Target Machine

• The target computer is a type of byte-addressable machine. It has 4 bytes to a word.
• The target machine has n general purpose registers, R0, R1,...., Rn-1. It also has two-address instructions of the form:

1. op source, destination  

Where, op is used as an op-code and source and destination are used as a data field.

• It has the following op-codes:
    ADD (add source to destination)
      SUB (subtract source from destination)
      MOV (move source to destination)
• The source and destination of an instruction can be specified by the combination of registers and memory location with address modes.

MODE	FORM	ADDRESS	EXAMPLE	ADDED COST
absolute	M	M	Add R0, R1	1
register	R	R	Add temp, R1	0
indexed	c(R)	C+ contents(R)	ADD 100 (R2), R1	1
indirect register	*R	contents(R)	ADD * 100	0
indirect indexed	*c(R)	contents(c+ contents(R))	(R2), R1	1
literal	#c	c	ADD #3, R1	1

• Here, cost 1 means that it occupies only one word of memory.
• Each instruction has a cost of 1 plus added costs for the source and destination.
• Instruction cost = 1 + cost is used for source and destination mode.

Example:

1. Move register to memory R0 → M

1. MOV R0, M  
2. cost = 1+1+1    (since address of memory location M is in word following the instruction)  

2. Indirect indexed mode:

1. MOV * 4(R0), M  
2.      cost = 1+1+1   (since one word for memory location M, one word  
3. for result of *4(R0) and one for instruction)  

3. Literal Mode:

1. MOV #1, R0  
2. cost = 1+1+1 = 3   (one word for constant 1 and one for instruction)