1 accumulator, 2 index register, Accumulator – Freescale Semiconductor MC68HC908MR32 User Manual
Page 80: Index register
Central Processor Unit (CPU)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
80
Freescale Semiconductor
Figure 7-1. CPU Registers
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.
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 7
6
5
4
3
2
1
Bit 0
Read:
Write:
Reset:
Unaffected by reset
Figure 7-2. Accumulator (A)
Bit
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Bit
0
Read:
Write:
Reset:
0
0
0
0
0
0
0
0
X
X
X
X
X
X
X
X
X = Indeterminate
Figure 7-3. Index Register (H:X)
ACCUMULATOR (A)
INDEX REGISTER (H:X)
STACK POINTER (SP)
PROGRAM COUNTER (PC)
CONDITION CODE REGISTER (CCR)
CARRY/BORROW FLAG
ZERO FLAG
NEGATIVE FLAG
INTERRUPT MASK
HALF-CARRY FLAG
TWO’S COMPLEMENT OVERFLOW FLAG
V 1 1 H
I
N Z C
H
X
0
0
0
0
7
15
15
15
7
0