Explaining the Instruction Set for the
PIC12c508A

Note: 00 to 1F represents the files.  (1F is equal to thirty-two files). These files are separated into two parts. The 7 Special Purpose Registers and 25 General Purpose Files. 

Files 00 to 06 are Special Purpose Registers - there are 7 of these (see below for definition of files and registers - we have given them different names to make them easy to identify.  Registers 00 to 06 are mainly used by the micro. Files 07 to 1F are used by you, the programmer.)

Files 07 to 1F are General Purpose files - there are 25 of these

Files 0C to 1F match up with PIC16F84 and these are the files we recommend you use when programming. The only files you lose are: 07, 08, 09, 0A & 0B.

The following is a detailed description of each instruction for the PIC12c508A, in alphabetical order:

ADDLW  This means: Add Literal to W. The word Literal means "NUMBER" such as the number 9 or 16h. The number 16h is a hex number and in normal values it represents the number: "twenty-two."
ADDLW 00 to FF    A fixed number (called the literal) is added to the W register (the Working register). The literal (number) can be 00 to FF.  Three flags in the STATUS register are affected by the ADD command.    See below.
This instruction is not available for the '508A. If you want this instruction or a program requires or contains it, use the following 3 instructions:

ADDWF This means: Add W and a file 
ADDWF 00 to 1F,0 the result is stored in the W register.
ADDWF 00 to 1F,1 the result is stored back in the file.
ADD the contents of the W register with a file. The result can be held in the W register (the designator=0) or put back into the named file (the designator=1).  Three flags in the STATUS register are affected by the ADD command:    Carry, Zero and Digit Carry.
If the result of an ADD instruction is greater than 255, the Carry flag is set (1), otherwise it is 0.
If the result of an ADD instruction is zero, the zero flag is set (1), otherwise it is 0.
The PIC monitors the lowest nibble in an addition and if the result is higher than "F," the Digit Carry flag is set (1), otherwise it is 0. For instance, if 21 is added to 3C, the result is 5D and this
does not affect the carry flag. But if 2D is added to 3E, the result is 6B and the Digit Carry flag is set (1).

ANDLW This means: AND Literal with W   
ANDLW 00 to FFThe purpose of the AND operation is to detect when the same bits in two binary numbers are set (1). If either or both bits are zero, the result is zero and the XOR instruction is used in this case.
In this instruction, the contents of the W register is ANDed with a fixed number (called the literal) and the result is placed in the W register. The literal can be 00 to FF.
The AND operation can be used to mask (remove) any bits as needed.
For example: 0F will mask the high nibble and F0 will remove the low nibble. E.g:

using 0F:
First number:                1001 0011
Second number (0F):     0000 1111
ANDed answer:             0000 0011

using F0:
First number:                 1001 0011
Second number (F0):      1111 0000
ANDed answer:              1001 0000

ANDWF  This means: AND W with f  
ANDWF 00 to 1F,0 the result is stored in the  W register. 
ANDWF 00 to 1F,1  the result is stored back in the  file. 
The W register is ANDed with a file.  Two files cannot be ANDed together. One file must be moved into W and ANDed with the second file.
See above for masking operations for the AND operation.
With the three AND instructions, only the Zero flag is affected. If the answer is zero, the Zero flag in the STATUS register is set (1). If the answer is greater than zero, the flag is 0.

BCF This means: Bit Clear in the file named
BCF 00 to 1F,bit There are nearly 300 instructions (hidden in the micro's internal workings) for this command.
There are 25 files in the PIC12C508A. The first 06h files are called Special Function Registers (SFR's) and the next nineteen files are called General Purpose Files (GPR's). These GPR's are called 07
to 1F to avoid confusion. Each file has 8 bits.
BCF means to clear (make 0) the identified bit in a named register.
E.g: BCF 6,5 means that bit 5 of file 6 is to be cleared. (File 6 contains the in/out lines - commonly called the in/out port).

BSF This means: Bit Set in the file named
BSF 00 to 1F,bit There are nearly 300 instructions for this command.
There are 32 files in the PIC12C508A. The first seven files are called Special Function Registers (SFR's) and the next twenty five files are called General Purpose Files (GPR's). These GPR's are  07 to 1F. Each file has 8 bits.
BSF means to set (make logic 1) the identified bit in a named file.
E.g: BSF 6,5 means that bit 5 of file 6 is to be set. (File 6 contains the in/out lines - commonly called the in/out port).

