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PIC Assembly Language Reference - 243

Appendix D: PIC Assembly Language Reference

PIC Assembly is the lowest-level programming lanugage for Microchip PIC microcontrollers. These

processors are used on the LogoChip and LogoChip modules, the LogoBoard, the PIC Foundation,

and every layer in the Tower system.

When using PIC Assembly, user programs are entered on a desktop computer and assembled into byte

codes that are transferred to the PIC processor through the serial port of the host computer.

Our assembly language differs in some ways from the commercially-available one. It is designed to be

more human-readable than the standard instruction set, and can be easily modied to support other

processors while retaining its user-friendly naming scheme.

As in all assembly-level programming, computation is performed in an “accumulator” scratch register, which is modied as needed by the different processor instructions.

The basic instruction format uses square brackets to frame each instruction:

[instruction argument]

Program location labels are dened by plain text, with no brackets:

program-label

Constants are used to dene memory locations for both built-in registers and variable storage. A con-

stant is dened like this:

[const name address]

Memory addresses can be denoted either in decimal or hexidecimal format, as indicated by placing a

“$” character in front of the number.

The status register, at memory location “$03”, is a special one on the processor. It has user-readable

ags that represent different conditions, which can be triggered by the results of various instructions.

In most cases, the two status ags that one would be most concerned with are:

• C - The carry ag, which is set when an instruction overows a register.

• Z - The zero ag, which is set when the result of an instruction is “0”.



244 - PIC Assembly Language Reference

Program Flow

bsr - branch subroutine 246

rts - return from subroutine 246

rtv - return from subroutine with value 247

bra - unconditional branch 247

rti - return from interrupt 248

Register Manipulation

lda - load accumulator 248

ldan - load accumulator with number 248

sta - store accumulator 249

tst - test 249

clr - clear 250

clr - clear accumulator 250

inc - increment 251

linc - load and increment 251

incsz - increment skip if zero 252

lincsz - load and increment skip if zero 252

dec - decrement 253ldec - load and decrement 253

decsz - decrement skip if zero 254

ldecsz - load and decrement skip if zero 254

rol - rotate left 255

lrol - load and rotate left 255

ror - rotate right 256

lror - load and rotate right 256

com - complement 257

lcom - load and complement 257

swap - swap 258

lswap - load and swap 258

What follows, is a list of all instructions currently supported in the PIC Assembler. The instructions

are organized by their functionality, for ease of reference when the exact name is not known. The list

below represents the order in which their detailed descriptions and examples of use can be found on

the subsequent pages:




rtv - return from subroutine with valueProgram Flow

Description

Returns from a subroutine with a value stored in the accumulator. The top value on the stack

is popped and loaded into the program counter, returning to the program location of the most

recently called bsr instruction.

Usage Format

[rtv number]

The number argument is the value to return.

Status Flags Affected

None

bra - unconditional branchProgram Flow

Description

Branches to a given memory location.

Usage Format

[bra address]

The address argument is a number corresponding to a program memory location, usually

specied by the name of a location label.

Status Flags Affected None




rti - return from interruptProgram Flow

Description

Returns from an interrupt. The top value on the stack is popped and loaded into the program

counter, returning to the program location at the time of interrupt.

Usage Format

[rti]


None

lda - load accumulatorRegister Manipulation

Description

Loads the accumulator with the contents of a register.

Usage Format

[lda register]

The register argument is the address of any register.


Z

ldan - load accumulator with numberRegister Manipulation

Description

Loads the accumulator with a number.




Usage Format

[ldan number]

The number argument is the value to be loaded into the accumulator.


None

sta - store accumulatorRegister Manipulation

Description

Stores the contents of the accumulator in a register.

Usage Format

[lda register]



Z

tst - testRegister Manipulation

Description

Tests the value of a register and sets the zero ag if the contents of the register is equal to“0”.

Usage Format

[tst register]






Z

clr - clearRegister Manipulation

Description

Sets the contents of a register to “0”.

Usage Format

[clr register]



Z

clra - clear accumulatorRegister Manipulation

Description

Sets the contents of the accumulator to “0”.

Usage Format

[clra]


Z




inc - incrementRegister Manipulation

Description

Increments the contents of a register and stores the result in the register.

Usage Format

[inc register]



Z

linc - load and incrementRegister Manipulation

Description

Increments the contents of a register and stores the result in the accumulator.

Usage Format

[linc register]



Z




dec - decrementRegister Manipulation

Description

Decrements the contents of a register and stores the result in the register.

Usage Format

[dec register]



Z

ldec - load and decrementRegister Manipulation

Description

Decrements the contents of a register and stores the result in the accumulator.

Usage Format

[ldec register]



Z




decsz - decrement skip if zeroRegister Manipulation

Description

Decrements the contents of a register and stores the result in the register. If the result of the

operation is zero, the next program instruction is skipped.

