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【正文】 bly the two most significant inventions of the 20th century.Some employ the split program/data memory of the Harvard architecture, shown in , others follow the philosophy, widely adapted for generalpurpose puters and microprocessors, of making no logical distinction between program and data memory as in the Princeton architecture, shown in . In general terms a singlechip microputer is characterized by the incorporation of all the units of a puter into a single device, as shown in Fig35A3. These two types of architecture are found in singlechip microputer. Fig. A1 A Harvard type Fig. A2 A conventional Princeton puterRead only memory (ROM)ROM is usually for the permanent, nonvolatile storage of an applications program .Many microputers and microcontrollers are intended for highvolume applications and hence the economical manufacture of the devices requires that the contents of the program memory be mitted permanently during the manufacture of chips. Clearly, this implies a rigorous approach to ROM code development since changes cannot be made after manufacture .This development process may involve emulation using a sophisticated development system with a hardware emulation capability as well as the use of powerful software tools.Some manufacturers provide additional ROM options by including in their range devices with (or intended for use with) user programmable memory. The simplest of these is usually device which can operate in a microprocessor mode by using some of the input/output lines as an address and data bus for accessing external memory. This type of device can behave functionally as the single chip microputer from which it is derived albeit with restricted I/O and a modified external circuit. The use of these ROM less devices is mon even in production circuits where the volume does not justify the development costs of custom onchip ROM。 there can still be a significant saving in I/O and other chips pared to a conventional microprocessor based circuit. More exact replacement for ROM devices can be obtained in the form of variants with 39。 EPROM (Erasable programmable ROM ) sockets or devices with EPROM instead of ROM .These devices are naturally more expensive than equivalent ROM device, but do provide plete circuit equivalents. EPROM based devices are also extremely attractive for lowvolume applications where they provide the advantages of a singlechip device, in terms of onchip I/O, etc. with the convenience of flexible user programmability.Random access memory (RAM)RAM is for the storage of working variables and data used during program execution. The size of this memory varies with device type but it has the same characteristic width (4,8,16 bits etc.) as the processor ,Special function registers, such as stack pointer or timer register are often logically incorporated into the RAM area. It is also mon in Harvard type microputers to treat the RAM area as a collection of register。 in most a mechanism is provided to at least allow some flexibility of choosing which pins are outputs and which are inputs. This may apply to all or some of the ports. Some I/O lines are suitable for direct inter
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