Basic Addressing Modes Information

We have load and store instructions to get data to and from memory. To make certain types of programming easier, such as accessing an array of data, the HC11 supports several different addressing modes, or ways of accessing memory locations.

Inherent: The locations of data are implied by the instruction name

Example: ABA, which adds register B to A, implies the registers in the instruction name
Note: no operands are needed to indicate memory to work with or constant value to load.

Immediate: The actual data is located in memory immediately following the instruction's opcode

Format: the instruction name is followed by a constant value or label with a # sign in front.
Example: LDAA #22 generates two bytes of machine code $86 $16, where the second byte is equivalent to decimal 22, the constant to be loaded into the A register.

Extended: The address of the data is located in memory immediately following the instruction's opcode

Format: the instruction name is followed by a label that is in the assembly program somewhere, or is followed by a constant value, without a # sign.
Example: LDAA param2, where param2 is a label in the program
Example: LDAA $ff32, which means load the byte at $ff32 into the A register

Direct: A one-byte address of the data is located immediately following the opcode. This is the low-order byte of the address. The high order byte is assumed to be $00

Format, same as extended, except that labels and addresses must begin with RAM (High byte is always $00)
Example, LDAA $43, which means load the byte at $0043 into the A register
This is somewhat redundant, given that Extended addressing can do the same thing, but it is more efficient because only one byte of addressing is fetched
Remember: our RAM is all located in the "direct" area!

Relative: The value in the operand is computed from the relationship between where you are currently in the code in memory to where you are going to, the label.

Format: the instruction name followed by a 8 bit value that indicates the distance to move (add to PC) from where currently in code.
Example: BEQ 05 branches 6 memory locations forward from the memory location where the value $05 is located (adds $05 to the current PC value)
Example: BRA fe branches 1 memory locations backwards from the memory location where the value $fe is located (adds $fe, subtracts 2, to the current PC value)
The relative address value is in two's complement form so it can only indicate a change of -128 memory locations backwards and +127 memory locations forwards.

Indexed (not responsible for yet): The value in an index register (X or Y) is added to an offset value, which provides the final effective address of the data.

Format: the instruction name followed by a value,register pair
Example: LDAA var,X loads the byte found at address (X+var) into register A
The offset is a positive 1-byte value, so it is limited to values from 0 to 255

Ok, we've concentrated on load/store instructions, but the HC11 actually kind-of cheats. It allows memory accesses on most any form of instruction. So, the addressing modes apply to many, many instructions.