Datasheet

. . Bits . .

Byte

Word (High Byte)

Word (Low Byte)

xxxAh

xxx9h

xxx8h

xxx7h

xxx6h

xxx5h

xxx4h

xxx3h

Address Space

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1.4.2 RAM

RAM starts at 0200h. The end address of RAM depends on the amount of RAM present and varies by

device. RAM can be used for both code and data.

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

Introduction SLAU144J–December 2004–Revised July 2013

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