MC68HC711D3_05 Datasheet, PDF (38/138 Pages) Freescale Semiconductor, Inc – Microcontrollers

Central Processor Unit (CPU)
A byte is eight bits wide and can be accessed at any byte location. A word is composed of two consecutive
bytes with the most significant byte at the lower value address. Because the M68HC11 is an 8-bit CPU,
there are no special requirements for alignment of instructions or operands.
3.4 Opcodes and Operands
The M68HC11 Family of microcontrollers uses 8-bit opcodes. Each opcode identifies a particular
instruction and associated addressing mode to the CPU. Several opcodes are required to provide each
instruction with a range of addressing capabilities. Only 256 opcodes would be available if the range of
values were restricted to the number able to be expressed in 8-bit binary numbers.
A 4-page opcode map has been implemented to expand the number of instructions. An additional byte,
called a prebyte, directs the processor from page 0 of the opcode map to one of the other three pages.
As its name implies, the additional byte precedes the opcode.
A complete instruction consists of a prebyte, if any, an opcode, and zero, one, two, or three operands.
The operands contain information the CPU needs for executing the instruction. Complete instructions can
be from one to five bytes long.
3.5 Addressing Modes
Six addressing modes can be used to access memory:
1. Immediate
2. Direct
3. Extended
4. Indexed
5. Inherent
6. Relative
These modes are detailed in the following paragraphs. All modes except inherent mode use an effective
address. The effective address is the memory address from which the argument is fetched or stored or
the address from which execution is to proceed. The effective address can be specified within an
instruction, or it can be calculated.
3.5.1 Immediate
In the immediate addressing mode, an argument is contained in the byte(s) immediately following the
opcode. The number of bytes following the opcode matches the size of the register or memory location
being operated on. There are 2-, 3-, and 4- (if prebyte is required) byte immediate instructions. The
effective address is the address of the byte following the instruction.
3.5.2 Direct
In the direct addressing mode, the low-order byte of the operand address is contained in a single byte
following the opcode, and the high-order byte of the address is assumed to be $00. Addresses $00–$FF
are thus accessed directly, using 2-byte instructions. Execution time is reduced by eliminating the
additional memory access required for the high-order address byte. In most applications, this
256-byte area is reserved for frequently referenced data. In M68HC11 MCUs, the memory map can be
configured for combinations of internal registers, RAM, or external memory to occupy these addresses.
MC68HC711D3 Data Sheet, Rev. 2.1
38
Freescale Semiconductor