COP8TAB9 datasheet(47/59 Pages) NSC

47 / 59 page

19.0 Instruction Set (Continued)

erand addressing modes and transfer-of-control addressing

modes. Operand addressing modes are the various meth-

ods of specifying an address for accessing (reading or writ-

ing) data. Transfer-of-control addressing modes are used in

conjunction with jump instructions to control the execution

sequence of the software program.

19.3.1 Operand Addressing Modes

The operand of an instruction specifies what memory loca-

tion is to be affected by that instruction. Several different

operand addressing modes are available, allowing memory

locations to be specified in a variety of ways. An instruction

can specify an address directly by supplying the specific

address, or indirectly by specifying a register pointer. The

contents of the register (or in some cases, two registers)

point to the desired memory location. In the immediate

mode, the data byte to be used is contained in the instruction

itself.

Each addressing mode has its own advantages and disad-

vantages with respect to flexibility, execution speed, and

program compactness. Not all modes are available with all

instructions. The Load (LD) instruction offers the largest

number of addressing modes.

The available addressing modes are:

•

Direct

•

Indirect

with

Post-Incrementing/

Decrementing

•

Immediate

•

Immediate Short

•

Indirect from Program Memory

The addressing modes are described below. Each descrip-

tion includes an example of an assembly language instruc-

tion using the described addressing mode.

Direct. The memory address is specified directly as a byte in

the instruction. In assembly language, the direct address is

written as a numerical value (or a label that has been defined

elsewhere in the program as a numerical value).

Example: Load Accumulator Memory Direct

LD A,05

Reg/Data

Contents

Memory

Before

After

Accumulator

XX Hex

A6 Hex

Memory Location

A6 Hex

0005 Hex

by the contents of the B Register or X register (pointer

fies which register serves as the pointer.

Example: Exchange Memory with Accumulator, B Indirect

X A,[B]

Reg/Data

Contents

Memory

Before

After

Accumulator

01 Hex

87 Hex

Memory Location

87 Hex

01 Hex

0005 Hex

B Pointer

05 Hex

Decrementing. The relevant memory address is specified

by the contents of the B Register or X register (pointer

or decremented after execution, allowing easy manipulation

of memory blocks with software loops. In assembly lan-

guage, the notation [B+], [B−], [X+], or [X−] specifies which

be incremented or decremented.

Example: Exchange Memory with Accumulator, B Indirect

with Post-Increment

X A,[B+]

Reg/Data

Contents

Memory

Before

After

Accumulator

03 Hex

62 Hex

Memory Location

62 Hex

03 Hex

0005 Hex

B Pointer

05 Hex

06 Hex

Intermediate. The data for the operation follows the instruc-

tion opcode in program memory. In assembly language, the

number sign character (#) indicates an immediate operand.

Example: Load Accumulator Immediate

LD A,#05

Reg/Data

Contents

Memory

Before

After

Accumulator

XX Hex

05 Hex

Immediate Short. This is a special case of an immediate

instruction. In the “Load B immediate” instruction, the 4-bit

immediate value in the instruction is loaded into the lower

nibble of the B register. The upper nibble of the B register is

reset to 0000 binary.

Example: Load B Register Immediate Short

LD B,#7

Reg/Data

Contents

Memory

Before

After

B Pointer

12 Hex

07 Hex

Indirect from Program Memory. This is a special case of

an indirect instruction that allows access to data tables

stored in program memory. In the “Load Accumulator Indi-

rect” (LAID) instruction, the upper and lower bytes of the

Program Counter (PCU and PCL) are used temporarily as a

pointer to program memory. For purposes of accessing pro-

gram memory, the contents of the Accumulator and PCL are

exchanged. The data pointed to by the Program Counter is

loaded into the Accumulator, and simultaneously, the original

contents of PCL are restored so that the program can re-

sume normal execution.

Example: Load Accumulator Indirect

LAID

Reg/Data

Contents

Memory

Before

After

PCU

04 Hex

PCL

35 Hex

36 Hex

Accumulator

1F Hex

25 Hex

Memory Location

25 Hex

041F Hex

COP8TAB9 Datasheet(PDF) 47 Page - National Semiconductor (TI)

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