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Central Processor Unit (CPU)
15
15
15
7.3.1 Accumulator
The accumulator is a general-purpose 8-bit register. The CPU uses the accumulator to hold operands and
the results of arithmetic/logic operations.
Read:
Write:
Reset:
7.3.2 Index Register
The 16-bit index register allows indexed addressing of a 64-Kbyte memory space. H is the upper byte of
the index register, and X is the lower byte. H:X is the concatenated 16-bit index register.
In the indexed addressing modes, the CPU uses the contents of the index register to determine the
conditional address of the operand.
The index register can serve also as a temporary data storage location.
Bit
15
Read:
Write:
Reset:
0
X = Indeterminate
80
7
H
7
V 1 1 H I N Z C
Figure 7-1. CPU Registers
Bit 7
6
5
Figure 7-2. Accumulator (A)
14
13
12
11
10
0
0
0
0
0
Figure 7-3. Index Register (H:X)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
0
ACCUMULATOR (A)
0
X
INDEX REGISTER (H:X)
0
STACK POINTER (SP)
0
PROGRAM COUNTER (PC)
0
CONDITION CODE REGISTER (CCR)
CARRY/BORROW FLAG
ZERO FLAG
NEGATIVE FLAG
INTERRUPT MASK
HALF-CARRY FLAG
TWO'S COMPLEMENT OVERFLOW FLAG
4
3
2
Unaffected by reset
9
8
7
6
5
0
0
X
X
X
1
Bit 0
Bit
4
3
2
1
0
X
X
X
X
X
Freescale Semiconductor
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