I've been shamed!!!!!

Jan, it was obvious there was room for optimisation in my code. In that fragment of code, it was not necessary to save the PSW as you point out but for the purposes of instruction the code I wrote was rather generic to illustrate a point. You can see from the responses that the 'seasoned' ones amongst us would push the psw if the acc was touched - probably a fallback to the old days of Z80's! I would've preferred Jeroen to ask about possible optimisations as that would demonstrate learning on his part. Hopefully Jeroen understands a bit more about interrupts and the psw now.