The PIC Microcontroller: Your Personal Introductory Course, Third Edition

"PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division. PICs are popular with developers due to their low cost, wide availability, large user base, extensive collection of application notes, availability of low cost or free development tools, and serial programming (and re-programming with flash memory) capability.

All PICs feature Harvard architecture, so the code space and the data space are separate. PIC code space is generally implemented as EPROM, ROM, or FLASH ROM.

In general, external code memory is not directly addressable due to the lack of an external memory interface. The exceptions are PIC17 and select high pin count PIC18 devices.

PICs have a set of register files that function as general purpose ram and as special purpose control registers for on-chip hardware resources. The addressability of memory varies depending on device series, and all PIC devices have some banking mechanism to extend the addressing to additional memory. Later series of devices incorporate move instructions which can cover the whole addressable space, independent of the selected bank.

External data memory is not directly addressable except in some high pin count PIC18 devices.

PIC microcontrollers have a very small set of instructions, leading some to consider them as RISC devices, however many salient features of RISC CPU's are not reflected in the PIC architecture. For example:

it does not have a load-store architecture ie. there is no concept of "main memory" as being separate from register space operations between immediate constants and registers is generally disallowed PICs instructions vary in number from about 35 instructions for the low-end PICs to about 70 instructions for the high-end PICs. The instruction set includes instructions to perform a variety of operations on the accumulator and a constant or the accumulator and a memory location, as well as for conditional execution, and program branching.

PICs have excellent support for bit-wise operations. Bits may be individually set, cleared, and tested.

Most literal operations involve the accumulator register, for example, to add a constant to a register you must first load the constant into the accumulator and then add the accumulator to the register. Other operations, such as bit setting and testing, can be performed on any register.

Conditional jumps are implemented as "skip" instructions.

Notably, the PIC architecture has no (or very meager) hardware support for saving processor state when servicing interrupts. The 18 series improved this situation by implementing shadow registers which save several important registers during an interrupt."
Information found at: Wikipedia

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Susan Dietel
Web Programmer
Email: susansdesign@yahoo.com
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