Encoding of Machine Instructions in CAO

• Assembly language program needs to be converted into machine instructions. (ADD = 0100 in ARM instruction set)

• In the previous section, an assumption was made that all instructions are one word in length.

• OP code: the type of operation to be performed and the type of operands used may be specified using an encoded binary pattern

 • Suppose 32-bit word length, 8-bit OP code (how many instructions can we have?), 16 registers in total (how many bits?), 3-bit addressing mode indicator. 

• Add  R1, R2 

• Move  24(R0), R5 

• LshiftR  #2, R0 

• Move  #$3A, R1 

• Branch>0  LOOP

 

•What happens if we want to specify a memory operand using the Absolute addressing mode?

•Move  R2, LOC

•14-bit for LOC – insufficient

•Solution – use two words

 

• Then what if an instruction in which two operands can be specified using the Absolute addressing mode? 

• Move  LOC1, LOC2 

• Solution – use two additional words 

• This approach results in instructions of variable length. Complex instructions can be implemented, closely resembling operations in high-level programming languages – Complex Instruction Set Computer (CISC)

• If we insist that all instructions must fit into a single 32-bit word, it is not possible to provide a 32-bit address or a 32-bit immediate operand within the instruction. 

• It is still possible to define a highly functional instruction set, which makes extensive use of the processor registers. 

• Add  R1, R2 ----- yes 

• Add  LOC, R2 ----- no 

• Add  (R3), R2 ----- yes