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rnanavaty · Beginner Joined: 12 Apr 2006 Posts: 29 Topics: 20

Hi,

Can you tell me what is the difference between L and LA instructionin MVS assembely while using constant as second operand even though they perform the same operation ?

For example: L R3,5
and LA R3,5

Both will load value 5 in register R3

And what should be used at what time?

Bill Dennis

rnanavaty · Beginner Joined: 12 Apr 2006 Posts: 29 Topics: 20

See the link http://publib.boulder.ibm.com/infocenter/tivihelp/v1r1/index.jsp?topic=/com.ibm.itsm390n.doc/anrzgd55664.htm

Here also in the sample code instruction
LA R7,8 Gonna loop 8 times

is used to move 8 to register R7. can you explain it pls?

RonB · Posted: Mon Nov 02, 2009 7:43 am Post subject:

R7 is being initialized in preperation for the

BCT R7,LOOP1 Loop 8 times (Reg7 init to 8 )

instruction that follows.
The BCT ( Branch on Count ) instruction subtracts 1 from the indicated Register, and, if the resulting value in the Register is NOT zero, takes the indicated branch. So, in this case, since the instruction set following the LA R7,8 is performed 8 times - once before hitting the BCT, and seven more times following the BCT ( that is, when its execution results in values of 7,6,5,4,3,2, and 1 ).

papadi · Supermod Joined: 20 Oct 2009 Posts: 594 Topics: 1

You need to do as Bill suggested and spend some time doing the proper research.

The Load (L) and Load Address (LA) instructions are NOT the same - even though in some situations the same result happens. . .

As we said many years ago:

Rookie: What
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All the best,

di

semigeezer · Posted: Mon Nov 02, 2009 3:29 pm Post subject:

I think what people are referring to is that you should study how the the different instruction formats are represented in machine code and how assembler instructions get translated to machine code. Each instruction in the Principle of Operations has a description of the various parts of the machine code that is generated.

When you see how the instructions are represented in storage, you will see why this appears to work the same, but why it is a dangerous thing to assume. (hint : see how register 0 works in addressability). It will also be the 1st step in being able to read dumps without the original source code (if you ever care to do that -- most people don't Smile

).
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