Addressing Modes

1
Addressing Modes

• In order to provide programmers with flexibility
  and optimizations, microcontrollers offer a
  variety of addressing modes.
• The HC11 offers six
• Inherent
• Immediate
• Direct
• Extended
• Indexed X or Indexed Y
• Relative

2
Definitions and Observations

• Operation The abstract task the
  microcontroller is trying to accomplish
• Opcode The machine code instruction
• Operand Additional arguments for the operation
• Many operation have more than one opcode. Why?
• To select what addressing mode will be used
• Example
• LDAA immediate ? 0x86
• LDAA direct ? 0x96
• Most opcodes are 1 byte. Some are two. Why?
• HC11 supports 308 unique operations/addressing
  modes combinations.
• Some operations will have a pre-opcode
• Example
• LDAA ind,y ? 0x18 0xA6
• Depending on the operation and addressing mode, a
  given opcode may fetch 0, 1, or 2 additional
  bytes from memory. What are the implications?
• Some operations use more memory
• Some operations take longer to complete
• Is there a pattern to the operation / opcode
  assignments?

3
Inherent

• Not really an addressing mode
• Applies to operations that do not require an
  address or argument (operand) to operate
• Examples
• INX increment index register X
• INY increment index register Y
• DECA decrement accumulator A

4
Immediate

• Operation (opcode) is followed by immediately by
  target of operation.
• Definition of target varies by operation.
• Example Load the next byte of memory into
  accumulator A.
• logic M ? A
• opcode 0x86
• operand M the next byte in memory after the
  opcode
• results 0x86 0x55, will cause 0x55 to be loaded
  into accumulator A.
• Some operations may need two bytes of data.
• LDD loads double accumulator with next two bytes
  of memory.

5
Reflect

• Why do we need more addressing modes (let along
  six)?
• Where is the 0x55 coming from in this case?
• A fixed location
• Likely in ROM
• How can we load a variable (RAM) location into
  the accumulator?
• We need the equivalent of pointers (from C)
• instead of A M, we need A M, or load
  whatever value is in the memory location M is
  referring to.
• Direct and Extended Addressing Modes
• Operand becomes a pointer to, or the address
  of, the value to be loaded into the accumulator.

6
Direct

• Operand of opcode is the 8-bit direct address
  from memory.
• But HC11 has 16-bit address bus.
• High address byte assumed
• 0x0000 0x00FF (Page 0)
• Example
• Load whatever value is in address 0x0045 into the
  accumulator
• 0x96 0x45

7
Extended

• Two bytes are loaded which form a 16-bit address
• Example Load whatever is in address 0x0003 into
  accumulator A.
• 0xB6 0x00 0x03

8
Reflect

• Why are there two versions that accomplish the
  exact same thing?
• Performance
• Page 0 RAM becomes a more efficient location to
  use for frequently used variables
• Note, for LDAA
• Direct version operates in 3 clocks, and consumes
  2 bytes
• Extended version operates in four clocks and
  consumes 3 bytes.
• 25 - 33 improvement
• Page 0 RAM is a constrained resource
• 256 bytes
• Some may be used for other purposes.
• Use it wisely

9
Indexed

• Similar to direct and extended addressing
• Use index register X or Y to form base address
• Load an 8-bit (unsigned) offset from memory to
  add to base to get 16-bit address
• Advantages
• 16 bit address anywhere in memory
• Only have to load 8-bit offset better
  performance
• Example
• Load contents of 0x1403 into accumulator A, index
  register X contains 0x1400.
• 0xA6 0x03
• Index Y operation have a pre-opcode
• 0x18 0xA6 0x03
• Avoid if possible
• Index registers can be easily incremented and
  decremented. Fewer memory accesses to iterate
  memory.

10
Relative

• Similar to indexed addressing
• Only used for branches
• Use current program counter (address following
  location of operand) to form base address
• Load an 8-bit (signed) offset from memory to add
  to base to get 16-bit address
• Limitations
• Can only jump ahead 127 locations, or back 128
• Example Loop forever, use BRA (Branch Always)
• 0x20 0xFE
• What if we want to branch further than Relative
  addressing allows?

11
Next Time

• Demo, Instruction Set