RDD - Round Down 
Round Down uses similar logic to Round Nearest, but rounds down to the last level specified in Operand 3
Examples			
Operand 2		Operand 3	Result (stored in Operand 1)
1.10011		 1	1
1.10011		0.001	1.100
1.1		0.5	1.0
1.0		0.5	1.0
1.9		0.5	1.5
As with RDN, it is acceptable to round down to the next .25, or any other amount such as to the next 2, 3, etc. 
Statement:			Round the value of PC 500 down to the nearest quarter hour and store it back into itself.
Line	CMD	OT	Operand 1	OPER	OT	Operand2	OT	Operand3	If Goto	Else GoTo
00010	LET	PC	500	RDD	PC	500	N	.25	99999	
99999	EXIT								99999