MC68HC711xxxx Motorola, MC68HC711xxxx Datasheet - Page 74

• Manufacturer Part Number: MC68HC711xxxx
• Description: Microcontrollers
• Manufacturer: Motorola

Central Processor Unit (CPU)
4.2.1 Accumulators A, B, and D
Data Sheet
74
15
7
Accumulators A and B are general-purpose 8-bit registers that hold operands and
results of arithmetic calculations or data manipulations. For some instructions,
these two accumulators are treated as a single double-byte (16-bit) accumulator
called accumulator D. Although most instructions can use accumulators A or B
interchangeably, these exceptions apply:
A
•
•
•
•
The ABX and ABY instructions add the contents of 8-bit accumulator B to
the contents of 16-bit register X or Y, but there are no equivalent instructions
that use A instead of B.
The TAP and TPA instructions transfer data from accumulator A to the
condition code register or from the condition code register to accumulator A.
However, there are no equivalent instructions that use B rather than A.
The decimal adjust accumulator A (DAA) instruction is used after
binary-coded decimal (BCD) arithmetic operations, but there is no
equivalent BCD instruction to adjust accumulator B.
The add, subtract, and compare instructions associated with both A and B
(ABA, SBA, and CBA) only operate in one direction, making it important to
plan ahead to ensure that the correct operand is in the correct accumulator.
Figure 4-1. Programming Model
0
SP
PC
D
IX
IY
7
7
Central Processor Unit (CPU)
S
X
H
I
B
N
Z
V
C
0
0
0
8-BIT ACCUMULATORS A & B
OR 16-BIT DOUBLE ACCUMULATOR D
INDEX REGISTER X
INDEX REGISTER Y
STACK POINTER
PROGRAM COUNTER
CONDITION CODES
CARRY/BORROW FROM MSB
OVERFLOW
ZERO
NEGATIVE
I-INTERRUPT MASK
HALF CARRY (FROM BIT 3)
X-INTERRUPT MASK
STOP DISABLE
M68HC11E Family — Rev. 5
MOTOROLA