Target Machine

TARGET MACHINE

 Familiarity with the target machine and its instruction set is a prerequisite for designing a good code generator.

 The target computer is a byte-addressable machine with 4 bytes to a word.

 It has ngeneral-purpose registers, R₀, R₁, . . . , R_n-1.

 It has two-address instructions of the form:

o op source, destination

o       where, opis an op-code, and sourceanddestinationaredatafields.

·                                                     It has the following op-codes :

o       MOV (move sourcetodstination)

For example : MOV R₀, M stores contents of Register R₀ into memory location M.

Instruction costs :

•  Instruction cost = 1+cost for source and destination address modes. This cost corresponds to the length of the instruction.

• Address modes involving registers have cost zero.

• Address modes involving memory location or literal have cost one.

•  Instruction length should be minimized if space is important. Doing so also minimizes the time taken to fetch and perform the instruction.

For example : MOV R0, R1 copies the contents of register R0 into R1. It has cost one, since

occupies only one word of memory.

• The three-address statement a : = b + c can be implemented by many different instruction sequences :

i) MOV b, R0

ADD c, R0 cost = 6 MOV R0, a

ii) MOV b, a

ADD c, a cost = 6

iii) Assuming R0, R1 and R2 contain the addresses of a, b, and c : MOV *R1, *R0

ADD *R2, *R0 cost = 2

• In order to generate good code for target machine, we must utilize its addressing capabilities efficiently.