LC-3：JSR不会修改PC

Question

I'm working on a program in LC-3 assembly, and I am having an issue with JSR . Instead of modifying the PC, jumping to the label, and writing the previous PC to R7, it merely continues to the next instruction. Here is the relevant section:

....
NXTNODE     LDR     R2, R2, #0  ; get next node
BRz DONE    ; now that node is loaded in R2, compare the first and last name

AND R6, R6, #0
AND R3, R3, #0
AND R3, R4, #0  ; copy search term pointer to R3
ADD R6, R2, #0  ; copy node pointer to R6
ADD R6, R6, #3  ; point to last name
AND R5, R5, #0
ADD R5, R5, #1  ; init loop counter

OUTP    JSR     COMPARE         ; uses R6 to compare first and last name to search term.
                                ; if they match, continue (R0 = 1). Otherwise, go to NXTNODE.
ADD R0, R0, #-1
BRnp    NXTNODE         ; no match, don't print

LDR R0, R6, #0  ; there was a match. Load first name to R0      

TRAP    x22
LEA R0, SPACE
TRAP    x22
....

(The indentation is a little messy from the markdown)

COMPARE is located at a different location, in this line:

COMPARE ST  R1, ST1     ; R6 --> current node

The instruction is never moved to the COMPARE label. The simulator just steps by it, on to the next instruction.

It was working as expected before, but I must've changed something, and I have no idea what that is, or why it isn't working. Any help would be greatly appreciated.

Answer 1

The simulator just steps by it, on to the next instruction.

Just an idea:

Is the code at the COMPARE label executed?

Or - to ask differently: Do the registers after the JSR instruction have the values you would expect after returning from the subroutine?

If yes the simulator might step in "step over" mode you might know from high-level languages, too.

In this mode a function (high-level language) or subroutine (assembler) is not stepped but it is executed and the debugger stops when the function or subroutine has finished.