Basic instructions such as Load have two operands, often a target (register or storage base/displacement) and source (register or storage). These are always Op1 and Op2. This is also the case with LAA, for example. Some instructions such as CLM have long had an additional Mask operand, which is Op3. LAA has an additional third operand for the value (a register) to be added to the second (source) operand which is then re-stored (I.e., source is also a secondary target).
To number these three operands differently would make things maybe less consistent in terms of what the operands actually do, as opposed to their textual position in assembly language. Maybe the better question is why the extra third operand is not written at the end, but asking won't change it of course. OTOH some more recent instructions may have a fourth operand (e.g. branch address) and there's things like CLIB that have the operands neatly 1,2,3,4, the 2 being the immediate source operand which has apparently generally adopted position 2 lexically, perhaps nobody was really ensuring consistency here. Or maybe there's a subtle deeper logic to it all. I long ago gave up trying to keep up with memorizing all the new instructions anyway. There's a lot of cliquishness and talking down to people here, nothing to really bother about. -----Ursprüngliche Nachricht----- Von: IBM Mainframe Assembler List [mailto:[email protected]] Im Auftrag von Bodoh John Robert [Contractor] Gesendet: Dienstag, 19. Februar 2013 14:10 An: [email protected] Betreff: Re: Load and Add John, I have been involved in assembler since 1970...although not extensively. It has only been a few years that it is the only language I use. Can you tell me why the POP dos not follow a sequential format such as R1,R2,D3(R3) instead of R1,R3,D2(R2). At least then there would have been no confusion on my part. I also don't know why everybody is making such a big deal about my having been confused about a small point regarding the POP. I now understand that I have to read the POP from an assembler programmer point of view rather than an English major's point of view. John -----Original Message----- From: IBM Mainframe Assembler List [mailto:[email protected]] On Behalf Of John Gilmore Sent: Monday, February 18, 2013 5:58 PM To: [email protected] Subject: Re: Load and Add Mr Bodoh, You are of course free to disagree. I tried to make it clear, politely, that conventions of this sort are arbitrary. Your response was full of provincial moralism This usage is an old one that you would already be familiar with if you were at all experienced. Get used to it or take up another language, RPG say. It will not be changed at your behest. John Gilmore, Ashland, MA 01721 - USA
