How does LEA instruction store address of A?

Question

This is based off this question LEA instruction

Here is the code segment I have a question about

.ORIG X3700
 LEA R0, A
 .....
  A .FILL X1234


        

Answer 1

Anytime you see a PCoffset as an opcode operand

LEA R2, A    ; Loads the memory location of A into R2
             ; LEA, DR, PCoffset9

It's telling you that when your code is assembled it doesn't actually place the label 'A' into your LEA command.

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Take the following code:

.ORIG X3700     ; Not a code line in the simulator, only used by the assembler
LEA R0, A       ; Line 1, starting at x3700
HALT            ; Line 2
A .FILL X1234   ; Line 3, located at x3702
.END            ; Not a code line in the simulator, only used by the assembler

Now in the simulator the LEA line actually looks like this:

1110 000 000000001    ; Opcode 1110 is LEA
                      ; 000 is our register R0
                      ; 000000001 gives us how many memory locations away A is

And that's what the offset means. It asks the question "How many blocks of memory is A away from me?" and if our variable 'A' is too many blocks away then we will get an error because we cannot represent that value in the offset.