Central Processor Unit (CPU)
7
15
H
15
15
X
0
STACK POINTER (SP)
0
PROGRAM COUNTER (PC)
7
0
V 1 1 H I N Z C
CONDITION CODE REGISTER (CCR)
0
ACCUMULATOR (A)
0
INDEX REGISTER (H:X)
CARRY/BORROW FLAG
ZERO FLAG
NEGATIVE FLAG
INTERRUPT MASK
HALF-CARRY FLAG
TWO’S COMPLEMENT OVERFLOW FLAG
Figure 6-1. CPU Registers
6.3.1 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.
Bit 7
Read:
Write:
Reset:
Unaffected by reset
6
5
4
3
2
1
Bit 0
Figure 6-2. Accumulator (A)
6.3.2 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
0
0
0
0
0
0
0
X
X
X
X
X
X
X
X
X = Indeterminate
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Bit
0
Figure 6-3. Index Register (H:X)
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 5
68
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
