Specifications

Chapter 3 - Instruction Set

Arithmetic and logic

Of all arithmetic operations, PIC like most microcontrollers supports only subtraction and addition.

Flags C, DC and Z are set depending on a result of addition or subtraction, but with one exception:

since subtraction is performed like addition of a negative value, C flag is inverse following a

subtraction. In other words, it is set if operation is possible, and reset if larger number was

subtracted from a smaller one.

Logic unit of PIC has capability of performing operations AND, OR, EX-OR, complementing (COMF)

and rotation (RLF and RRF).

Instructions which rotate the register contents move bits inside a register through flag C by one

space to the left (toward bit 7), or to the right (toward bit 0). Bit which "comes out" of a register is

written in flag C, and value of C flag is written in a bit on the "opposite side" of the register.

Bit operations

Instructions BCF and BSF do setting or cleaning of one bit anywhere in the memory. Even though

this seems like a simple operation, it is executed so that CPU first reads the whole byte, changes

one bit in it and then writes in the entire byte at the same place.

Directing a program flow

Instructions GOTO, CALL and RETURN are executed the same way as on all other microcontrollers,

only stack is independent of internal RAM and limited to eight levels.

'RETLW k' instruction is identical with RETURN instruction, except that before coming back from a

subprogram a constant defined by instruction operand is written in W register. This instruction

enables us to design easily the Look-up tables (lists). Mostly we use them by determining data

position on our table adding it to the address at which the table begins, and then we read data from

that location (which is usually found in program memory).

Table can be formed as a subprogram which consists of a series of 'RETLW k' instructions, where 'k'

constants are members of the table.

We write the position of a member of our table in W register, and using CALL instruction we call a

subprogram which creates the table. First subprogram line ADDWF PCL, f adds the position of a W

data address in program memory. When returning from a subprogram we will have in W register the

contents of an addressed table member. In a previous example, constant 'k2' will be in W register

following a return from a subprogram.

RETFIE (RETurn From Interrupt - Interrupt Enable) is a return from interrupt routine and differs from

a RETURN only in that it automatically sets GIE (Global Interrupt Enable) bit. Upon an interrupt, this

bit is automatically cleared. As interrupt begins, only the value of program counter is put at the top

of a stack. No automatic storing of register values and status is provided.

Conditional jumps are synthesized into two instructions: BTFSC and BTFSS. Depending on a bit

status in 'f' register that is being tested, instructions skip or don't skip over the next program

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