###########################################
#
#  A modular stack data structure using
#  a vf-based calling convention.
#  This approach can only be extended up to
#  a size 16 stack.
#
###########################################

: scratch 0 0 0 0 0 0 0 0
: under   0
: stack   0
: over    0 0 0 0 0 0 0

: push
	# here we take our argument from vf.
	# and do not corrupt any other registers.
	# we can eliminate the scratch saving
	# and restoring if low registers don't
	# need to be preserved.

	i := scratch
	save v6

	i := stack
	load v6
	i := over
	save v6
	i := stack
	v0 := vf
	save v0

	i := scratch
	load v6
;

: pop
	# result is left in vf.
	# again, we don't corrupt any other registers.

	i := scratch
	save v7

	i := stack
	load v7
	vf := v0
	i := under
	save v7
	
	i := scratch
	load v7
;

###########################################
#
#  Usage Example:
#
###########################################

: print
	# takes an arg in vf	
	i := hex vf
	sprite va vb 5
	va += 6
;

: main
	va := 3
	vb := 3

	vf := 5 push
	vf := 3 push
	vf := 1 push
	pop print
	vf := 9 push
	pop print
	pop print
	pop print

	# should print '1935'