Introduction Remote Procedure Call RPC is an inter-process communication technique to allow client and server software to communicate on a network. The client makes a procedure call that appears to be local but is actually run on a remote computer.
A DMA controller can generate memory addresses and initiate memory read or write cycles. It contains several hardware registers that can be written and read by the CPU.
These include a memory address register, a byte count register, and one or more control registers. The CPU then commands peripheral device to initiate data transfer. Each time a byte of data is ready to be transferred between the peripheral device and memory, the DMA controller increments its internal address register until the full block of data is transferred.
Bus mastering[ edit ] In a bus mastering system, also known as a first-party DMA system, the CPU and peripherals can each be granted control of the memory bus. Where a peripheral can become bus master, it can directly write to system memory without involvement of the CPU, providing memory address and control signals as required.
Some measure must be provided to put the processor into a hold condition so that bus contention does not occur. Transfer types[ edit ] DMA transfers can transfer either one byte at a time or all at once in burst mode.
If they transfer a byte at a time, this can allow the CPU to access memory on alternating bus cycles — this is called cycle stealing since the CPU and either the DMA write access unavailable cannot proceed because microsoft or the bus master contend for memory access.
Modes of operation[ edit ] Burst mode[ edit ] In burst mode, an entire block of data is transferred in one contiguous sequence. Once the DMA controller is granted access to the system bus by the CPU, it transfers all bytes of data in the data block before releasing control of the system buses back to the CPU, but renders the CPU inactive for relatively long periods of time.
The mode is also called "Block Transfer Mode". Cycle stealing mode[ edit ] The cycle stealing mode is used in systems in which the CPU should not be disabled for the length of time needed for burst transfer modes.
However, in cycle stealing mode, after one byte of data transfer, the control of the system bus is deasserted to the CPU via BG. It is then continually requested again via BR, transferring one byte of data per request, until the entire block of data has been transferred.
By continually obtaining and releasing the control of the system bus, the DMA controller essentially interleaves instruction and data transfers. On the one hand, the data block is not transferred as quickly in cycle stealing mode as in burst mode, but on the other hand the CPU is not idled for as long as in burst mode.
Cycle stealing mode is useful for controllers that monitor data in real time. Transparent mode[ edit ] Transparent mode takes the most time to transfer a block of data, yet it is also the most efficient mode in terms of overall system performance.
In transparent mode, the DMA controller transfers data only when the CPU is performing operations that do not use the system buses. The primary advantage of transparent mode is that the CPU never stops executing its programs and the DMA transfer is free in terms of time, while the disadvantage is that the hardware needs to determine when the CPU is not using the system buses, which can be complex.
When the CPU accesses location X in the memory, the current value will be stored in the cache. Subsequent operations on X will update the cached copy of X, but not the external memory version of X, assuming a write-back cache.
If the cache is not flushed to the memory before the next time a device tries to access X, the device will receive a stale value of X. Similarly, if the cached copy of X is not invalidated when a device writes a new value to the memory, then the CPU will operate on a stale value of X.
This issue can be addressed in one of two ways in system design: Cache-coherent systems implement a method in hardware whereby external writes are signaled to the cache controller which then performs a cache invalidation for DMA writes or cache flush for DMA reads.
Non-coherent systems leave this to software, where the OS must then ensure that the cache lines are flushed before an outgoing DMA transfer is started and invalidated before a memory range affected by an incoming DMA transfer is accessed.Recovering a Microsoft email account is simple, in theory.
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