30.3.1. Concrete Examples of Displacement Addressing Modes

So, let us write an instruction LOAD IMMEDIATE 20 that is may be the opcode can be something like LDI. Generally here I have discussed in a more linguistic manner, but we always have a mnemonic for that so it may be LDI that is load immediate or you can put some times a person may call it load indirect also.

The LOAD IMMEDIATE instruction loads a specific value directly into the accumulator without referencing any external memory locations. In this example, the immediate value of 20 is part of the instruction itself, meaning the computer can use it immediately without looking elsewhere.

Imagine you have a toolbox (the accumulator) and you receive a specific tool (the number 20) in your hand (the instruction). You can use it right away for tasks without needing to check other boxes or shelves.

LOAD DIRECT 3 is a direct addressing mode. This means, this 3 actually refers to the memory location 3 where the data is present. So, load accumulator 3; that means, to the accumulator you load the data which is present in memory location 3 that is 20.

In LOAD DIRECT mode, the instruction specifies the exact memory location (3) from which data should be loaded. Here, the value stored at memory location 3 is 20, which gets loaded into the accumulator.

Think of it like a library, where you have an exact reference to a book. When you go to shelf 3 (memory location 3), you can directly pick up the book titled '20' without any confusion or searching.

LOAD INDIRECT 5 means the accumulator will be loaded with something which is present in memory location 5. Memory location 5 has value 1 now it’s an indirect.

In LOAD INDIRECT mode, the instruction points to a location (5) that contains an address (1). The real data is at that address (memory location 1), which contains the value 49. Hence, the accumulator gets loaded with 49 after following this indirection.

Imagine you get a letter (LOAD INDIRECT 5) telling you to find another letter at a particular desk (memory location 5). That desk only has a note leading you to the final letter which has the hidden message (49) that you need.

LOAD REGISTER DIRECT 2 means you load to the accumulator what is available in register number 2. So, register 2 has the value 3, that value will be loaded.

With LOAD REGISTER DIRECT, the instruction indicates that data from a specific register (2) should be loaded into the accumulator. Here, the value stored in register 2 (3) is directly brought into the accumulator.

Think of registers as drawers in a filing cabinet. When you ask for documents from drawer 2 (REGISTER 2), you simply pull out whatever is inside that drawer (the value 3) and use it directly.

Displacement addressing has 2 address fields; one explicitly refers to a memory address and the other to a register. These two components are added together to get the effective address.

Displacement addressing uses two fields: one specifying a direct memory address and another specifying a register's content. When the values in both fields are combined, they yield the effective address where the actual data can be found, providing flexibility and efficiency in how data is accessed.

Imagine you have a treasure map with a starting point (memory address) and a clue about how many steps to take (the register's value) before you find the treasure (the data). Adding these together allows you to pinpoint exactly where the treasure is buried.

The register actually contains the PC. If this register is involved, we say it’s relative addressing. The effective location is obtained by adding the content of the PC to the displacement value.

Relative addressing modes reference memory locations based on the current position of the program counter (PC). By adding a displacement value to the PC's current location, the effective address for the next instruction is determined, allowing for jumps and branches in program execution.

Imagine you're on a train (the PC) that moves along a track. If you know a signal (the displacement) is set to allow you to stop 5 stations ahead, you simply add that to your current station to reach your intended destination.

Base register addressing uses a register containing a main memory address and an address field that contains a displacement. Indexed addressing uses a general-purpose register as the index.

Base register addressing refers to using a particular register to hold a starting address, and a displacement offsets that address for data retrieval. Indexed addressing functions similarly but uses a general-purpose register, often for array or iterative access, allowing efficient looping through data.

Picture a GPS system. In this case, the base register is like your home address—it gives you a starting point. The displacement can be your planned detour. Indexed addressing is like a map index that helps you navigate quickly to various points in an area without getting lost.

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