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In the indexed, 8-bit offset addressing mode, the effective address is obtained by adding the contents
of the byte following the opcode to the contents of the index register. This mode of addressing is useful
for selecting the kth element in an n element table. To use this mode, the table must begin in the
lowest 256 memory locations and may extend through the first 511 memory locations (IFE is the last
location which the instruction may access). Indexed 8-bit offset addressing can be used for ROM,
RAM, or I/O. This is a 2-byte instruction with the offset contained in the byte following the opcode. The
content of the index register (X) is not changed. The offset byte supplied in the instruction is an
unsigned 8-bit integer.
7.1.7 Indexed, 16-bit Offset Addressing Mode
In the indexed, 16-bit offset addressing mode, the effective address is the sum of the contents of the
8-bit index register and the two bytes following the opcode. The content of the index register is not
changed. These instructions are three bytes, one for the opcode and two for a 16-bit offset.
7.1.8 Relative Addressing Mode
The relative addressing mode is used only for branch instructions. Branch instructions, other than the
branching versions of bit-manipulation instructions, generate two machine-code bytes: one for the
opcode and one for the relative offset. Because it is desirable to branch in either direction, the offset
byte is a signed twos-complement offset with a range of –127 to +128 bytes (with respect to the
address of the instruction immediately following the branch instruction). If the branch condition is true,
the contents of the 8-bit signed byte following the opcode (offset) are added to the contents of the
program counter to form the effective branch address; otherwise, control proceeds to the instruction
immediately following the branch instruction.
7.2 Instruction Type
There are 65 instructions in CPU, and can be divided into 5 types.
1) Register/Memory Instructions
2) Read/Modify-Write Instructions
3) Branch Instructions
4) Control Instructions
5) bit manipulate Instructions
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