The following message is a courtesy copy of an article that has been posted to bit.listserv.ibm-main as well.
[email protected] (Rick Fochtman) writes: > And if you opened the covers of the old 2880 Block Multiplexor Channel > box, you found a downsized version of the 360/44 front panel. Fancy > that! :-) the next phase was the 303x channel director ... which was 370/158 engine with just the integrated channel microcode (and w/o the 370 instruction microcode). a 3031 was a 370/158 engine with just the 370 instruction microcode (and w/o the integrated channel micrcode), configured to operate with channel director. 3032 was 370/168 configured to run with channel director (instead of 28x0 channel boxes). 3033 started out as 370/168 wiring diagram mapped to 20% faster chips. the 3033 chips also had ten times the number of circuits (as chips used in 370/168) but started out with the additional circuits not being used. during the development cycle, some critical sections were redesigned to make better use of the additional on-chip circuits ... and the 3033 eventually came out 50% faster than 168 (leveraging higher integrated, on-chip operations). there use to be some technology laying out data records on 3330 cylinders with "dummy" spacer records that would allow for channel program processing latency to do a head switch operation (on the same cylincer) between the end of a data record (on one track) and the start of the (next) data record (on another track) ... without a rotational miss. Several 370s; 145, 148, & 168, the channel processing was fast enough to execute the head-switch in the time it took a 3330 disk to rotate the dummy spacer record amount. The problem was that 158 channels had higher latency and would only make the head-switch (w/o a miss & additional revolution) 20-30% of the time (the rest of the time, the head-switch would miss picking the next record and have to may a complete revolution before trying again). The 3330 track size wasn't large enuf to make the dummy record sizes larger (using 4k data records). It turned out that the same rotational miss rates was true for the 303x channel directors (regardless of the machine they were attached to; since they all used the same 158 integrated channel processing). misc. past posts discussing dummy records & channel program head-switch latency: http://www.garlic.com/~lynn/2000d.html#7 4341 was "Is a VAX a mainframe?" http://www.garlic.com/~lynn/2001j.html#3 YKYGOW... http://www.garlic.com/~lynn/2002b.html#17 index searching http://www.garlic.com/~lynn/2003g.html#22 303x, idals, dat, disk head settle, and other rambling folklore http://www.garlic.com/~lynn/2004d.html#64 System/360 40 years old today http://www.garlic.com/~lynn/2004d.html#65 System/360 40 years old today http://www.garlic.com/~lynn/2004d.html#66 System/360 40 years old today http://www.garlic.com/~lynn/2005p.html#38 storage key question http://www.garlic.com/~lynn/2005s.html#22 MVCIN instruction http://www.garlic.com/~lynn/2006w.html#8 Why these original FORTRAN quirks? -- 40+yrs virtualization experience (since Jan68), online at home since Mar70 ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
