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CHAPTER 2 CPU
2.7.1 Accumulator (A)
The accumulator (A) register consists of 2 16-bit arithmetic operation registers (AH and
AL), and is used as a temporary storage for operation results and transfer data.
■ Accumulator (A)
During 32-bit data processing, AH and AL are used together. 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 2.7-3 and Figure
2.7-4 ). The data stored in the A register can be operated upon with the data in memory or registers (Ri,
Rwi, or RLi). In the same manner as with the F
2
MC-8L, when a word or shorter data item is 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 high-order eight bits of AL
before operation are ignored. The high-order eight bits of the operation result all become zeroes.
The A register is not initialized by a reset. The A register holds an undefined value immediately after a
reset.
Figure 2.7-3 32-bit Data Transfer
Figure 2.7-4 AL-AH Transfer
XXXXH XXXXH
8F74H 2B52H
DTB
A6
H
6
RW1
A61540
H
A6153EH
MSB LSB
8F
H
2BH
15H
74H
52H
38H
AH AL
MOVL A,@RW1+6
+
A before
execution
A after
execution
+6
RW1
A61540
H
A6153EH
XXXXH
1234H
1234H 1234H
DTB
A6
H
8FH
2BH
15H
74H
52H
38H
MSB
LSB
MOVW A,@RW1+6
A before
execution
A after
execution