Freescale Semiconductor, Inc.
Central Processing Unit
A
B
7
0 7
0
0
8-BIT ACCUMULATORS A & B
OR
16-BIT DOUBLE ACCUMULATOR D
15
D
15
15
15
15
IX
IY
0
0
0
0
INDEX REGISTER X
INDEX REGISTER Y
STACK POINTER
SP
PC
PROGRAM COUNTER
CONDITION CODE REGISTER
S X H I N Z V C
Figure 2-1. Programming 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 accumulator
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 autoincrement/decrement.
Technical Data
MC68HC912DG128 — Rev 3.0
Central Processing Unit
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