as 11 0000 0101 1010. Since each he

as 11 0000 0101 1010. Since each hex digit represents four binary bits, the leading two bits are zero, and the leading digit will only range from 0 to 3 for a 14-bit number.

In the instruction set (data sheet, Table 13-2), it is seen that the first 4 bits (11 00) are the instruction code, the next two are unused (xx, appearing as 00 in the binary code) and the last 8 are the literal value (5A). The literal is represented as ‘kkkk kkkk’ since it can have any value from 00000000 to 11111111 (00–FF).

The format of other instructions depends mainly on the number of bits required for the operand (data to be processed). The number of op-code bits can vary from 3 to all 14, depending on the number of bits needed for the operand. This is different from a conventional processor, such as the Pentium, where the op-code and operand are each created as a whole number of bytes. The PIC instruction is more compact, as is the instruction set itself, for greater speed of operation. This defines it as a RISC (Reduced Instruction Set Computer) chip.


Program BIN4

The program BIN4 contains many of the basic program elements, and the list file (Program 1.2) shows the source code, machine code, memory address and list file line number as before. There are additional comments to aid program analysis and debugging.

Note that two types of labels are used in program to represent numbers. Label equates are used at the top of the program to declare labels for the file registers which will be used in the program. Address labels are placed in the first column to mark the destination for GOTO and CALL instructions.


Chip Configuration Word

In Program BIN4, the assembler directive CONFIG is included at the top of the program, which sets up aspects of the chip operation which cannot be sub- sequently changed without reprogramming. A special area of program memory outside the normal range (address 2007h) stores a chip configuration word; the clock type, and other MCU options detailed below, are set by loading the con- figuration bits with a suitable binary code. The function of each bit is shown in Table 1.2, along with some typical configuration settings. Details can be found in the data sheet, Section 12.

CODE PROTECTION

Normally, the program machine code can be read back to the programming host computer, be disassembled and the original source program recovered. This can be prevented if commercial or security considerations require it. The