very well explained. thanks Em qua., 15 de nov. de 2023 às 12:08, Jonathan Scott < jonathan_sc...@vnet.ibm.com> escreveu:
> I doubt whether there is much difference between methods of > zeroing a register now. I would assume that if you want the > condition code preserved, LHI is best, otherwise use XR, but SR > on modern hardware probably takes the same time as XR anyway. > I would expect current hardware to detect XR with identical > operands and to treat it as a "Zero this register" instruction. > > Back in the mists of time, XR was a bad idea because some > machines (370/135 comes to mind) handled it byte by byte as a > logical operation, and SR could be slower than SLR, especially > when fixed point overflow was enabled, so SLR was preferred. > However, the condition code following SLR reg,reg is 2 rather > than 0 which could be confusing. So a common approach was to > define a macro to do it, which could be updated to whatever was > best at the time. > > Jonathan Scott, HLASM > IBM Hursley, UK >
