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--
--		File:
--			Doubling.ssbc
--
--		Author:
--			Clayton Seelenmayer
--
--		Date:
--			Oct 10th 2018
--
--		Behaviour:
--			Main read a value from Port B and store f(n) to Port A.
--			Subroutine "f" computes f(n) = f(n-1)*2 where n >= 1 and
--				f(0)=1.
--
--		Assumptions:
--			Program status word (PSW) is destroyed.
--			Subroutine "f" assumes input n >= 1, and f(0) = 1.
--			Use Big Endian binary notation.
--
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-- ============================ Main =================================

		pushimm     RL            Main return address (low byte)
		pushimm     RH                                (high byte)
		pushext     B             Save n from Port B.
		pushimm     0             Clear Z flag.
		popext      PSW
		jnz         SR            Jump to subroutine f.
		popext      A             Store f(n) to Port A.
		halt


-- ============================ Subroutine f =========================

SR:	pushimm     0             Subroutine.
		add
		jnz         RS (!base)    Jump to RR if it is not the base case.
		popext      PSW           Clear Z flag.
		popext      RH            Pop return address. (high byte)
		popext      RL                                (low byte)
		pushimm     1             Return value 1 (1 = 2^0) as base case.
		jnz         return        Return

RS:	popext      Hold          Recursive Subroutine.
		pushimm     RRL           Recursive Return Address. (low byte)
		pushimm     RRH                                     (high byte)
		pushimm     1             Decrement n.
		pushext     Hold
		sub
		pushimm     0             Clear Z flag
		popext      PSW
		jnz         SR            Jump to Subroutine.

RR:	popext      Hold          Recursive Return.
		pushext     Hold          n = n * 2.
		pushext     Hold
		add
		popext      Hold
		pushimm     0             Clear Z flag
		popext      PSW
		popext      RH            Pop return address (high byte)
		popext      RL                               (low byte)
		pushext     Hold          Save parameter in hold.
		jnz                       Recursive call.


-- ============================ Data =================================

		Hold                      Temporary storage variable address.