###########################################
#
#  A modular stack data structure using
#  a v0-based calling convention.
#  vf is used as a working temporary
#  register because it is "fragile".
#  maximum stack size is 256 bytes.
#
###########################################

# the first value is the pointer index
: stack 0 0 0 0 0 0 0 0 0

: push
	# modifies vf, v0
	# takes argument in v0
	# stack[++stack[0] + 1] := v0
	
	i  := stack
	vf := v0
	load  v0
	i  += v0
	v0 := vf
	save  v0

	i  := stack
	load  v0
	v0 +=  1
	i  := stack
	save  v0
;

: pop
	# modifies vf, v0
	# returns result in v0
	# v0 := stack[stack[0]-- + 1]

	i  := stack
	load  v0
	v0 += -1
	vf := v0
	i  := stack
	save  v0
	i  += vf
	load  v0
;

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

: print
	# takes an arg in v0	
	i := hex v0
	sprite va vb 5
	va += 6
;

: main
	va := 3
	vb := 3

	v0 := 5 push
	v0 := 3 push
	v0 := 1 push
	pop print
	v0 := 9 push
	pop print
	pop print
	pop print

	# should print '1935'