© Copyright 2004 Dr. Phillip A. Laplante 1 Memory  Memory access  Memory technologies  Memory organization

© Copyright 2004 Dr. Phillip A. Laplante 2 Memory access Illustration of the clock synchronized memory transfer process between a device and the CPU. The symbolism “<>” shown in the data and address signals indicates that multiple lines are involved during this period in the transfer.

© Copyright 2004 Dr. Phillip A. Laplante 3 Memory technologies Selection of the appropriate technology is a systems design issue.

© Copyright 2004 Dr. Phillip A. Laplante 4 Memory organization Typical memory map showing designated regions. Not to scale.

© Copyright 2004 Dr. Phillip A. Laplante 5 I/O  Programmed I/O  Direct memory access (DMA)  Memory-mapped I/O  Interrupts

© Copyright 2004 Dr. Phillip A. Laplante 6 Direct memory access (DMA) The DMA controller prevents collisions by requiring each device to issue a DMA request signal (DMARQ) that will be acknowledged with a DMA acknowledge signal (DMACK). Until the DMACK signal is given to the requesting device its connection to the main bus remains in a tri-state condition. Any device that is tri-stated cannot affect the data on the memory data lines.

© Copyright 2004 Dr. Phillip A. Laplante 7 Memory-mapped I/O Input from an appropriate memory-mapped location involves executing a LOAD instruction on a pseudomemory location connected to an input device. Output uses a STORE instruction.

© Copyright 2004 Dr. Phillip A. Laplante 8 Memory-mapped I/O A bit map (packed binary word) describes a view of a set of devices that are accessed by a single (discrete) signal and organized into a word of memory for convenient access either by DMA or memory mapped-addressing.

© Copyright 2004 Dr. Phillip A. Laplante 9 Internal CPU handling of interrupts Step 1: finish the currently executing macroinstruction. Step 2: save the contents of the program counter to the interrupt return location. Step 3: load the address held in the interrupt handler location into the program counter. Resume the fetch and execute sequence. Single interrupt support

© Copyright 2004 Dr. Phillip A. Laplante 10 Internal CPU handling of interrupts Step 1: complete the currently executing instruction. Step 2: save the contents of PC to interrupt return location i. Step 3: load the address held in interrupt handler location i into the PC. Resume the fetch-execute cycle. Multiple interrupt support

© Copyright 2004 Dr. Phillip A. Laplante 11 Watchdog timers A watchdog timer. Software issues a reset signal via memory-mapped or programmed I/O to reset the timer before it can overflow, issuing a watchdog timer interrupt.