;; Main routine
                      .ORIG x3000
MAIN                  ;; setup stack
                      LD    R6, STACKBASE       ; set the stack base
                      ADD   R5, R6, #0          ; set main's frame pointer

                      LD    R0, LZ_AMOUNT       ; load param 1
		      LD    R1, LZ_IN		; load param 2

                      ;; push parameters
                      ADD   R6, R6, #-1         ;
                      STR   R1, R6, #0          ; push param 2
		      ADD   R6, R6, #-1		;
		      STR   R0, R6, #0		; push param 1
		      JSR   SUB_FILL_LZ         ; call subroutine
                      LDR   R0, R6, #0          ; pop return value
                      ADD   R6, R6, #1          ;

                      ST    R0, RESULT          ; store the result

                      ;; cleanup stack
                      ADD   R6, R6, #2          ; 
                      HALT

;; subroutine: Fill_Leading_Zeroes
;; desc: Takes a number LZ_IN and sets LZ_AMOUNT bits from MSB to LSB to 0
;; function signature in C might look something like: lc3_word_t fill_LZ(int lz_amount, lc3_word_t lz_in); 
SUB_FILL_LZ
                      ;; setup stack frame
                      ADD   R6, R6, #-1         ; allocate return value
                      ADD   R6, R6, #-1         ;
                      STR   R7, R6, #0          ; push caller's return address (R7)
                      ADD   R6, R6, #-1         ;
                      STR   R5, R6, #0          ; push caller's frame pointer (R5)
                      ADD   R5, R6, #-1         ; set current frame pointer FP = SP - 1
		      ADD   R6, R6, #-2		; allocates memory for two local variables (if this function required it)

                      ;; SUB_FILL_LZ does its work... 

		      ;; get parameters 1 and 2
        	      LDR   R0, R5, #4          ; get LZ_AMOUNT
                      LDR   R1, R5, #5		; get RZ_IN

                      ;; select the appropriate mask 
		      LEA R2, MASK16 		; pointer to mask xFFFF
		      BRnz FILL_LZ_DONE		; no work required if LZ_IN <= 0
SELECTMASK	      ADD R2, R2, #1            ; mask pointer += 1
		      ADD R0, R0, #-1		; decrement counter
	              BRp SELECTMASK		; lather, rinse, repeat

		      ;; load and apply the mask
	              LDR R3, R2, #0 		; load the selected mask
		      AND R1, R1, R3		; apply the selected mask to LZ_IN

		      ;; SUB_FILL_LZ work complete.

                      ;; store result
FILL_LZ_DONE	      STR   R1, R5, #3          ;

                      ;; unwind the stack
                      ADD   R6, R5, #1          ; reset stack pointer (SP = FP + 1)
                      LDR   R5, R6, #0          ; pop caller's frame pointer
                      ADD   R6, R6, #1          ;
                      LDR   R7, R6, #0          ; pop caller's return address
                      ADD   R6, R6, #1          ;
                      RET
	
;; constants used by FILL_LZ
MASK16		.FILL xFFFF
MASK15		.FILL x7FFF
MASK14		.FILL x3FFF
MASK13		.FILL x1FFF
MASK12		.FILL x0FFF
MASK11		.FILL x07FF
MASK10		.FILL x03FF
MASK9		.FILL x01FF
MASK8		.FILL x00FF
MASK7		.FILL x007F
MASK6		.FILL x003F
MASK5		.FILL x001F
MASK4		.FILL x000F
MASK3		.FILL x0007
MASK2		.FILL x0003
MASK1		.FILL x0001
MASK0		.FILL x0000

;; INPUT/OUTPUT VARIABLES
STACKBASE             .FILL     x30FF
LZ_AMOUNT	      .FILL     #8
LZ_IN		      .FILL	xFFFF
RESULT                .BLKW     1

                      .END