In a message dated 6/22/2005 1:37:35 P.M. Central Daylight Time, [EMAIL PROTECTED] writes:
My recollection is that the 3340, 3344 and 3350 were the first. I'll have to see whether I still have my 3330 manuals. Could well be. I hesitated on saying 3330. I think the 3340 and 3344 had 3 SDs and the 3350 had 5. Too many decades ago to remember clearly. They always seemed to go up by 2 whenever they increased. 3375/3380/3390s had the most - 7 SDs. The way these worked was pretty clever. The first thing ever permanently recorded on a track is the Home Address. Once skip displacements were invented, the SDs were saved in a normally unreadable part of the Home Address. To read them in or write them back out, you had to use the Read/Write Special Home Address CCW. This read in 27 bytes on the 3375 and 28 on 3380/3390, 14 bytes of which were 7 different 2-byte SDs. Then when you wrote R0 after the HA, the controller copied the 7 SDs that it found in the HA into an unreadable part of R0 (a glorified count field, IIRC). Then whenever you wrote the first record after R0, the controller copied the 7 SDs into an unreadable part of this new record. Thus the same 7 SDs were propagated into each new record written on the track, and no matter where you were on the track when you first established orientation, the controller would have all 7 SDs available the next time it sensed any count field. Bill Fairchild ---------------------------------------------------------------------- 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
