14.2.2. Control Instructions for Addressing Modes

In this unit, we will be looking at different addressing modes like immediate, direct, indirect, register indirect, and some generic addressing modes, along with the control signals generated.

Addressing modes are methods for identifying the locations of operands in memory. In this section, we'll discuss several types of addressing modes such as immediate, direct, indirect, and register indirect. Each addressing mode has a specific way of referencing the data needed for an instruction. Additionally, we will explore how different control signals are generated for each mode to execute instructions appropriately.

Think of addressing modes like different ways you can describe a friend’s house. For example, you might say, "It's the blue house at 123 Main Street" (direct), or "It's the house at the end of the road that belongs to John" (indirect). Similarly, addressing modes describe how to locate data in memory.

The first three steps that consist of instruction fetch are similar for all types of instructions. In the first step, the program counter value is loaded into the bus, which is then read into the memory address register.

In instruction fetching, there are three key steps that are consistent regardless of the type of instruction being executed. First, the program counter (PC) sends its value to the bus. This value is then stored in the memory address register (MAR). This means that the CPU is identifying which memory location it needs to read from. Next, the CPU operates in 'read mode', indicating to memory that it intends to fetch data. Finally, the ALU (Arithmetic Logic Unit) increments the PC by one, preparing for the next instruction.

Imagine you're at a library. You first look up a book's location in the library catalog (the MAR), which tells you which aisle to check. Then, like reading a specific book (fetching), you go to that aisle to find your next book (incrementing your search).

For a direct instruction, in step 4, we extract the memory location M directly from the instruction register to indicate where to read the operand from memory.

In direct addressing mode, the actual address of the operand is specified directly in the instruction. During step 4, the CPU fetches this memory location (M) from the instruction register (IR) and places it in the memory address register (MAR). The memory is set to read mode, and after a brief wait, the operand located at memory address M is retrieved into the memory data register (MDR). Finally, in step 6, the operand can be transferred to a register for further processing.

Think of direct addressing like a delivery instruction on a package. The address directly tells the delivery person where to go to drop off the package (the operand). It’s straightforward and doesn’t need them to figure out where to look.

In indirect mode, the memory location M points to another memory address, which contains the operand. Here, we have to access memory twice to get the operand.

In indirect addressing mode, the instruction specifies a memory location (M) which itself contains the address of another memory location where the actual operand resides. The steps are slightly different from direct addressing. First, the register value M is loaded in the MAR. Once the memory address is accessed, the value from that memory location (let's say it yields address 72) is then loaded into the MAR again to fetch the final operand. Thus, this addressing mode requires two memory accesses instead of one.

This is like getting a forwarding address. If you order something to be sent to a friend's house (the first address), but it redirects to another location (the second address). Thus, your item has to go through the first house to get to the final destination.

In immediate mode, the operand is specified directly in the instruction itself, allowing for the quickest execution with only four micro-operations.

Immediate addressing mode is the simplest type of addressing, where the operand is embedded directly in the instruction. This enables quick execution because fewer steps are required. After fetching the instruction (which includes the operand), the operation is directly performed using the operand without needing to access memory. This mode only requires four micro-operations: fetch from memory, increment PC, and transfer the immediate value to the desired register.

This is like carrying cash for buying items directly. When you pay with cash (immediate value), you don't need to go through banks (memory accesses) to get it—the money is in your pocket (instruction).

In summary, each addressing mode impacts the number of steps required to execute an instruction, with immediate mode requiring the least and indirect mode requiring more.

Different addressing modes affect the complexity and number of operations needed to execute an instruction. Immediate mode is most efficient, requiring only four steps, while indirect addressing takes more time, usually around seven steps. Consequently, understanding these modes is essential for designing effective instruction executions in any computer architecture.

Consider your morning routine. Some tasks take just one step, like grabbing a coffee from the counter (immediate). Others, like making breakfast, might require multiple steps (like indirect), taking more time to complete.

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