BTFSC This means: Bit Test, Skip if Clear 
BTFSC 00 to 1F,bit There are nearly 300 instructions for this command to cover the 25 files, each with 8 bits.
BTFSC means to test the identified bit in the named register and skip the next instruction if it is 0.
BTFSC 6,4 means to test bit 4 in file 6 and skip the next instruction if bit 4 is zero.

BTFSS This means: Bit Test, Skip if Set
BTFSS 00 to 1F,bit There are nearly 300 instructions for this command to cover the 25 files, each with 8 bits.
BTFSS means to test the identified bit in the named register and skip the next instruction if it is 1. BTFSS 3,2 means to test bit 2 in file 3 and skip the next instruction if bit 2 is 1.

CALL    This means: Call Subroutine. In a program it is written as: CALL Spike1 or CALL Tone1 etc, where Spike1 and Tone1 are called Labels. A RETURN should be placed at the end of the sub-routine being CALLed so that the micro comes back to the next instruction after the CALL instruction.  

CALL Label For the PIC12C508A, the CALL instruction can only access addresses: 000 to 0FF. This is effectively only half the memory and if your program is more than approx 256 bytes long,(0FFh)  you will have to consider how you will structure the program. See Structuring the program.
Programs should be written so that the first few instructions take the micro to the MAIN PROGRAM. The "Main Program" is located at the end of your program and this may be at the end of memory or half-way through, if your program is about 250 bytes long. 
After the first few instructions that tell the micro to GOTO Main, you place all the sub-routines required for the program. The sub-routines are called by a CALL command and the end of each sub-routine must have a RETLW 00 instruction as the '508A does not have a RETURN instruction.
A further call can be made from the sub-routine to another sub-routine but this second sub-routine cannot call another sub-routine. Each time a CALL instruction is executed, an address value is placed (PUSHED) into the stack so the micro knows where to come back to after the sub-routine is executed and the stack can only be 2 high. So care is needed not to run out of stack.
See also GOTO, RETURN and RETLW.

CLRF  This means: Clear f 
CLRF 00 to 1F The contents of a file is cleared and the Z bit is set. That is: all 8 bits become 0.
The first 6 files are Special Function Registers (SFR's) and the next nineteen files are called General Purpose files (GPR's). These GPR's are called 07h to 1F (they can also be called registers 7 to thirty-two).

CLRW  This means: Clear W (the working register -called the accumulator in other microprocessors)
CLRW The working register (W) is cleared. All bits are made (0's). When a bit is zeroed, the zero flag (Z flag ) is set (1). In other words the Z flag is 1. 

CLRWDT  This means: Clear Watchdog Timer
CLRWDT CLRWDT instruction resets the Watchdog Timer. It also resets the prescaler of the WDT.

GOTO  This means: Unconditional Branch
GOTO Label GOTO is an unconditional branch. The micro is sent to the Label specified.

IORLW This means: Inclusive OR Literal with W
IORLW 00 to FF The contents of the W register is OR'ed with a number. The result is placed in the W register. The literal number can be 00 to FF.
This is simply an `OR' operation and the purpose of performing it is to change two or more bits to `1." If a bit is ORed with 0, the answer is unchanged.
If a bit is ORed with 1, the result is 1.
If the working register (W) is loaded with 1111 0000 and a number such as 0100 1110 is ORed with W, the result is 1111 1110.

MOVLW  This means: Move Literal to W 
MOVLW 00 to FF A fixed number (called the Literal) is loaded into W register. The literal can be  00 to FF.

MOVWF  This means: Copy W to the named file 
MOVWF 00 to 1F This instruction copies data from W register to file F.

OPTION      This means: Load OPTION register
The contents of the W register is loaded into the OPTION register.

RETLW  This means: Return with Literal in W
RETLW 00 to FF The W Register is loaded with the literal value. The Program Counter is loaded from the top of the stack (the return address).

RRF  This means: Rotate the file Right through Carry
RRF 00 to1F.0 (the result is stored in W)
RRF 00 to 1F,1 (the result is stored in f)
The contents of a file is rotated one bit to the right through the Carry Flag. This is a 9-shift to get the file back to original.

SUBLW     This means: Subtract W from Literal
SUBLW 00 to FF The W register is subtracted (2's complement method) from the literal value. The result is placed in the W register.
This instruction is not available in the '508A instruction-set. Use the following  5 instructions:

TRIS 06 or just TRIS   This means: Load TRIS Register 
(See also OPTION 0DFh in the Library of Routines to make sure GP2 is an output line.)
This instruction loads TRIS with the contents of  W. We use TRIS 06 to remind you that you are referring to PortB (determining the input/output nature of each of the lines) when the program is transferred to a PICF84.