Where do you find such detailed descriptions? Patent application? System journal? NDA material? You can tell me, but then you'ld have to kill me?
Bill Fairchild Rocket Software -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@bama.ua.edu] On Behalf Of Edward Jaffe Sent: Friday, July 22, 2011 1:19 PM To: IBM-MAIN@bama.ua.edu Subject: Re: running Assembler I/O macro code as AMODE 31, RMODE ANY The last time I thoroughly studied/investigated System z branch prediction logic (BPL) was on the z9. On that model, the BPL runs early in the pipeline, before instruction decode, and it essentially runs asynchronously to the rest of the pipeline. It prefetches instructions and predicts direction and target based on the path it thinks the rest of the pipeline will be later executing. It puts those prefetched streams into fairly large instruction buffers awaiting when they might be needed by the decode logic. The BPL logic itself uses what appears to be a unified Branch Target Buffer and Direction Buffer (they are physically built of separate arrays but are logically the same). The BTB contains 8K entries. Being clever, IBM decided not to 'remember' not-taken conditional branches, thus allowing the BTB to appear much bigger than it really is. This was considered a reasonable performance trade-off (the rationale for keeping not taken branches in the BTB is to more accurately handle branches that frequently change direction). The z9 BTB uses a strongly-taken, weakly-taken approach for each branch. Another clever thing IBM did on the z9 was to implement "just in time" prefetching down non-predicted branch paths to enhance performance. So if the hardware predicts a branch will be not taken, it prefetches the taken-path just before the branch direction is resolved in the execution stage of the pipeline. So if it was predicted incorrectly, the taken-path will be in a "recovery instruction buffer" where it could be sent into the decoder on the next cycle. Slick! :-) ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
