segmentation technique, for achieving virtual memory. The 80386 provided a new mode, virtual 8086 mode, in which real-mode programs could run while the processor was in protected mode. To support the concept of virtual memory to a grater extend it also has on-chip address translation unit. This, combined with a more flexible segmentation scheme and a larger addressable memory space (32 bits rather than 24, bring the total addressable memory to 4GB from 16MB and a virtual address space of 64 TB) has made 80386 protected mode the mode of choice for all modern operating systems.

Later IA-32 implementations have not made significant changes or enhancements to protected mode. IA-32 adds the extended registers EAX, ECX, EDX, EBX, EBP, ESP, ESI, EDI, EIP, and EFLAGS, as well as two additional, segment registers FS and GS. Originally all registers were special-purpose. For example, AX is originally an accumulator and could only be used as such. IA-32 lifted many of the restrictions on register usage, but some remain. For example, some instructions assume that a pointer in the EBX register is relative to the segment indexed by DS. In practice, 6 registers are available for generalpurpose use, far fewer than the number available in the ARM or IA-64 architectures.

The practical result of this register pressure is that IA-32 programs tend to make more frequent use of the stack for temporary storage. The 80386 has automatic self-test this feature is known as ‘Built-In-Self-Test’ (BIST). The BIST tests approximately one-half of the 80386 which includes the internal control ROM. After successful completion of BIST, the 80386 forms reset sequence after which it will start from the reset vector.



Microprocessor - 80386