Central Processing Unit

1. Central Processing Unit Chapter 2

2. Topics • Central processing unit • Internal architecture • Fetch and execute cycles • Microcontrollers • Instruction formats • Core instructions • Addressing modes • RISC vs CISC • Memory • Memory Access • Memory Technologies • Memory hierarchy • Memory Organization • Input/Output • Programmed IO • Direct memory access (DMA) • Memory mapped IO • Interrupts • Programmable Interrupt controller • Interfacing devices to CPU via interrupts

3. Internal Architecture of a CPU (Z80)

4. Fetch and Execute Cycles • Macro instructions are fetched from memory and executed. • Fetch and execute cycles: simple overlap.

5. Microcontroller approach (fig.2.7) • Microcontroller is a internal processor that is programmable via microinstructions. • This approach is highly relevant to real-time systems and devices since each macro instruction is mapped into a set of microinstructions that can be directly executed with decoding etc. • Fine grained instructions are in micro memory and are wider than macro instructions. • These microinstructions directly control the logic gates of the microcontroller. • Microcontroller can take direct input from devices and can signal directly to devices. • High level language support is available for software development using microcontrollers. • Examples: Intel 8051, 8748, MCS 51/251 • See http://www.intel.com/design/embcontrol/index.htm

6. Instruction formats • 1, 0 address forms (implicit operands, destination); example: pop • 2-address forms: arithmetic operations • 3-address forms • Core instructions: • Horizontal bit operations: XOR, AND, NOT • Vertical bit operations: rotate right, rotate left • Control: trap, cli, epi, dpi, halt • Data movement: store/load • Math/special processing: add, sub, bts (lock)

7. Addressing Modes • Immediate • Direct memory • Indirect memory • Register • Auto-increment, auto-decrement

8. RISC vs CISC • CISC (complex instruction set computer) • Complex instructions: multiple cycles • Any instruction can ref memory • No instructions are pipelined • Micro-program for native instruction • Variable format instructions • Multiple instructions and addressing mode • Single set of registers • Complexity is in the micro-program and hardware

9. RISC vs. CISC • RISC (reduced instruction set computer) • Complementary set of first eight CISC principles • Simple single cycle instructions • Load/store only can reference memory • Highly pipelined • Instructions directly executed by hardware • Fixed-format instructions • Few instructions and addressing modes • Large multiple set of registers • Complexity handled by the compiler and software • Advantage for real-time system: average instruction execution time is lower than cisc • Disadvantage: associated with cache and elaborate multistage pipeline, while this improves performance for average case, pathological (extreme) situations may not be handled satisfactorily. Many real-time systems typically encounter extreme conditions.

10. Memory • Memory access time: interval between when a datum is requested and when it is available to the CPU. • This can have profound effect on real-time performance. • Real access time is decided by the physical characteristic of the memory chosen. • Effective access time depends on type, memory technology, memory layout (ex: memory interleaving), and buffering etc.

11. Memory Access • Typical microprocessor read cycle embodies the handshaking between the processor and main memory store. • CPU places address on address bus and allows signal to settle. • Then it places appropriate data on the data bus. • CPU asserts ALE (Address Latch enable) • CPU sends out WR signal • Memory device is selected and the data is written to the addressed location • All are synchronized to the system clock.