Q. Sequence of micro -operations to perform a specific function?

A digital system executes a sequence of micro-operations on data stored in registers or memory. Specific sequence of micro-operations performed is predetermined for an instruction. So an instruction is a binary code specifying a definite sequence of micro-operations to perform a specific function.

For illustration a C program instruction sum = sum + 7 will first be converted to equivalent assembly program:

• Move data from memory location 'sum' to register R1 (LOAD R1, sum)
• Add an immediate operand to register (R1) and store the results in R1

(ADD R1, 7)

• Store data from R1 register to memory location 'sum' (STORE sum, R1).

So several machine instructions may be required (this will differ from machine to machine) to execute a simple C statement. However how will every one of these machine statements be executed with the help of micro-operations? Let's try to explain the execution steps:

• Fetch instructions.
• Pass address of Program Counter (PC) to Memory Address Register (MAR).
• Issue the memory read operation to fetch instruction in Buffer Register for data like (MBR).
• Increment Program Counter to refer to subsequent instruction in sequence and bring instruction to Instruction Register (IR).

• Execute instruction
• Decode instruction to ascertain operation.
• As one of the operands is already available in R1 register and second operand is an immediate operand thus fetch operand step is not needed. The immediate operand is available in address part of instruction.
• Perform ALU based addition with R1 and buffer register store result in R1.

So we may have to execute instruction in several steps. For later discussion, for simplicity let's presume that every micro-operation can be completed in one clock period though some micro-operations need memory write/read which may take more time.