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This manual describes the features and operation of the CPU12 processing unit used
in all M68HC12 microcontrollers.
1.1 CPU12 Features
The CPU12 is a high-speed, 16-bit processing unit that has a programming model
identical to that of the industry standard M68HC11 CPU. The CPU12 instruction set is
a proper superset of the M68HC11 instruction set, and M68HC11 source code is ac-
cepted by CPU12 assemblers with no changes.
The CPU12 has full 16-bit data paths and can perform arithmetic operations up to 20
bits wide for high-speed math execution.
Unlike many other 16-bit CPUs, the CPU12 allows instructions with odd byte counts,
including many single-byte instructions. This allows much more efficient use of ROM
space.
An instruction queue buffers program information so the CPU has immediate access
to at least three bytes of machine code at the start of every instruction.
In addition to the addressing modes found in other Motorola MCUs, the CPU12 offers
an extensive set of indexed addressing capabilities including:
•
Using the stack pointer as an index register in all indexed operations
•
Using the program counter as an index register in all but auto inc/dec mode
•
Accumulator offsets allowed using A, B, or D accumulators
•
Automatic pre- or post-increment or pre- or post-decrement (by
•
5-bit, 9-bit, or 16-bit signed constant offsets
•
16-bit offset indexed-indirect and accumulator D offset indexed-indirect ad-
dressing
1.2 Readership
This manual is written for professionals and students in electronic design and software
development. The primary goal is to provide information necessary to implement con-
trol systems using M68HC12 devices. Basic knowledge of electronics, microproces-
sors, and assembly language programming is required to use the manual effectively.
Because the CPU12 has a great deal of commonality with the M68HC11 CPU, prior
knowledge of M68HC11 devices is helpful, but is not essential. The CPU12 also in-
cludes features that are new and unique. In these cases, there is supplementary ma-
terial in the text to explain the new technology.
CPU12
REFERENCE MANUAL
SECTION 1
INTRODUCTION
INTRODUCTION
8 to +8)
–
MOTOROLA
1-1
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