14.4.2. Recap of Single Bus Organization

Basically, we have a program counter, memory address register, and memory data register, which is taking the data from memory. We also have the ALU, which has a Y as the input from the bus. There is a multiplexer that takes a constant as the input required to increment the program counter. The output of the ALU is stored in a temporary register called Z. This is your register bank, instruction register, and it directly goes into the instruction decoder, generating the control signals for the instruction.

In a single bus organization, we use a shared communication path (the bus) to connect all the components – such as the program counter and registers. Data flows through this bus, which simplifies the architecture by reducing the number of connections needed between components. The program counter holds the address of the next instruction to be executed, while the memory address register holds the address of memory locations for data retrieval. The ALU performs arithmetic and logic operations, using inputs from the bus. The multiplexer facilitates selecting between different inputs, such as a constant value to increment the program counter.

Think of this system like a single-lane road where multiple cars (data) can use the same lane (bus) to get to their destination (memory/ALU). Each car takes turns driving along the road to reach either a restaurant (data register) or a parking lot (instruction register) where they will do their job (fetch or execute instructions).

We have seen that the first 3 steps consist of instruction fetch. In step 1, the program counter value is loaded into the bus, and the bus sends this value to the memory address register. Memory is set to read mode. The ALU adds program counter value with a constant (the next memory location), and this value is loaded back into the program counter in the next iteration. In step 2, the accumulator holds the value of PC = PC + 1, and the instruction from the memory location is dumped to the memory data register. Step 3 confirms the memory is ready and now transfers the instruction from the memory data register to the instruction register via the bus.

Instruction fetching involves getting the instruction from memory to be executed. In the first three steps: 1) the program counter’s address is placed on the bus to access memory, 2) the program counter is then incremented to point to the next instruction, and 3) the fetched instruction is moved from the memory to the instruction register. This is vital for the processor to know what task it needs to perform next. The first three steps are essential and uniform for all instructions, setting the stage for further instruction processing.

Imagine this process as a chef (CPU) preparing to cook (execute an instruction). The chef first fetches the recipe (instruction) from a cookbook (memory), checks the next page (incrementing the PC), and prepares to begin cooking. Only once the recipe is fetched can the cooking start!

The next stages depend on the type of instruction. If the instruction is direct, in step 4, we get the value of M from the instruction register, which indicates the specific memory location with the operand. This value is then loaded into the memory address register, and the memory is set to read mode. Similarly, if it’s an indirect addressing mode, it requires more steps compared to direct mode. In immediate addressing mode, the operand is retrieved directly from the instruction.

Addressing modes define how the instruction specifies the operand needed for execution. In direct addressing, the instruction contains the memory address directly. Indirect addressing involves an additional lookup, as the address points to another location where the operand is stored. The control signals generated vary accordingly, with indirect mode needing more steps to navigate the extra layer of memory. Immediate addressing, however, is most efficient as it requires simply pulling the value directly from the instruction.

Consider this as selecting vegetables for cooking. Using direct addressing is like selecting an onion by pointing directly to it. In indirect addressing, you point to a basket that contains another basket that has the onion. With immediate addressing, you already have the chopped onion written into your recipe! Each mode represents a different way of determining where to get the necessary ingredients.

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