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MB90335 Series
2.6.1
Accumulator (A)
The accumulator (A) register consists of 2 × 16-bit arithmetic operation registers (AH
and AL), and is used as a temporary register for operation results and transfer data.
■ Accumulator (A)
During 32-bit data processing, AH and AL are used together (see Figure 2.6-3). Only AL is used for word
processing in 16-bit data processing mode or for byte processing in 8-bit data processing mode (see Figure
2.6-4). The data stored in the accumulator (A) register can be operated upon with the data in memory or
registers (Ri, RWi, and RLi). In the same manner as with the F
transferred to AL, the previous data item in AL is automatically sent to AH (data preservation function).
The data preservation function and operation between AL and AH help improve processing efficiency.
When a byte or shorter data item is transferred to AL, the data is sign-extended or zero-extended and stored
as a 16-bit data item in AL. The data in AL can be handled either as word or byte long. When a byte-
processing arithmetic operation instruction is executed on AL, the upper 8 bits of AL before operation are
ignored. The upper 8 bits of the operation result all become "0". The A register is not initialized by a reset
and holds an undefined value right after the reset.
MOVL A,@RW1+6
A before
execution
A after
execution
A before
execution
A after
execution
CM44-10137-6E
Figure 2.6-3 An Example of 32-bit Data Transfer
XXXX
XXXX
H
H
A6
DTB
H
8F74
2B52
H
H
AH
AL
Figure 2.6-4 An Example of AL to AH Transfer
MOVW A,@RW1+6
XXXX
1234
H
H
A6
DTB
H
1234
2B52
H
H
FUJITSU MICROELECTRONICS LIMITED
2
MC-8L, when a word or shorter data item is
MSB
A61540
H
8F
A6153E
2B
H
+
6
RW1
15
MSB
A61540
8F
H
2B
A6153E
H
+6
15
RW1
CHAPTER 2 CPU
2.6 Registers
LSB
74
H
H
52
H
H
38
H
H
LSB
74
H
H
52
H
H
38
H
H
31
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