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CHAPTER 3 CPU
3.2.2
Accumulator (A)
The accumulator (A) consists of two 16-bit length operation registers (AH and AL) used
for temporary storage of the operation result or data.
The accumulator can be used as a 32-, 16-, or 8-bit register to perform various
operations between the AH and AL registers and memory or other registers.
I Accumulator (A)
G
Data transfer to accumulator
The accumulator can process 32-bit data (long word), 16-bit data (word), and 8-bit data (byte).
• When processing 32-bit data, the AH register and the AL register are concatenated and used.
• When processing 16- or 8-bit data, only the AL register is used.
Data retention function
When data of word length or less is transferred to the AL register, data stored in the AL register is
transferred automatically to the AH register.
Code-extended function and zero-extended function
When transferring data of byte length or less to the AL register, the data is code-extended (MOVX
instruction) or zero-extended (MOV instruction) to be the 16-bit length and stored in the AL register. Data
in the AL register can also be treated in word and byte lengths.
Figure 3.2-3 "Data transfer to accumulator" shows data transfer to the accumulator and a concrete example.
32-bit data transfer
16-bit data transfer
8-bit data transfer
34
Figure 3.2-3 Data transfer to accumulator
32-bit
AH
Data transfer Data transfer
AH
Data saving
AH
Data saving
00
or FF
H
* 0 extension or sign extension
AL
AL
Data transfer
AL
Data transfer
H
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