[ned1] / software / assembly_fragments / subroutines / sr_subtract.asm

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
Commit	Line	Data
bc5b63cf AT	1	subtract
	2	# Description:
	3	# Performs X-Y and returns result on TOS.
	4	# Stack Requirements:
	5	# Y Operand
	6	# X Operand
	7	# Return PC <-- TOS
	8
	9	# Place Return PC at bottom of stack.
	10	LDSP+2
	11	LDSP+1
	12	STSP+3
	13	STSP+0
	14
	15	# Stack now looks like:
	16	# Return PC
	17	# X Operand
	18	# Y Operand <-- TOS
	19
	20	# Negate Y
	21	NOT
	22	IM_1
	23	ADD
	24
	25	ADD # Perform X + (-Y)
	26
	27	SWAP # Put result behind Return PC on the stack.
	28	RTS
	29