subtract
# Description:
#   Performs X-Y and returns result on TOS.
# Stack Requirements:
#   Y Operand
#   X Operand
#   Return PC <-- TOS

# Place Return PC at bottom of stack.
LDSP+2
LDSP+1
STSP+3
STSP+0

# Stack now looks like:
#   Return PC
#   X Operand
#   Y Operand <-- TOS

# Negate Y
NOT
IM_1
ADD

ADD # Perform X + (-Y)

SWAP # Put result behind Return PC on the stack.
RTS