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7
CHAPTER 2 MEMORY ARCHITECTURE
2.1.1 Direct Address Area
The lower portion of the address space is used for the direct address area. Instructions
that specify direct addresses allow you to access this area without the use of general-
purpose registers, using only the operand information in the instruction itself. The size
of the address area that can be specified by direct addressing varies according to the
length of the data being transferred.
■ Direct Address Area
The size of the address area that can be specified by direct addressing varies according to the length of the
data being transferred, as follows:
• Transfer of byte data: 0000 0000
H
to 0000 00FF
H
• Transfer of half-word data: 0000 0000
H
to 0000 01FF
H
• Transfer of word data: 0000 0000
H
to 0000 03FF
H
■ Use of Operand Information Contained in Instructions
The 8-bit address information contained in the instruction has the following significance.
• In byte data: Value represents the lower 8 bits of the address.
• In half-word data: Value is doubled and used as the lower 9 bits of the address.
• In word data: Value is multiplied by 4 and used as the lower 10 bits of the address.
Figure 2.1-2 shows the relationship between the length of the data that designates the direct address, and
the actual address in memory.
Figure 2.1-2 Relation between Direct Address Data and Memory Address Value
[Example 1] Byte data: DMOVB R13,@58H
[Example 2] Half-word data: DMOVH R13,@58H
[Example 3] Word data: DMOV R13,@58H
Object code:1A58H
0000 0058HR13 12345678
0000 0058
HR13 12345678
0000 0058
HR13 12345678
Right 1-bit shift
Right 2-bit shift
Memory space
Memory space
Memory space
78
5678
1345678
58
H
No data shift
Object code:192C
H 58HLeft 1-bit shift
Object code:1816
H 58HLeft 2-bit shift