Ive been trying to figure out how the division by subtraction works but there are no resources online that make it clear. Also i need a good example of how subroutines should look like in terms of syntax.
Answer
There are two ways you can approach division in LC3. If you're looking for an example on how to do division through subtraction take a look at this post:
How do i do a bitshift right in binary
But if you don't have to use the subtraction method, I would recommend doing 15 bit-shifts to the left. It's more efficient because it requires less run time, and its speed isn't impacted by the number we want to bit shift.
The code below shows you how to preform a bit-shift right by using 15 bit shifts to the left:
.ORIG x3000
LD R0, VALUE
SHIFT_RIGHT
AND R2, R2, #0 ; Clear R2, used as the loop counter
SR_LOOP
ADD R3, R2, #-15 ; check to see how many times we've looped
BRzp SR_END ; If R2 - 15 = 0 then exit
LD R3, BIT_15 ; load BIT_15 into R3
AND R3, R3, R0 ; check to see if we'll have a carry
BRz #3 ; If no carry, skip the next 3 lines
ADD R0, R0, R0 ; bit shift left once
ADD R0, R0, #1 ; add 1 for the carry
BRnzp #1 ; skip the next line of code
ADD R0, R0, R0 ; bit shift left
ADD R2, R2, #1 ; increment our loop counter
BRnzp SR_LOOP ; start the loop over again
SR_END
ST R0, ANSWER ; we've finished looping 15 times, store R0
HALT
BIT_15 .FILL b1000000000000000
VALUE .FILL x3BBC ; some random number we want to bit shift right
ANSWER .BLKW 1
.END