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Computer System Organization

Computer System Organization. S H Srinivasan shs@cs.ucsd.edu. von Neumann Architecture. CPU arithmetic logical unit control unit Memory I/O Bus. von Neumann Architecture. CPU. Control Unit. ALU. Address bus. Data bus. Memory. Device controller. Control Unit. Operation fetch

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Computer System Organization

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  1. Computer System Organization S H Srinivasan shs@cs.ucsd.edu

  2. von Neumann Architecture • CPU • arithmetic logical unit • control unit • Memory • I/O • Bus

  3. von Neumann Architecture CPU Control Unit ALU Address bus Data bus Memory Device controller

  4. Control Unit • Operation • fetch • decode • execute • Registers • PC (program counter) • IR (instruction register) • PS (processor status) • SP (stack pointer)

  5. Control unit operation PC = <machine start address>; IR = memory[PC]; haltFlag = FALSE; while (!haltFlag) { execute(IR); PC = PC + sizeof(INSTRUCTION); IR = memory[PC]; }

  6. Memory • MAR • MDR • command: Read/Write

  7. ALU • General purpose registers • Functional units: addition, multiplication, etc. • Status flags: carry, overflow, etc.

  8. Devices • Control register • accepts commands • Status register: ready, busy • polling • Data registers • I/O: character, block

  9. I/O operation while (device.busy || device.done); device.data[0] = <value to write> device.command = WRITE; while(device.busy); device.done = TRUE; The above illustrates “busy-wait” operation This can be speed up using interrupts DMA

  10. Interrupts • Asynchronous input to CPU • Processor state (PC) needs to be saved • where? Stack • one more register: SP • Priorities: each interrupt has a priority • Masking • IPL: interrupt priority level • if (input IPL < current IPL) ignore the interrupt

  11. Interrupt processing /* assuming the interrupt is not masked */ (1) push PC and PS on the stack (2) set current IPL = input IPL (3) handle interrupt /* jump to service routine */ (4) restore the IPL (5) restore the PC and PS (from the stack)

  12. DMA • Device <===> Memory data transfer is usually through the CPU • read device, write memory or read memory, write device • Huge data transfer between memory and device: lot of overhead • DMA: Direct device <===> memory data transfer without CPU intervention

  13. Bootstrapping • Power-up: CPU starts executing from a fixed location • This program reads a fixed number of bytes from a fixed location in disk (boot sector) • The program in the boot sector is called the bootstrap loader • The bootstrap loader loads the OS

  14. Hardware support • Processor modes • Special instructions • traps • Atomic operations • test and set • Exceptions

  15. Processor Modes • Two modes: supervisor and user • Supervisor • all instructions can be executed, e.g., halt • all memory can be accessed • User • subset of instructions and memory

  16. Traps • “trap” is a software instruction with a single parameter • example, “trap 40” • can be executed in the user mode • when executed, changes the mode to supervisor and calls a subroutine associated with the number (“40”)

  17. Traps (cont’d) • These subroutines are written by the OS developer and not the user • system calls are basically C language wrappers for traps

  18. Exceptions • Exceptions signal error conditions • illegal memory access • divide by zero • Exceptions are equivalent to traps except that they are invoked by the hardware

  19. traps, exceptions: synchronous • caused by the current instruction • interrupts: asynchronous • caused by external events like user typing • traps: invoked by user • exceptions: invoked by hardware because of error

  20. System parameters • Data, Address size • Clock speed • Bandwidth: memory, device

  21. What does the OS provide? • Multiple processors • Unlimited memory • Easy I/O • How?

  22. What does a program really want? • Current instruction (locality) • Register values: PC, SP, SR, other registers • Nothing else! • We can execute the program one instruction at a time, keeping only one instruction in memory.

  23. What does the kernel do?(Highly simplified) • Take one program • restore the register values • execute one instruction • save the register values • Take another program • repeat the same operation • In practice, kernel executes the program for one time slice and keeps several pages in memory. • Kernel needs some machinery to do this

  24. Timer • Timer is absolutely essential for time slice allocation, error detection (timeout), etc. • Most systems also have a realtime clock • timer vs. cpu clock vs. wall clock

  25. Programming abstraction for processes if ((pid = fork()) == 0) { /* child code */ /* begins independent execution */ } else { /* error */ } /* rest of parent code */ /* if there is no error, there are two processes here */

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