Central Processor Unit (CPU)
Addressing Modes
MC68HC11E Family
—
Rev. 4
Technical Data
MOTOROLA
Central Processor Unit (CPU)
55
3.6.2 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.
3.6.3 Extended
In the extended addressing mode, the effective address of the argument
is contained in two bytes following the opcode byte. These are 3-byte
instructions (or 4-byte instructions if a prebyte is required). One or two
bytes are needed for the opcode and two for the effective address.
3.6.4 Indexed
In the indexed addressing mode, an 8-bit unsigned offset contained in
the instruction is added to the value contained in an index register (IX or
IY). The sum is the effective address. This addressing mode allows
referencing any memory location in the 64-Kbyte address space. These
are 2- to 5-byte instructions, depending on whether or not a prebyte is
required.
3.6.5 Inherent
In the inherent addressing mode, all the information necessary to
execute the instruction is contained in the opcode. Operations that use
only the index registers or accumulators, as well as control instructions
with no arguments, are included in this addressing mode. These are
1- or 2-byte instructions.