uh@bsiao.UUCP (Uul Haanstra) (11/19/86)
Cybers and Real Time Computing
I always thougt that the Cyber used 60 bit words so that the counting
of seconds and minutes would be easier. Starting at second 1, you say MX5 1,
and get one 1 bit in register X5. After a second you say AX5 1, this does an
arithmetic right shift and extends the sign bit. We now have two bits that
are 1. Now to test for 60 of those bits, you say ZR,X5 MINUTE which will jump
to label MINUTE if there are 60 one bits. This is also the reason the Cyber
works in one's complement: 60 one bits is (minus) zero.
If you ever want to display the time, use the PX6 X5 instruction
(population count), and you get the number of 1 bits in register X6.
Of course, counting the hours is done by one of the Peripheral Processor Units,
which have 12 bit words. The difference between AM and PM can be maintained
as follows: during AM hours, the CPU runs in user mode; when 12 bits are one,
issue an XJ and switch to monitor mode, and start the same process.
I may be wrong though...
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Uul Haanstra, Postbank N.V. Amsterdam ...!mcvax!bsiao!uh
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In article <197@bsiao.UUCP> uh@bsiao.UUCP (Uul Haanstra) writes: >Cybers and Real Time Computing > >I always thougt that the Cyber used 60 bit words so that the counting >of seconds and minutes would be easier. ... > The difference between AM and PM can be maintained >as follows: during AM hours, the CPU runs in user mode; when 12 bits are one, >issue an XJ and switch to monitor mode, and start the same process. > >I may be wrong though... Except on some models - at least of the 6400 - there wasn't a monitor mode or an XJ instruction. It cost an extra $10,000 to have CDC hook up the wire that allowed XJ. Oh, well... -- Ed Gould mt Xinu, 2560 Ninth St., Berkeley, CA 94710 USA {ucbvax,decvax}!mtxinu!ed +1 415 644 0146 "A man of quality is not threatened by a woman of equality."