Question

Assume that our MDR register is 16 bits wide while our machine language instructions are 32 bits wide. How might the fetch phase of the Von Neumann cycle have to be changed from what was described in the text?

Short answer

Split the fetch phase to perform two 16-bit reads using MDR, then combine for a 32-bit instruction.

Step-by-step solution

Understanding the Problem

We need to adjust the fetch phase of the Von Neumann cycle. Given that the Memory Data Register (MDR) can only hold 16 bits at a time, and the machine instructions are 32 bits wide, the original fetch phase designed for 32-bit instructions must be adapted.

Splitting the Instruction Fetch

To handle 32-bit instructions with a 16-bit MDR, the instruction fetch phase needs to be divided into two parts. First, we fetch the first 16 bits of the instruction and store it in the MDR.

Fetching the Remaining Bits

After the initial fetch, a second fetch operation is required to gather the remaining 16 bits of the instruction from memory, storing them again in the MDR.

Combining the Instruction

With two partial fetches completed, the system must combine the two 16-bit segments from the MDR into a single 32-bit instruction. This combined result is then available for execution during subsequent processing phases.

Updating the Program Counter

During fetch operations, the program counter must be adjusted correctly. Since two fetch operations are needed for one full instruction, it should be incremented appropriately to account for this during the instruction retrieval process.

Key concepts

Fetch Phase

The fetch phase is a crucial part of the Von Neumann architecture's cycle. During this phase, the CPU retrieves an instruction from memory for processing. The goal is to get the necessary instruction into a format that the CPU can understand and execute. Typically, this involves accessing memory, retrieving the required data, and temporarily storing it in an appropriate register.

In a standard process, the fetch phase would seamlessly involve retrieving a complete machine instruction in one go. However, challenges like a mismatch in data width between the instruction set and the memory register can complicate this phase, necessitating changes to ensure accurate instruction processing. That’s where understanding each component of this phase becomes critical.

Memory Data Register

The Memory Data Register (MDR) is an essential component in a computer's architecture, specifically within the fetch phase. Its primary role is to hold data that has been fetched from memory, or data that is to be written back to memory. However, issues arise when there's a size mismatch between the MDR and the instruction width.

For example, if the MDR is 16 bits wide and the machine instruction is 32 bits wide, the standard fetch procedure should adapt. The MDR would initially store the first 16 bits of the instruction. Since it cannot store the entire 32-bit instruction at once, the fetch phase would have to be adjusted to accommodate this limitation by splitting the instruction into manageable segments.

Instruction Fetch

Instruction fetch is another critical concept in the Von Neumann architecture. It refers to the process of retrieving a machine code instruction from memory. In scenarios where the width of the instruction exceeds the MDR's capacity, a two-step approach becomes necessary.

This involves two separate fetch operations to capture the complete instruction:

• First, fetch the first half of the instruction (16 bits in the 32-bit example) into the MDR.
• Next, perform a second fetch for the remaining bits, storing these in the MDR as well.

After completing both fetches, these segments are then combined to form the full instruction.

This approach ensures that even with constraints, the CPU has access to the complete instruction needed for processing.

Program Counter

The program counter is an essential component that tracks where the CPU is in its program sequence. It always points to the memory location of the next instruction to be fetched. This register gets incremented as each instruction is retrieved and processed.

In scenarios where multiple fetch operations are required for a single instruction, as with our 16-bit MDR and 32-bit instruction issue, the program counter has to update accordingly. Simply put, if two fetch operations are needed, the program counter must be incremented appropriately between these operations.

Thus, while one part of the fetch phase focuses on obtaining and combining parts of an instruction, the role of the program counter is to ensure continuity and correctness in sequence by maintaining the correct address for fetching upcoming instructions.