Datasheet

© 2007 Microchip Technology Inc. DS39605F-page 191
PIC18F1220/1320
20.0 INSTRUCTION SET SUMMARY
The PIC18 instruction set adds many enhancements to
the previous PIC instruction sets, while maintaining an
easy migration from these PIC 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 20-1 lists
byte-oriented, bit-oriented, literal and control
operations. Table 20-1 shows the opcode field
descriptions.
Most byte-oriented instructions have three operands:
1. The file register (specified by ‘f’)
2. The destination of the result
(specified by ‘d’)
3. The accessed memory
(specified by ‘a’)
The file register designator ‘f’ specifies which file
register is to be used by the instruction.
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:
1. The file register (specified by ‘f’)
2. The bit in the file register
(specified by ‘b’)
3. 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 20-1 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 20-1,
lists the instructions recognized by the Microchip
Assembler (MPASM
TM
). Section 20.2 “Instruction
Set” provides a description of each instruction.
20.1 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 “BCF PORTB,1” instruction will read
PORTB, clear bit 1 of the data, then write the result
back to PORTB. The read operation would have the
unintended result that any condition that sets the RBIF
flag would be cleared. The R-M-W operation may also
copy the level of an input pin to its corresponding output
latch.