Designing Embedded Hardware, 2nd Edition

Addressing Modes

The different ways in which an instruction can reference a register or memory location are known as the addressing modes of the processor. The types of addressing modes available within different architectures vary. To illustrate some of the more common modes, let's take a look at those available in the 68HC11 architecture:

Inherent

The instruction deals purely with registers. CLRB clears the B accumulator, for example.

Immediate/Literal

The instruction has a literal number as an operand.

Direct

The instruction accesses a memory location, specified by a short address. In other words, direct addressing provides access to a subset of the total address space. On a given processor with a 16-bit address bus, a direct access may, for example, specify an address within the first 256 bytes. On a 32-bit processor, a direct access may specify an address within the first 64K of memory, for example. Direct addressing is used (when possible) to reduce the length of instruction-referencing memory. This can reduce code size, and therefore instruction fetch time, in time-critical applications. Many processors, especially RISC, do not use direct addressing.

Extended

The instruction accesses a memory location, specified by the full address. So LDAA $B098 means load accumulator A with the contents of address 0xB098 in memory.

Indexed

The instruction uses the contents of a register as a pointer into memory. If, for example, the X register is equal to 0x5034, then LDAA 0,X means "load ...