COP8SE datasheet(36/47 Pages) NSC

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12.0 Instruction Set (Continued)

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 in-

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

mediate 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 Pro-

gram 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

12.3.2 Tranfer-of-Control Addressing Modes

Program instructions are usually executed in sequential or-

der. However, Jump instructions can be used to change the

normal execution sequence. Several transfer-of-control ad-

dressing modes are available to specify jump addresses.

A change in program flow requires a non-incremental

change in the Program Counter contents. The Program

Counter consists of two bytes, designated the upper byte

(PCU) and lower byte (PCL). The most significant bit of PCU

is not used, leaving 15 bits to address the program memory.

Different addressing modes are used to specify the new ad-

dress for the Program Counter. The choice of addressing

mode depends primarily on the distance of the jump. Farther

jumps sometimes require more instruction bytes in order to

completely specify the new Program Counter contents.

The available transfer-of-control addressing modes are:

•

Jump Relative

•

Jump Absolute

•

Jump Absolute Long

•

Jump Indirect

The transfer-of-control addressing modes are described be-

low. Each description includes an example of a Jump in-

struction using a particular addressing mode, and the effect

on the Program Counter bytes of executing that instruction.

Jump Relative. In this 1-byte instruction, six bits of the in-

struction opcode specify the distance of the jump from the

current program memory location. The distance of the jump

can range from −31 to +32. A JP+1 instruction is not allowed.

The programmer should use a NOP instead.

Example: Jump Relative

JP 0A

Reg

Contents

Before

After

PCU

02 Hex

PCL

05 Hex

0F Hex

Jump Absolute. In this 2-byte instruction, 12 bits of the in-

struction opcode specify the new contents of the Program

Counter. The upper three bits of the Program Counter re-

main unchanged, restricting the new Program Counter ad-

dress to the same 4 kbyte address space as the current in-

struction.

(This restriction is relevant only in devices using more than

one 4 kbyte program memory space.)

Example: Jump Absolute

JMP 0125

Reg

Contents

Before

After

PCU

0C Hex

01 Hex

PCL

77 Hex

25 Hex

Jump Absolute Long. In this 3-byte instruction, 15 bits of

the instruction opcode specify the new contents of the Pro-

gram Counter.

Example: Jump Absolute Long

JMP 03625

Reg/

Contents

Memory

Before

After

PCU

42 Hex

36 Hex

PCL

36 Hex

25 Hex

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