A performance hit is only a factor if this will run many times an hour in an application.
However as a possible future example of how to code a TESTCB that might end up in a heavily used subroutine, I believe the standard form of TESTCB should never be used without lots of warnings if anyone else will ever see it. TESTCB has list, execute, and generate forms; they generate reentrant code. On 2020-10-26 2:07 p.m., Christopher Y. Blaicher wrote:
This will not have the same performance hit. The problem with the MVI was the CPU had to 1) bring the cache line into the data cache; 2) apply the MVI data; 3) refresh the cache line in the instruction cache; and finally 4) execute the instruction. Doing an EX skips steps 1, 2 and 3. EX has its own set of overheads, but nowhere near what all the cache hits have. Chris Blaicher Technical Architect Precisely.com
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From: IBM Mainframe Assembler List [mailto:[email protected]] On Behalf Of Melvyn Maltz Sent: Monday, October 26, 2020 11:16 AM To: [email protected] Subject: Is TESTCB a bad boy ? This message originated Externally. Use proper judgement and caution with attachments, links, or responses. In ancient times it was common practice to do this sort of thing... SWITCH NOP GO MVI SWITCH+1,X'F0' I believe this clears the cache and causes severe performance hits In my research into TESTCB for the z390 Project I found that it dynamically builds the subject instruction for an EX Would this have the same performance hit ? Melvyn Maltz.
