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Address Space
1-5
Introduction
1.4.3 Peripheral Modules
Peripheral modules are mapped into the address space. The address space
from 0100 to 01FFh is reserved for 16-bit peripheral modules. These modules
should be accessed with word instructions. If byte instructions are used, only
even addresses are permissible, and the high byte of the result is always 0.
The address space from 010h to 0FFh is reserved for 8-bit peripheral modules.
These modules should be accessed with byte instructions. Read access of
byte modules using word instructions results in unpredictable data in the high
byte. If word data is written to a byte module only the low byte is written into
the peripheral register, ignoring the high byte.
1.4.4 Special Function Registers (SFRs)
Some peripheral functions are configured in the SFRs. The SFRs are located
in the lower 16 bytes of the address space, and are organized by byte. SFRs
must be accessed using byte instructions only. See the device-specific data
sheets for applicable SFR bits.
1.4.5 Memory Organization
Bytes are located at even or odd addresses. Words are only located at even
addresses as shown in Figure 1−3. When using word instructions, only even
addresses may be used. The low byte of a word is always an even address.
The high byte is at the next odd address. For example, if a data word is located
at address xxx4h, then the low byte of that data word is located at address
xxx4h, and the high byte of that word is located at address xxx5h.
Figure 1−3. Bits, Bytes, and Words in a Byte-Organized Memory
15
7
14
6
. . Bits . .
. . Bits . .
9
1
8
0
Byte
Byte
Word (High Byte)
Word (Low Byte)
xxxAh
xxx9h
xxx8h
xxx7h
xxx6h
xxx5h
xxx4h
xxx3h