==================================================== -----Original Message----- From: "Binyamin Dissen" <[EMAIL PROTECTED]> Sent: 7/27/2006 9:11 AM To: "[email protected]" <[email protected]> Subject: Doing a LM of two words on a double word bounday - is it serialized?
I would like to guarantee a concurrent load of two words on a double word boundary. Will LM serialize the doubleword? Or must I something hokey like a CDS which appears to do a concurrent load if the result is not equal. Doing a L followed a PLO/CompareLoad seems the absolute worst choice. -- Binyamin Dissen <[EMAIL PROTECTED]> ==================================================== The PoPs guarantees it. A "LM" for a double word-aligned operand will perform a concurrent 8-byte access. Same for STM. This is spelled out in the "Storage-Operand Consistency" specification in Chapter 5. Look at "Block-Concurrent References". If you are really paranoid about it, then CDS will also perform a concurrent 8-byte load, but it may also store it back unchanged. Your operand in storage must not be protected against storing. CDS R2,R2,operand ; check-point synch and possible store. will concurrently load R2,R3 and it may concurrently store the unchanged value back into the operand location. It also performs a check-point synch before and after the instruction. The check-point synch may be necessary to purge your local cache of stale data. LM R2,R3,operand ; no check-point synch, no store. will concurrently load R2,R3. The values are fetched 8-bytes in parallel. If another CPU is concurrently storing 8-bytes, then your CPU will see the entire old 8-bytes or the entire new 8-bytes and never a mixture of old and new bytes. Note that if another CPU is storing with a narrower alignment, like a 4-byte store, then you will get inconsistent results with either CDS or LM/STM. Jeffrey D. Smith Farsight Systems Corporation 24 BURLINGTON DR LONGMONT, CO 80501 303-774-9381 direct 303-709-8153 cell 303-484-6170 fax ---------------------------------------------------------------------- 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
