MC68HC912BL16 Datasheet, PDF (6/128 Pages) Freescale Semiconductor, Inc – 16-Bit Microcontroller

Freescale Semiconductor, Inc.
2 Central Processing Unit
The CPU12 is a high-speed, 16-bit processing unit. It has full 16-bit data paths and wider internal reg-
isters (up to 20 bits) for high-speed extended math instructions. The instruction set is a proper superset
of the M68HC11instruction set. The CPU12 allows instructions with odd byte counts, including many
single-byte instructions. This provides efficient use of ROM space. An instruction queue buffers pro-
gram information so the CPU always has immediate access to at least three bytes of machine code at
the start of every instruction. The CPU12 also offers an extensive set of indexed addressing capabilities.
2.1 Programming Model
CPU12 registers are an integral part of the CPU and are not addressed as if they were memory loca-
tions.
7
A
07
B
15
D
0 8-BIT ACCUMULATORSA&B
OR
0 16-BIT DOUBLE ACCUMULATORD
15
IX
0 INDEX REGISTER X
15
IY
0 INDEX REGISTERY
15
SP
0 STACKPOINTER
15
PC
0 PROGRAMCOUNTER
S X H I N Z V C CONDITION CODE REGISTER
Figure 2 Programming Model
HC12 PROG MODEL
Accumulators A and B are general-purpose 8-bit accumulators used to hold operands and results of
arithmetic calculations or data manipulations. Some instructions treat the combination of these two 8-
bit accumulators as a 16-bit double accumulator (accumulator D).
Index registers X and Y are used for indexed addressing mode. In the indexed addressing mode, the
contents of a 16-bit index register are added to 5-bit, 9-bit, or 16-bit constants or the content of an ac-
cumulator to form the effective address of the operand to be used in the instruction.
Stack pointer (SP) points to the last stack location used. The CPU12 supports an automatic program
stack that is used to save system context during subroutine calls and interrupts, and can also be used
for temporary storage of data. The stack pointer can also be used in all indexed addressing modes.
Program counter is a 16-bit register that holds the address of the next instruction to be executed. The
program counter can be used in all indexed addressing modes except auto-increment/decrement.
Condition Code Register (CCR) contains five status indicators, two interrupt masking bits, and a
STOP disable bit. The five flags are half carry (H), negative (N), zero (Z), overflow (V), and carry/borrow
(C). The half-carry flag is used only for BCD arithmetic operations. The N, Z, V, and C status bits allow
for branching based on the results of a previous operation.
MC68HC912BL16
6
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