8086
In 1978, Intel introduced the first major processor for personal computers—the
8086—which had a 16-bit data bus, 16-bit registers, and a 20-bit address bus. A 20-
bit address bus meant that the 8086 could access 1MB of RAM. The speed of the
8086 ranged from 4.77 MHz to around 10 MHz, which is extremely slow considering
today’s standards (approximately 1.5 GHz).
8088
The 8086 chip was too expensive for PC manufacturers to use in their systems and
still sell the system at a reasonable price to their customers. So Intel introduced the
cheaper 8088 chip a year after the 8086. Like the 8086, the 8088 processor had 16-
bit registers and a 20-bit address bus (which meant it could access 1MB of RAM).
However, the data bus was decreased from 16-bits to 8-bits. The 8088 ran at the
same speed as the 8086, keeping its speeds at 4.77 MHz and 8 MHz.
The 8086/8088 did not include a built-in cache, nor did it have a built-in math co-processor.
If you wanted to add a math co-processor to your system, you would purchase
an 8087 chip to sit on the system board beside the CPU. The 8087 chip was
specifically designed as the math co-processor for the 8086 and 8088 processors.
80286
In 1982, Intel produced the 80286 chip, which had 16-bit registers and a 16-bit data
bus, and ran at speeds ranging between 6 MHz and 20 MHz. Other than the speed
increase, these characteristics matched that of the 8086—this time, however, the
market was there.
The 80286 also increased the size of the address bus to 24-bits, which meant that it
could access up to 16MB of RAM. Like the 8086/8088, the 80286 processor did not
contain its own internal cache, nor did it include a math co-processor. Processors
prior to the 80286 chip ran in real mode, while the 80286 processor introduced what
is known as protected mode. The following sections compare real mode with protected
mode.
Real mode
Real mode meant that the processor accessed memory as a whole and dealt with it
as a single entity. In other words, real-mode processors did not have any multitasking
capabilities—the capacity to divide memory up into multiple parts and run a
different application (or task) in each part, switching back and forth between them.
Protected mode
Protected-mode processors support the dividing up of system memory into different
parts and assigning a different application to each part of memory. Therefore,
protected-mode processors support multitasking and multitasking operating systems,
such as Windows 95/98, Windows NT, and Windows 2000.
Protected-mode processors also support virtual memory, which is the process of
using hard disk space as emulated memory. This means you could have 2MB of
RAM while the system is also using 10MB of hard disk space as “pretend” RAM. In
this case—as far as the applications that are running are concerned—there is
12MB of RAM.
80386DX
In 1985, Intel released its first 32-bit processor, the 80386DX, which had a 32-bit data
bus, a 32-bit address bus, and 32-bit registers. The 32-bit address bus meant that
the 80386DX processor could access 4 gigabytes (GB) of RAM, which is an amazing
improvement over previous processors (unfortunately, most people can’t afford to
purchase 4 GB of RAM).
The speed of the 80386DX processor ranged between 16 MHz and 40 MHz. The
80386DX contains no built-in cache, and the math co-processor (the 80387 chip) has
to be purchased separately. Once again, the math co-processor would be inserted
on the system board in the math co-processor socket.
80386SX
Three years after the 80386DX chip was out, Intel released the 80386SX, which was
a lower-end 386 chip. The 80386SX was a 16-bit processor, meaning it had a 16-bit
data bus. It could also only access 16MB of RAM, so the address bus had been
reduced to 24-bit. The speed of the 80386SX processor ranged from 16 MHz to 33
MHz. The registers were maintained as 32-bit registers.
Although both flavors of the 80386 chips support real and protected mode, they
have taken this support to the next level. These chips enable on-the-fly switching
between the two modes, whereas the 80286 processor had to be reset before it
could switch from one mode to another.
The major difference between the 80386DX processor and the 80386SX processor is
that the DX flavor is a 32-bit processor, while the SX flavor is a 16-bit processor.
When you compare the characteristics of the 80286 and the 80386SX chip, you realize
that the 80286 chip is competitive with the 80386SX chip. In reality, an 80386SX
chip is nothing more than a glorified 80286 chip with a bigger price tag.
80486DX
In 1989, major advancements were made in the performance of the computer system
when Intel released the 80486DX chip. This chip had a 32-bit data bus, a 32-bit
address bus (4GB of RAM), and 32-bit registers.
The 80486DX chip introduced two major advancements in CPU technology. The first
was the idea of integrating cache directly into the chip. The 80486DX had 8 kilobytes
(K) of built-in cache, or what is called L1 cache. The second major advancement
was that the math co-processor was integrated inside the 80486DX chip. Now,
instead of buying a math co-processor chip and inserting it onto the system board,
the chip was integrated and working as long as it was enabled in the system BIOS.
The speed of the 80486DX ranged from 20 MHz to 50 MHz (20 MHz, 25 MHz, and 33
MHz were the most popular speeds). After the original 486s, a second generation of
the 80486DX arrived that were marketed as 80486DX2-50, 80486DX2-66, and
80486DX4-100. The following sections discuss the DX2 and DX4 model processors.
80486DX2
The “80486DX” portion of “80486DX2-50” means that the processor is the DX flavor
of the 80486. The “2” after the “DX” implies “clock double,” a term indicating that
the CPU is working at twice the speed of the system board. In our example, the CPU
works at a speed of 50 MHz, while the system board runs at 25 MHz. So, as information
travels out of the CPU and hits the system board, the data slows down to half
the speed. The same could be said for the 80486DX2-66. The CPU works at a speed
of 66 MHz, while the system board runs at 33 MHz.
80486DX4
The DX4 model works exactly the same way as the clock double, only the clock
double is actually a clock triple. In other words, the CPU works at three times the
speed of the system board. So, why call it a “DX4” if it’s really a clock triple?
Because one extra enhancement, other than the clock triple, was added to the DX4:
8K more of L1 cache to the chip. The idea is that our clock triple plus the extra 8K
of cache memory gives us a DX4 chip: (clock)3 plus 8(K) equals (DX)4.
All 80486 chips—except the DX4 chips, which have 16K of L1 cache—have 8K of
L1 cache. Also, all processors created after the 80486 chip use L1 cache, though
they may differ in the actual amount. This becomes one of the selling points of the
different processors.
80486SX
Two years after the success of the 80486DX chip, Intel decided to market a lowerend
80486 chip. This new chip, released in 1991, was called the 80486SX.
What did the 80486SX chip have that made it so special? Or maybe a better question
to ask is: What didn’t it have? The 80486SX chip was a full-blown 80486DX with
the integrated math co-processor disabled. This time, Intel was trying to attract
customers who could not justify the price of the more functional chip, so Intel simply
downgraded the 80486DX, marketing it as a different chip and selling it at a
lower price.
Because the 80486SX chip did not have an integrated and functioning math co-processor,
there was a place on the system board to add a math co-processor chip (the
80487SX).
|
Disclaimer
