Hardware Reference
In-Depth Information
capability that makes using protected mode more practical. In addition, the 386 includes a new mode,
called
virtual real mode
, which enables several real mode sessions to run simultaneously under
protected mode.
The protected mode of the 386 is fully compatible with the protected mode of the 286. Intel extended
the memory-addressing capabilities of 386 protected mode with a new MMU that provided advanced
memory paging and program switching. These features were extensions of the 286 type of MMU, so
the 386 remained fully compatible with the 286 at the system-code level.
The 386 chip's virtual real mode was also new. In virtual real mode, the processor could run with
hardware memory protection while simulating an 8086's real mode operation. Multiple copies of
DOS and other OSs, therefore, could run simultaneously on this processor, each in a protected area of
memory. If the programs in one segment crashed, the rest of the system was protected.
Numerous variations of the 386 chip were manufactured, some of which are less powerful and some
of which are less power hungry.
To learn more about the members of the 386 chip family, see Chapter 3 of
Upgrading and Repairing
PCs,
19
th
Edition, available in its entirety on the disc packaged with this topic.
P4 (486) Processors
Although fourth-generation processors were more about refinement than redesign, the Intel 80486
(normally abbreviated as 486) was another major leap forward in the race for speed. The additional
power available in the 486 fueled tremendous growth in the software industry. Tens of millions of
copies of Windows, and millions of copies of OS/2, have been sold largely because the 486 finally
made the graphical user interface (GUI) of Windows and OS/2 a realistic option for people who
work on their computers every day.
The 486 is a family of processors, consisting of DX, SX, and a number of other variations. Four main
features make 486 processors roughly twice as fast as an equivalent MHz 386 chip:
•
Reduced instruction-execution time
—A single instruction in the 486 takes an average of only
two clock cycles to complete, compared to an average of more than four cycles on the 386.
•
Internal (Level 1) cache
—The built-in cache has a hit ratio of 90-95%, which describes how
often zero-wait-state read operations occur. External caches can improve this ratio further.
•
Burst-mode memory cycles
—A standard 32-bit (4-byte) memory transfer takes two clock
cycles. After a standard 32-bit transfer, more data up to the next 12 bytes (or three transfers)
can be transferred with only one cycle used for each 32-bit (4-byte) transfer. Thus, up to 16
bytes of contiguous, sequential memory data can be transferred in as little as five cycles instead
of eight cycles or more. This effect can be even greater when the transfers are only 8 bits or 16
bits each.
•
Built-in (synchronous) enhanced math coprocessor (some versions)
—The math coprocessor
runs synchronously with the main processor and executes math instructions in fewer cycles than
previous designs did. On average, the math coprocessor built into the DX-series chips provides
two to three times greater math performance than an external 387 math coprocessor chip.
The 486 chip is about twice as fast as the 386 at the same clock rate. You can see why the arrival of
the 486 rapidly killed off the 386 in the marketplace. The die for the 486 is shown in
Figure 3.24
.
