PIC18F258-ESLASHLSQTP datasheet(283/402 Pages) MICROCHIP

283 / 402 page

DS41159E-page 281

PIC18FXX8

25.0

INSTRUCTION SET SUMMARY

The PIC18 instruction set adds many enhancements to

the previous PICmicro instruction sets, while maintaining

an easy migration from these PICmicro instruction sets.

Most instructions are a single program memory word

(16 bits) but there are three instructions that require two

program memory locations.

Each single-word instruction is a 16-bit word divided

into an opcode, which specifies the instruction type and

one or more operands, which further specify the

operation of the instruction.

The instruction set is highly orthogonal and is grouped

into four basic categories:

• Byte-oriented operations

• Bit-oriented operations

• Literal operations

• Control operations

The PIC18 instruction set summary in Table 25-2 lists

byte-oriented,

bit-oriented,

literal

and

control

operations.

Table 25-1

shows

the

opcode

field

descriptions.

Most byte-oriented instructions have three operands:

The file register (specified by ‘f’)

The destination of the result

(specified by ‘d’)

The accessed memory

(specified by ‘a’)

The file register designator ‘f’ specifies which file

The destination designator ‘d’ specifies where the

result of the operation is to be placed. If ‘d’ is zero, the

result is placed in the WREG register. If ‘d’ is one, the

result is placed in the file register specified in the

instruction.

All bit-oriented instructions have three operands:

The file register (specified by ‘f’)

The bit in the file register

(specified by ‘b’)

The accessed memory

(specified by ‘a’)

The bit field designator ‘b’ selects the number of the bit

affected by the operation, while the file register desig-

nator ‘f’ represents the number of the file in which the

bit is located.

The literal instructions may use some of the following

operands:

• A literal value to be loaded into a file register

(specified by ‘k’)

• The desired FSR register to load the literal value

into (specified by ‘f’)

• No operand required

(specified by ‘—’)

The control instructions may use some of the following

operands:

• A program memory address (specified by ‘n’)

• The mode of the CALL or RETURN instructions

(specified by ‘s’)

• The mode of the table read and table write

instructions (specified by ‘m’)

• No operand required

(specified by ‘—’)

All instructions are a single word, except for three

double-word instructions. These three instructions

were made double-word instructions so that all the

required information is available in these 32 bits. In the

second word, the 4 MSbs are ‘1’s. If this second word

is executed as an instruction (by itself), it will execute

as a NOP.

All single-word instructions are executed in a single

instruction cycle, unless a conditional test is true or the

program counter is changed as a result of the instruc-

tion. In these cases, the execution takes two instruction

cycles, with the additional instruction cycle(s) executed

as a NOP.

The double-word instructions execute in two instruction

cycles.

One instruction cycle consists of four oscillator periods.

Thus, for an oscillator frequency of 4 MHz, the normal

instruction execution time is 1

μs. If a conditional test is

true, or the program counter is changed as a result of

an instruction, the instruction execution time is 2

μs.

Two-word branch instructions (if true) would take 3

μs.

Figure 25-1 shows the general formats that the

instructions can have.

All examples use the format ‘nnh’ to represent a hexa-

decimal number, where ‘h’ signifies a hexadecimal

digit.

The Instruction Set Summary, shown in Table 25-2,

lists the instructions recognized by the Microchip

MPASMTM Assembler.

Section 25.2 “Instruction Set” provides a description

of each instruction.

25.1

Read-Modify-Write Operations

Any instruction that specifies a file register as part of

the instruction performs a Read-Modify-Write (R-M-W)

operation. The register is read, the data is modified and

the result is stored according to either the instruction or

the destination designator ‘d’. A read operation is

performed on a register even if the instruction writes to

that register.

For example, a “CLRF

PORTB

” instruction will read

PORTB, clear all the data bits, then write the result

back to PORTB. This example would have the

unintended result that the condition that sets the RBIF

flag would be cleared.

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