big@hobbes.catt.ncsu.edu (Alan Porter) (01/28/91)
Forgive me, but I am still new at this game. How fast must a math coprocessor be to work and to be effective? I have a 286-16. Do I need a 287-16 or a 287-8 or what? Will a slower speed still work, but not as effectively? big@hobbes.catt.ncsu.edu Alan Porter NCSU Computer Theme Program Box 21536 NCSU / Raleigh, NC 27607 USA / +1 (919) 737-6121
acook@athena.mit.edu (Andrew R Cook) (01/28/91)
In article <1991Jan27.172506.15133@ncsuvx.ncsu.edu>, big@hobbes.catt.ncsu.edu (Alan Porter) writes: |> |> Forgive me, but I am still new at this game. |> How fast must a math coprocessor be to work and to be |> effective? |> |> I have a 286-16. Do I need a 287-16 or a 287-8 or what? |> Will a slower speed still work, but not as effectively? |> |> |> big@hobbes.catt.ncsu.edu Alan Porter |> NCSU Computer Theme Program |> Box 21536 NCSU / Raleigh, NC 27607 USA / +1 (919) 737-6121 The general rule of thumb for 86 and 286 machines is that the coprocessor runs at 2/3's of the main CPU speed. For your machine, you'll want a 12Mhz coprocessor. Getting a lower rated coprocessor is asking for trouble. The computer has no way of telling what speed your coprocessor is running at. The clock speed is set at something by the manufacturor. If the MPU cannot keep up with the set rate, it will trip up and can cause faulty answers, or in worst case, lock-up you machine. Check out AMD's coprocessor. It's only $99, and fully gauranteed to work. I got one, and it works great. Andy Cook acook@athena.mit.edu
scjones@thor.UUCP (Larry Jones) (01/29/91)
In article <1991Jan27.172506.15133@ncsuvx.ncsu.edu>, big@hobbes.catt.ncsu.edu (Alan Porter) writes: > Forgive me, but I am still new at this game. > How fast must a math coprocessor be to work and to be > effective? It must be rated for at least the clock speed that your motherboard supplies. In most cases, the external clock supplied to the 287 is the same as the external clock supplied to the 286 -- the 286 divides the external clock by 2 to get the internal clock (which is what the rating refers to) but the 287 divides the external clock by 3, so the 287 would need to be at least 2/3 the speed of the 286. Thus, a 12MHz 286 would need an 8Mhz 287 and a 16Mhz 286 would need a 12Mhz 287. Although this scheme is common, it is by no means universal, so you should really make an effort to find out the details for your particular motherboard. ---- Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH 45150-2789 513-576-2070 Domain: scjones@thor.UUCP Path: uunet!sdrc!thor!scjones Start tying the sheets together. We'll go out the window. -- Calvin
jpn@genrad.com (John P. Nelson) (01/29/91)
>The general rule of thumb for 86 and 286 machines is that the coprocessor >runs at 2/3's of the main CPU speed. While this is (usually) true of 80286 machines, it is definitely NOT true for 8086 or 8088 machines. The 8087 MUST run at the same clock rate as the main processor. 80387's and 80387SX's are generally run at the same speed as the CPU, also. Some 80286's use a seperate crystal to clock the 80287: On these, the 2/3 rule doesn't work: You really need to know the proper frequency for the co-processor. john nelson uucp: {decvax,mit-eddie}!genrad!jpn domain: jpn@genrad.com
dave@interlan.Interlan.COM (Dave Goldblatt) (01/29/91)
According to the Intel spec book I looked at, the 80287 (and, I assume, the
80387; not sure about the 8087) can be driver either off an external clock,
or, as is usually the case, off the system clock. When driven off of the
system clock, the coprocessor uses an internal divide-by-3 to knock down
the system clock speed. Thus, if your 80287 runs at 6MHz, the system clock
can be no greater than 18MHz.
The problem is that some people refer to say, an 80287-8 to mean an internal
clock of 8MHz, whereas others mean that it will only work on an 8MHz
machine.
Best bet is to talk to the manufacturer (or look in the data books) to
verify EXACTLY which coprocessor you'll need.
-dg-
--
"Dan Rather? NOT!" * Dave Goldblatt [dave@interlan.com]
- Wayne Campbell, * Diagnostic Engineering
"Wayne's World" * Racal InterLan
_SNL_ * Boxborough MA (508) 263-9929cjwein@watcgl.waterloo.edu (Chris J. Wein) (01/29/91)
I just ordered an AMD287 and the sales rep said that their chip was self- adjusting up to 20Mhz (ie. they make only one chip). I'll let you know when I get one if it mentions anything in the documentation. -- ============================================================================== Chris Wein | cjwein@watcgl.waterloo.edu Computer Graphics Lab, CS Dept. | cjwein@watcgl.uwaterloo.ca University of Waterloo | (519) 888-4548
scjones@thor.UUCP (Larry Jones) (01/31/91)
In article <1991Jan29.152533.5647@watcgl.waterloo.edu>, cjwein@watcgl.waterloo.edu (Chris J. Wein) writes: > I just ordered an AMD287 and the sales rep said that their chip was self- > adjusting up to 20Mhz (ie. they make only one chip). As will Intel's, Cyrix's, IIT's, etc. The rating is the maximum speed the chip will run, any of them will run slower than that without any problems. ---- Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH 45150-2789 513-576-2070 Domain: scjones@thor.UUCP Path: uunet!sdrc!thor!scjones These things just seem to happen. -- Calvin