Instruction set
A PIC's instructions vary from about 35 instructions for the low-end PICs to over 80 instructions for the high-end PICs. The instruction set includes instructions to perform a variety of operations on registers directly, the accumulator and a literal constant or the accumulator and a register, as well as for conditional execution, and program branching.
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Compiler development
These properties have made it difficult to develop compilers that target PIC microcontrollers. While several commercial compilers are available, in 2008, Microchip finally released their C compilers, C18 and C30, for their line of 18f 24f and 30/33f processors. By contrast, Atmel's AVR microcontrollers—which are competitive with PIC in terms of hardware capabilities and price, but feature a more traditional instruction set—have long been supported by the GNU C Compiler.
Also, because of these properties, PIC assembly language code can be difficult to comprehend. Judicious use of simple macros can make PIC assembly language much more palatable, but at the cost of a reduction in performance. For example, the original Parallax PIC assembler ("SPASM") has macros which hide W and make the PIC look like a two-address machine. It has macro instructions like "
mov b, a" (move the data from address a to address b) and "add b, a" (add data from address a to data in address b). It also hides the skip instructions by providing three operand branch macro instructions such as "cjne a, b, dest" (compare a with b and jump to dest if they are not equal).