NSC800D datasheet(26/76 Pages) NSC

26 / 76 page

110 Addressing Modes (Continued)

115 DIRECT ADDRESSING

Direct addressing is the most straightforward way of ad-

dressing supplies a location in the memory space Direct

addressing 16-bits of memory address information in two

bytes of data as part of the instruction The memory address

could be either data source of destination or a location for

program execution as in program control instructions

Example

Instruction Jump to location X’0377

Mnemonic JP

X’0377

Opcode

11000011

Defines jump opcode

01110111

Constant X’0377

00000011

(

This instruction loads the Program Counter (PC) is loaded

with the constant in the second and third bytes of the in-

struction The program counter contents are transferred via

direct addressing

116 REGISTER INDIRECT

Next to direct addressing register indirect addressing pro-

vides the second most straightforward means of addressing

memory In register indirect addressing a specified register

pair contains the address of the desired memory location

The instruction references the register pair and the register

contents define the memory location of the operand

Example

Instruction Add the contents of memory location X’0254 to

the A register The HL register contains X’0254

Mnemonic ADD

A(HL)

Opcode

10000110

This instruction uses implied addressing of the A and HL

registers and register indirect addressing to access the data

pointed to by the HL register

117 INDEXED

The most flexible mode of memory addressing is the in-

dexed mode This is similar to the register indirect mode of

addressing because one of the two index registers (IX or IY)

contains the base memory address In addition a byte of

data included in the instruction acts as a displacement to

the address in the index register

Indexed addressing is particularly useful in dealing with lists

of data

Example

Instruction Increment the data in memory location X’1020

The IY register contains X’1000

Mnemonic INC

(IYaX’20)

Opcode

TLC5171 – 54

The indexed addressing mode uses the contents of index

registers IX or IY along with the displacement to form a

pointer to memory

118 RELATIVE

Certain instructions allow memory locations to be ad-

dressed as a position relative to the PC register These in-

structions allow jumps to memory locations which are off-

sets around the program counter The offset together with

the current program location is determined through a dis-

placement byte included in the instruction The formation of

this displacement byte is explained more fully in the ‘‘In-

structions Set’’ section

Example

Instruction Jump to a memory location 7 bytes beyond the

current location

Mnemonic JR

$a7

Opcode

00011000

Defines relative jump

opcode

00000101

Displacement to be

applied to the PC

The program will continue at a location seven locations past

the current PC

NSC800D Datasheet(PDF) 26 Page - National Semiconductor (TI)

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