On Wed, 2 Sep 2015 07:32:15 -0400, John Eells wrote: >[email protected] wrote: ><snip> >> Mind you, max blocksize is 32768, but that info is useless. AMBLIST can >> *probably* help you, but ... > >The maximum block size for z/OS is actually 32760. (We have to reserve >space for those pesky RDWs and BDWs.) > No, the block size counts the BDW and RDWs, so the longest data portion is 32752.
The count in a CCW allows up to 65535. Originally, a smaller value was chosen because the s/360 hardware has no direct support for unsigned halfword arithmetic. It could have been done, SMOP, but with added code at a time when storage was precious. And using more than 32 Kib for a buffer was likewise unthinkable. Why not 32767? The best explanation I've gotten here is that the access method requires a few overhead bytes with each buffer, and there is (was?) a desire to obtain buffer storage page-aligned. Bad History. Otherwise, full track on a 3390 would be practical. And 32768 is reserved for LRECL=X. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
