Computer Hardware CPU / Micro processors / MicroControllers Architectures

From the Embedded Processor and Microcontroller primer and FAQ:

Von-Neumann Architecture

Microcontrollers based on the Von-Neuman architecture have a single "data" bus that is used to fetch both instructions and data. Program instructions and data are stored in a common main memory. When such a controller addresses main memory, it first fetches an instruction, and then it fetches the data to support the instruction. The two separate fetches slows up the controller's operation.

Harvard Architecture

Microcontrollers based on the Harvard Architecture have separate data bus and an instruction bus. This allows execution to occur in parallel. As an instruction is being "pre-fetched", the current instruction is executing on the data bus. Once the current instruction is complete, the next instruction is ready to go. This pre-fetch theoretically allows for much faster execution than a Von-Neuman architecture, but there is some added silicon complexity.

CISC

Almost all of today's microcontrollers are based on the CISC (Complex Instruction Set Computer) concept. The typical CISC microcontroller has well over 80 instructions, many of them very powerful and very specialized for specific control tasks. It is quite common for the instructions to all behave quite differently. Some might only operate on certain address spaces or registers, and others might only recognize certain addressing modes. The advantages of the CISC architecture is that many of the instructions are macro-like, allowing the programmer to use one instruction in place of many simpler instructions.

RISC

The industry trend for microprocessor design is for Reduced Instruction Set Computers (RISC) designs. This is beginning to spill over into the microntroller market. By implementing fewer instructions, the chip designed is able to dedicate some of the precious silicon real-estate for performance enhancing features. The benefits of RISC design simplicity are a smaller chip, smaller pin count, and very low power consumption. Among some of the typical features of a RISC processor: - Harvard architecture (separate buses for instructions and data) allows simultaneous access of program and data, and overlapping of some operations for increased processing performance - Instruction pipelining increases execution speed - Orthogonal (symmetrical) instruction set for programming simplicity; allows each instruction to operate on any register or use any addressing mode; instructions have no special combinations, exceptions, restrictions, or side effects

SISC

Actually, a microcontroller is by definition a Reduced Instruction Set Computer (at least in my opinion). It could really be called a Specific Instruction Set Computer (SISC). The [original] idea behind the microcontroller was to limit the capabilities of the CPU itself, allowing a complete computer (memory, I/O, interrupts, etc) to fit on the available real estate. At the expense of the more general purpose instructions that make the standard microprocessors (8088, 68000, 32032) so easy to use, the instruction set was designed for the specific purpose of control (powerful bit manipulation, easy and efficient I/O, and so on). Microcontrollers now come with a mind boggling array of features that aid the control engineer - watchdog timers, sleep/wakeup modes, power management, powerful I/O channels, and so on. By keeping the instruction set specific (and reduced), and thus saving valuable real estate, more and more of these features can be added, while maintaining the economy of the microcontroller.

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John von Neumann