PGIFFORD@ALLEGVM.BITNET (Pete Gifford) (03/04/91)
I was disassembling a program (not copyrighted!) a while ago and found it
starting with the instructions below. I understand all of it but the LA.
Can someone shed some light on its purpose?
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Pete Gifford PGIFFORD@ALLEGVM
Systems Manager PETE@MUSIC.ALLEG.EDU
Allegheny College (814) 332-5304
Meadville, PA (814) 332-5366
PROGRAM BALR 4,0 load base register
BCTR 4,0 back off one byte to point
BCTR 4,0 to label at start of program
LA 4,0(0,4) ???????XRJDM@SCFVM.BITNET (Joe McMahon) (03/05/91)
This practice (LA 4,0(0,4)) is Not Recommended in these days of MVS/XA and 31-bit addressing. On a 24-bit (MVS/370-mode) machine, this would clear out the high byte (which has stuff put in it by the BALR instruction - I've forgotten what, exactly, now). On a 31-bit machine, it simply clears the high bit. In the current case, it might not be so bad. But why the programmer didn't just use LR R4,R15 / USING WHATNOT,R4 is beyond me. Perhaps this is an entry into the Unclear Assembler contest? :-) --- Joe M.
REICHETZ@AWIIMC12.BITNET (Christian Reichetzeder) (03/05/91)
On Mon, 4 Mar 91 10:38:14 GMT Pete Gifford said: >..starting with the instructions below. I understand all of it but the LA. >Can someone shed some light on its purpose? >PROGRAM BALR 4,0 load base register > BCTR 4,0 back off one byte to point > BCTR 4,0 to label at start of program > LA 4,0(0,4) ??????? Whoever wrote this had something in mind - there're rumors that there's been a time when one of the major goals in software writing was "just to confuse the Russians". Well, the LA clears bits 0-7 (24bit mode) or bit 0 (31bit mode) to zeroes. Still this could be done without the BCTR instruction (given that you are happy with the base register offset 2 bytes from the entry point). Christian
cfor%CIBA-GEIGY.CH@OHSTVMA.IRCC.OHIO-STATE.EDU ("", Rainer Foeppl) (03/07/91)
RFC-822-HEADERS: Newsgroups: comp.lang.asm370 Organization: Ciba-Geigy AG, Basel, Switzerland ================== hello the only effect of the la instruction is to clear bit 0-7, if you are in 24- bit - addressing mode. if you are in 31-bit addressing mode, then it clears only bit 0. if you have any more questions, look at the ibm manual pricnciples of operation (sa22-7200-0). there you will find explanations to all valid instructions of the ibm 370 and 390 architexture. regards rainer
foster@ted.cs.uidaho.edu (03/08/91)
It's a trick. BALR loads the program mask into the lowest nibble of r4...or it used to. I'll bet the LA 4,0(0,4) is just there to zero out the program mask. This is not good code, as the program mask being loaded depends on the operating systems (doesn't work that way in MVS/XA or MVS/ESA). James
foster@TED.CS.UIDAHO.EDU (03/08/91)
It's a trick. BALR loads the program mask into the lowest nibble of r4...or it
used to. I'll bet the LA 4,0(0,4) is just there to zero out the
program mask.
This is not good code, as the program mask being loaded depends on the operating
systems (doesn't work that way in MVS/XA or MVS/ESA).
JamesUSERALVE@RPITSMTS.BITNET (Brian Eliot) (03/08/91)
If the instruction
LA x,0(0,x)
is understood to mean "zero all non-address bits in register x" then
it works perfectly well for all addressing modes and all operating
systems. In fact it is the best way to accomplish this since it
automatically adjusts to the addressing mode being used.
Brian Eliot
Information Technology Services
Rensselaer Polytechnic Institutegah@hood.hood.caltech.edu (Glen Herrmannsfeldt) (03/11/91)
PGIFFORD@ALLEGVM.BITNET (Pete Gifford) writes: >I was disassembling a program (not copyrighted!) a while ago and found it >starting with the instructions below. I understand all of it but the LA. >Can someone shed some light on its purpose? >- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - >Pete Gifford PGIFFORD@ALLEGVM >Systems Manager PETE@MUSIC.ALLEG.EDU >Allegheny College (814) 332-5304 >Meadville, PA (814) 332-5366 >PROGRAM BALR 4,0 load base register > BCTR 4,0 back off one byte to point > BCTR 4,0 to label at start of program > LA 4,0(0,4) ??????? Well, I believe it throws away the 8 high order bits. At least it used to, I don't know about in XA.