Understanding Addressing Modes in the 68HC11 Microcontroller, Slides of Microprocessor and Interfacing

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Address Modes

Lecture Overview

• The M68HC11 Addressing Modes
  • Special Consideration
  • Details of the various Addressing modes
    • (Note: And this is a very simple architecture)
• Material from Chapter 2 plus a 68HC reference manual.

Special Considerations

• To start, look at the programmers model of the architecture. What registers are available?
• Registers in the CPU

Special Consideration

• Consider that there is an Index Register and a Stack Pointer. - This indicates that these register will allow for more than simple load and store data transfers.
• Will now examine the modes of data transfer permitted.
• The 68HC11 architecture support addressing modes that allow the basis to understand the addressing modes on any architecture.

Some examples from text

• Load Immediate
  • LDAA #10 Loading a decimal value
  • Loads the binary for 10, i.e.,
  • a value of $0A into accumulator A
  • LDAA #$1C Loads the hexadecimal value $1C in A
  • LDAA #@03 Loads the octal value 3 into A
  • LDAA #%11101100 Loads a binary value
  • LDAA #’C’ Loads the ASCII code for the letter C

Extended Addressing Mode (EXT)

• This addressing mode introduces the concept of the effective address of an operand.
• The effective address of an operand is the address in memory of the operand and is usually a calculated value.
• This mode also introduces the use of an instruction prebyte in the machine code of the 68HC11. - Instructions that require a prebyte take 4 bytes of memory. Prebytes are either $18, $1A, or $CD

• • Ended here on Wed W2, Class

Direct Addressing (DIR)

• In direct addressing the least significant byte of the 16-bit address of the operand is in the instruction.
• The high order byte is taken to be $00. This is how you access the 256 bytes of RAM. (could also use extended)

Relative Addressing Mode (REL)

• Relative addressing is much like it sounds. The address is relative to something else.
• In the case of the 68HC11 relative addressing mode is used only for branch instructions.
• It is a 2 byte instruction with the second byte being the offset (-128 to +127) to take if the condition is TRUE.
• When the condition is not met, execution continues with the next instruction.

BCC example of relative (REL)

Indexed Mode example

Lecture summary

• Have covered
  • The addressing Modes of the 68HC
  • What the modes are and how they provide access to the operand of the instruction
  • What an effective address is.
• Knowledge base
  • What are the addressing modes
  • Where the operand (target data) for each operation comes from and where the result is stored.