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CHAPTER 3 CPU
3.7.1
Accumulator (A)
The accumulator (A) consists of two 16-bit arithmetic operation registers (AH and AL).
The accumulator is used to temporarily store the results of an arithmetic operation and
data.
The A register can be used as a 32-bit, 16-bit or 8-bit register. Various arithmetic
operations can be performed between memory and other registers or between the AH
register and the AL register. The A register has a data retention function that
automatically transfers pre-transfer data from the AL register to the AH register when
data of word length or less is transferred to the AL register. (Data is not retained with
some instructions.)
■ Accumulator (A)
●
Data transfer to the accumulator
The accumulator can process 32-bit (long word), 16-bit (word) and 8-bit (byte) data. The 4-bit data transfer
instruction (MOVN) is an exception. The explanation of 8-bit data also applies to 4-bit data.
• For 32-bit data processing, the AH register and AL register are combined.
• For 16-bit data and 8-bit data, only the AL register is used.
• When data of byte length or less is transferred to the AL register, data becomes 16 bits long by sign
extension or zero extension, and is stored in the AL register. Data in the AL register can be handled as
word-length or byte-length data.
Figure 3.7-2 shows data transfer to the accumulator. Figure 3.7-3 to Figure 3.7-6 show specific transfer
examples.
42
Figure 3.7-2 Data Transfer to the Accumulator
32-bit data transfer
16-bit data transfer
8-bit data transfer
* Becomes 000
or FFF
for a 4-bit transfer instruction.
H
H
32-bit
Data transfer
Data transfer
Data save
Data transfer
Data save
Data transfer
00
or FF
*
H
H
(Zero extension or sign extension)
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