80386: Making Protected Mode Better
The 386 had a fully 32-bit internal and external bus, and was able to switch between Real and Protected mode operation and vice versa without restarting the system, which paved the way for multi-tasking. In addition, “virtual real mode” was introduced.
Virtual real mode, as it would be known, allows a processor to create multiple real modes, or small virtual machines, or instances of real modes. For example, Windows 3.1 could run multiple instances of DOS at one time. Through partitioning, Virtual Real Mode allowed for multi-tasking to work better.
In order to actually concurrently run these programs in separate spaces, Windows needed the appearance of true multi-tasking; that is, you can “minimize” one program, per say, and work on the other, but still have the minimized one running and working. Although the processor can only handle one thing at a time (tick), the speed of ticks is so fast that it appears the applications are working simultaneously.
The 386DX: Intel’s First Derivative Chip
The 386DX and the DX line in general can be considered the “upper end” of a family of processors. In the 386 line, this meant it possessed the capability of full 32-bit computing, a 32-bit memory bus, clock speeds ranging from 16 to 33 MHz, and the ability to address 4 GB of physical RAM. In addition, the 386DX could address 64 terabytes of swap memory as a whole. This exorbitant figure basically represented an infinite number of possibilities and potential to software developers.
The 386SX: A Budget Choice
The SX was basically the update to the 286 line of processors. Offered at a low price, and restricted to a 16-bit external (but 32-bit internal) bus, the 386SX ran from 16 to 33 MHz, could only address 16 Megabytes of RAM, and was not pin-compatible to the original 286 sockets. Despite these shortcomings, the 386SX was able to run the same software as a 386/DX (provided the memory was available), meaning it too supported virtual real and enhanced mode (the name given to Protected Mode in the 386 by marketing).
Mobile Users and the 386SL
The 386SL, another 386 derivative, capitalized on the laptop market. By including such features as an external cache controller, a low-power design, power management features, and a relatively fast 25 MHz clock speed, it became obvious of the relative value of the chip in laptops, and manufacturers embraced the chip.
The FPU
The Floating Point Unit, AKA math coprocessor, became an option with the introduction of the 386. The FPU was a chip designated solely for fast arithmetic, and was clocked with its respective sister chip. The 386 33 MHz was introduced with an option 387 33 MHz. Thus, the naming scheme for this coprocessor became known as the 80X87 math coprocessor. An interesting note is that the FPU for the 486 was integrated into all 486 chips, as you will learn soon enough. However, on the 486SX, Intel continued to sell an optional 80487 FPU. They did so by disabling the FPU on the SX, and simply changing the socket shape on the FPU such that Intel could market a full featured 486 as an FPU which became the CPU once inserted into the special socket.
Review Time
Although we have only covered roughly half of the processor history we must cover, there is a lot of content and knowledge that has been given, and thus it is time to review what has been covered so far.
1. Which of the following processors does not feature some form of “Protected Mode”?
a. 80286
b. 80386
c. 80386DX
d. 80186
e. 80386SX
2. Which of the following motherboard form factors was introduced along with the 80286 processor?
a. ATX
b. XT
c. AT
d. NLX
e. Baby AT
3. True/False. The Pentium II was released two years before the groundbreaking 80386.
a.True
b. False
4. Which of the following processors is capable of addressing some 64 terabytes of page memory?
a. 386SX
b. 286DX
c. 486SX
d. 8086
e. 386DX
5. Which of the following is not a form factor used in smaller PC’s?
a. LPX
b. NLX
c. Mini-ATX
d. Mini-LPX
e. ATX
6. Which of the following enclosures would not ever be used with an ATX motherboard?
a. Tower
b. Mini-Tower
c. Mid-Tower
d. Laptop
e. Slimline
The 80486
The 80486 was another groundbreaking Intel processor. Featuring a full 32-bit bus (internal and external), a 32-bit address bus to address some four gigabytes of physical RAM, a level one internal cache, “burst mode,” and a built in FPU, along with upgrade capability and the ability to execute instructions at only two cycles, there were certainly many features loaded in the 486 to warranty a buy.
The question soon arises concerning what exactly a “level one internal cache” is. A cache is a small area of memory that the CPU believes will be needed in the near future and is set aside for data to fill that space. A cache can be found in RAM, on a hard disk, or on dedicated chips. Obviously, of these three, the hard disk is the slowest method, as you are inherently limited in the speed of the hard disk, whose transfer rate is much smaller and slower than that of the CPU’s. Although RAM is very fast, it still does not run nearly as fast as the CPU itself. And, dedicated chips are limited by the speed of the motherboard. The fastest and most efficient method of cache is internal, or built into the CPU itself.
The next question you may be asking yourself is “What in the world is burst mode?” Burst mode describes the method of which data is transferred into the cache. Synchronous burst mode is the burst mode where data is sent to the cache, “burst” by “burst,” until the cache is full, and the first “burst” in a block is overwritten. Think of this as storing files in cabinets, one by one, until the cabinets are full. You will take the stuff out of the oldest cabinet, put new files in, and discard the old (unless for some reason you need these old files). The cabinets represent cache, the files represent bursts, and the discarding mechanism is flexible in that in the case that cache memory is needed to be stored in some place, it can be kept safe in RAM, hard drive, or a lower level cache.
There is another burst mode. Pipeline burst mode utilizes a register, rather than a clock, to time the data transfer. In essence, both burst modes work on the same principle.
In addition to the cache, there is a FPU built into the 486. As you will soon observe, little difference lies between the 486 and the SX line of this family other than the lack of the FPU.
486DX/DX2/DX4
The DX lines of 486 chips operated at higher clock speeds than their non-DX counterparts. In addition, the DX2 and DX4 chips were capable of executing one instruction in one cycle. In all technicalities, there never was a straight-out 486 chip. The consumer model was introduced as the 486DX. Do not get the mistaken impression that the DX2 and DX4 operated twice or four times as fast. Rather, they had higher rated clock speeds, but the actual performance gains were not exceptional.
486SL
The 386SL was a major hit with laptop manufacturers. In the scheme of things, however, it is not very important to know much of its successor, the 486SL, other than the fact it existed, provided all of the benefits of the 486 line, and came will power management features that would eventually be extended to desktops. These features came through SMM, or System Management Mode. In addition, the Suspend/Resume function was very important.
486SX
The budget market was ripe for a new processor, and thus the 486SX was introduced as a cheaper alternative to the 486DX. However, the SX was in actuality not much slower than the 486DX (as it was missing only 15,000 transistors) and was a fully-loaded DX chip. One may ask then, why was there no FPU given with the SX. The answer lies in that the SX was simply a DX with less transistors and a disabled FPU.
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