Usage Format

[decsz register]



None

ldecsz - load and decrement skip if zeroRegister Manipulation

Description

Decrements the contents of a register and stores the result in the accumulator. If the resultof the operation is zero, the next program intsruction is skipped.

Usage Format

[ldecsz register]






rol - rotate leftRegister Manipulation

Description

The contents of a register are rotated to the left through the carry ag and the result is stored

in the register.

Usage Format

[rol register]



C

lrol - load and rotate leftRegister Manipulation

Description

The contents of a register are rotated to the left through the carry ag and the result is storedin the accumulator.

Usage Format

[lrol register]


Status Flags Affected C




swap - swapRegister Manipulation

Description

The upper and lower halves of a register are swapped and the result is stored in the register.

Usage Format

[swap register]



None

lswap - load and swapRegister Manipulation

Description

The upper and lower halves of a register are swapped and the result is stored in the accumula-

tor.

Usage Format

[lswap register]



None




bset - bit setBit Manipulation

Description

Makes the value of a given bit of a register equal to “1”.

Usage Format

[bset bit register]

The bit argument is any bit number (from 0 to 7) and the register argument is the address of

any register.


None

bclr - bit clearBit Manipulation

Description

Makes the value of a given bit of a register equal to “0”.

Usage Format

[bclr bit register]


any register.





btss - bit test skip if setBit Manipulation

Description

Tests the value of a given bit of a register. If it is equal to “1,” the next program instruction

is skipped.

Usage Format

[btss bit register]


any register.


None

btsc - bit test skip if clearBit Manipulation

Description Tests the value of a given bit of a register. If it is equal to “0,” the next program instruction

is skipped.

Usage Format

[btsc bit register]


any register.


None




add - add Arithmetic

Description

Adds the contents of the accumulator to a reguster and stores the result in the accumulator.

Usage Format

[add register]



Z, C

addm - add to memory Arithmetic

Description

Adds the contents of the accumulator to a register and stores the result in the register.

Usage Format

[addm register]



Z, C




addn - add number Arithmetic

Description

Adds the conents of the accumulator to a number and stores the result in the accumulator.

Usage Format

[addn number]

The number argument is the value to be added to.


Z, C

sub - subtract Arithmetic

Description

Subtracts the contents of the accumulator from a register and stores the result in the accu-

mulator.

Usage Format

[sub register]



Z, C (C=0 means result is negative)




subm - subtract from memory Arithmetic

Description

Subtracts the contents of the accumulator from a register and stores the result in the register.

Usage Format

[subm register]




subn - subtract number Arithmetic

Description

Subtracts the contents of the accumulator from a number and stores the result in the accu-

mulator.

Usage Format

[subn number]

The number argument is the value to be subtracted from.






and - andLogic

Description

Performs a bit-wise “and” of the contents of the accumulator with a reguster and stores the

result in the accumulator.

Usage Format

[and register]



Z, C

andm - and memoryLogic

Description

Performs a bit-wise “and” of the contents of the accumulator with a reguster and stores the

result in the register.

Usage Format

[andm register]



Z




andn - and numberLogic

Description

Performs a bit-wise “and” of the contents of the accumulator with a number and stores the


Usage Format

[andn number]

The number argument is the value to “and” the accumulator with.


Z

or - orLogic

Description

Performs a bit-wise “or” of the contents of the accumulator with a reguster and stores theresult in the accumulator.

Usage Format

[or register]


Status Flags Affected Z, C




orm - or memoryLogic

Description

Performs a bit-wise “or” of the contents of the accumulator with a reguster and stores the

result in the register.

Usage Format

[orm register]



Z

orn - or numberLogic

Description

Performs a bit-wise “or” of the contents of the accumulator with a number and stores theresult in the accumulator.

Usage Format

[orn number]

The number argument is the value to “or” the accumulator with.


Z




xor - exclusive orLogic

Description

Performs a bit-wise “xor” of the contents of the accumulator with a reguster and stores the


Usage Format

[xor register]



Z, C

xorm - exclusive or memoryLogic

Description

Performs a bit-wise “xor” of the contents of the accumulator with a reguster and stores theresult in the register.

Usage Format

[xorm register]



Z




xorn - exclusive or numberLogic

Description

Performs a bit-wise “or” of the contents of the accumulator with a number and stores the


Usage Format

[xorn number]

The number argument is the value to “xor” the accumulator with.


Z

nop - no operationSystem Commands

Description

Does nothing. Usually used to perform precise timing operations.

Usage Format

[nop]


None



sleep - sleepSystem Commands

Description

Puts the processor to sleep, a mode where its power consumption is signicantly reduced. It

cannot perform any computation while in this state, but can be awakened by interrupts.

Usage Format

[sleep]


None

clrwdt - clear watchdog timerSystem Commands

Description

Resets the value of the internal watchdog timer.

Usage Format

[clrwdt]


None

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