ECE 265 – Lecture 4

1. ECE265 ECE 265 – Lecture 4 The M68HC11 Address Modes

2. 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 68HC11 reference manual. ECE265

3. Special Considerations • To start, look at the programmers model of the architecture. What registers are available? ECE265

4. Special Considerations • To start, look at the programmers model of the architecture. What registers are available? • Registers in the CPU ECE265

5. 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. ECE265

6. Immediate Addressing (IMM) • In immediate addressing the instruction itself contains the data to be loaded into the destination. • Consider the instruction • LDAA #15 This instruction will load $0F into Accumulator A • In memory it will look like: (op code of LDAA is $86) the data immediately follows the instruction in memory. ECE265

7. 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 ECE265

8. 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 ECE265

9. Example of Extended addressing • Machine code and effect ECE265

10. Ended here on Wed W2, Class 4 ECE265

11. 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) ECE265

12. Inherent (INH) addressing mode • In this addressing mode all the information required for execution is contained in the instruction. • No other operand is required. • Examples: • Increment an Accumulator (either A or B) • Accumulator A+Accumulator B  Accumulator A ECE265

13. 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. ECE265

14. BCC example of relative (REL) ECE265

15. Indexed Addressing Mode • There are two index address registers, X and Y, providing two indexed addressing modes, INDX and INDY. • The value in the indexed register is added to an offset contained in the instruction to obtain the effective address of the operand. • This is best seen by an example ECE265

16. Indexed Mode example ECE265

17. Lecture summary • Have covered • The addressing Modes of the 68HC11 • 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. ECE265

18. Assignment • Read Chapter 3.1 through 3.6 • Problems – refer to web page • 2.4 • 2.6 • 2.19 • 2.21 ECE265