shonle@csu-cs.UUCP (Greg Shonle) (10/11/84)
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I just thought I'd set to rest all of the speculation going on about
IBM's new AT. I guarantee all material herein, because I've got an
AT sitting on my desk, and have carefully gone through Intel's manual
on the 80286 processor.
As far as speed goes, the AT really does run two to three times faster.
Basic programs run about twice as fast as the PC. Most other computation
bound programs run closer to three times faster (most programs run about
2.8 times faster). Disk-bound programs, such as most compilers, will run
about five times faster due to the very fast hard drive. The AT's hard
drive has an average access time of 40 ms, as opposed to the XT's 90 ms
average access time.
There are eight expansion slots on the AT. Two of them are completely
pin compatible with the IBM PC. The rest have a pin compatible area,
plus there is also additional pins to implement the larger address and
data busses in a second slot immediately in front of the standard slot.
Certain cards can still be plugged in the extended slots, such as the
Game Adapter. One of the 'original' slots is taken up by the disk
controller. The other must have a Color Graphics card, if present.
Otherwise, this slot usually contains the Monochrome card. The reason for
this is that the Color Graphics card is too tall (it extends below the
card level at the pin area) to fit in one of the extended slots. The
Monochrome card does not have this problem.
The 287 runs 16 % SLOWER than the 8087! Why is this? The system clock
rate is 12 mhz. The 286 internally divides this by two for an effective
clock rate of 6 mhz. The 287 can either divide the external clock by one
or by three. Since 12 mhz is too fast, it divides the clock by three for
an effective clock speed of 4 mhz. The 8087 by contrast runs at 4.77 mhz.
Other than that, the chips are essentially identical, except for some of
the interface lines.
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Greg Shonle
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