Why isn’t my assembly program setting r1 to the correct value?

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HALT is just a “pseudo-instruction” for a TRAP instruction used to halt the machine.

You could write:

TRAP x25  ;HALT the machine

But in this way you need to remember the position in the TRAP vector, in this case x25. So is better to just use HALT instead.

Others common TRAPs also have pseduo-instructions: IN, OUT, etc.

I asume you want to store your results somewhere. You could do something like:

;Sets pc to this address at start of program 
.ORIG x3000
;R1 will store the result lets clear it(ANd with 0)
AND R1,R1,x0
;R2 will be multiplied by R3, let's clear both of them 
AND R2,R2,x0
AND R3,R3,x0
;Test case 4 * 3 = 12;
ADD R2,R2,4
ADD R3,R3,3
;Add to increment zone 
LOOP Add R1,R1,R2;
;Decrement the counter, in this case the 3 or R3
ADD R3,R3,x-1
BrP LOOP
ST R1, Result         ;STORE R1 at Result
HALT
Result .FILL x0000    ;this will be x000C=12h after execution
.END

———————EDIT————————–

About you last question (in comments):

If HALT stops my program, how will Reslt .FILL x0000 directive run
then?

This is more a question about how assemblers works.

The answer is because: Assembly Time != Execution Time

Directives are considered at Assembly Time.

In fact, Assembly Time is composed in two passes:

  1. Resolve symbols creating a symbol table
  2. Convert instructions to “truly executable/machine code” using the symbol table.

This is a very common way to implement assemblers, and the LC3 assembler is not the exception.

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