MC 6811 and 6812 Processor Families

There are five (SIX) things to specify for an architecture:

Data Representation

• 8 bit signed integer
• 8 bit unsigned integer
• 16 bit signed integer
• 16 bit unsigned integer

Data Storage

Processor Block Diagram

• General and Special Purpose Registers
  • Two 8 bit accumulators, A and B (which can also be accessed as one 16 bit register, D.
  • One 8 bit Condition Code register, CC
  • Two 16 bit index registers, X and Y
  • One 16 bit Stack Pointer, SP
  • One 16 bit Program Counter, PC
• Memory Map
  

  MC68HC711E9	MC68HC812A4
  $0000 - $01FF RAM $1000 - $103F I/O $B600 - $B7FF EAPROM $F000 - $FFFF EPROM	$0000 - $01FF I/O $0800 - $0BFF RAM $F000 - $FFFF EPROM

Data Access

called "addressing mode"

• Inherent - opcode specifies register affected
  • clra
• Immediate - instruction contains data value
  • ldaa #$24
  • ldab #36
• Direct Page - instruction contains 8-bit address of data in address space
  • ldaa $24
  • ldab 36
• Extended Addressing - instruction contains 16-bit address of data in memory
  • ldaa $0801
• PC Relative - instruction contains 8-bit signed offset from PC for branch target.
  • loop: bra loop
  • 20 FE $F000: bra $F000
• Indexed Addressing (6811) - instruction contains 8-bit unsigned offset from either X or Y registers (Y is slower).
  • ldaa 4,X
• Additional Addressing Modes(6812)
  • Indexed - EA is calculated as a fixed signed offset from X, Y, SP or PC. (Y is no longer slow). Offset may be 5, 9 or 16 bit signed values.
    • ldaa -4, Y
    • ldaa $40, X
    • ldaa $200, SP
  • Auto Pre/Post Increment/Decrement Indexed - EA is calculated as a fixed signed offset from X, Y or SP (not PC). Maybe be incremented/decremented by values from 1 to 8.
    • ldaa 1,Y+
    • ldaa 4,-X
  • Accumulator Offset Indexed EA is calculated using A, B or D (offset) added to X, Y, SP or PC (index). Accumulator and Index registers unchanged.
    • ldaa B,Y
  • Indexed Indirect - EA loaded from memory (big endian) at the address calculated by adding a 16 bit signed offset to an index register (X, Y, SP or PC).
    • ldaa [-4,Y]
  • Accumulator D Offset Indexed Indirect - EA loaded from memory (big endian) at the address calculated by adding the 16 bit signed value in D to to an index register (X, Y, SP or PC).
    • ldaa [D,X]

Instruction Set

Instruction Encoding

Instructions are encoded as one (or two) byte Opcode followed by 0, 1 or 2 bytes of operand/operand specifier.

I/O Ports

As an example, here is the HC11A8 Data Sheet