Addressing Modes (Microcontrollers)

In the past two topics, we have introduced the instruction cycle and the instruction set. We have used a few addressing modes in order to understand those ideas. However, we did not attempt to convey the principles of addressing modes. We now complete our understanding of the instruction by describing the addressing modes used in the 6812.
Recall from topic 1 that an instruction generally consists of an operation with one address in memory for an operand and/or result. How that address is determined is called addressing, and the different ways that the address is determined are called addressing modes. The data are accessed in a program, that is, read or written, by an instruction with the addressing modes available in the computer. These modes correspond to the data structures that can be easily managed in that computer. If you want to handle a particular structure, such as a string of characters, an addressing mode such as postincrement is very useful, as we discuss in more detail in topic 9. This topic introduces the 6812\s addressing modes, which provide the tools that make handling the most useful data structures so easy on this machine. Learning the rich set of addressing modes here will also make it easier later to learn about the common data structures.
This topic introduces the following general aspects of addressing. We first discuss addressing modes that are determined by bits in the operation code byte, which is generally the first byte of the instruction. Indexing modes use a post byte and are discussed next. Relative modes are then discussed to show the important concept of position independence. We give examples that rework the addition program of topic 1 to illustrate data structure ideas and position independence using these addressing modes. Finally, we consider some architectural thoughts about addressing such as multiple address instructions and the effective address computation in the fetch execute cycle. We also discuss the level of addressing that indicates how many times an address must be read from memory to get the actual or effective address of the operand or result used with the instruction.
Upon completion of this topic, you should be able to use the addressing modes described here with any instruction that has been introduced in topic 2. You should be able to determine what has been done to compute the effective address, what that effective address will be, and what side effects are generated where some modes are used. This will prepare you to use good data structures in your programs and thus to write shorter, faster, and clearer programs as you progress through this material.