Intel mcs-48 manual Memory Write, Input/Output, Interrupts

INTRODUCTION

Memory Write

Amemory write operation is similar to a read except for the direction of data flow. The CPU issues a write signal, sends the proper memory address, then sends the data word to be written into the addressed data memory location.

Input/Output

Input and Output operations are similar to memory read and write operations with the exception that an I/O port is addressed instead of a memory location. The CPU issues the appropriate input or output control signal, sends the proper address and either receives the data being input or sends the data to be output.

Data can be input/output in either parallel or serial form. All data within a digital computer is represented in binary coded form. A binary data word consists of a group of bits; each bit is either a one or Ii zero. Parallel I/O consists of transferring all bits in the word at the same time, one bit per line. Serial I/O consists of transferring one bit at a time on a single line. Naturally serial I/O is much slower, but it requires considerable less hardware than does parallel I/O.

Interrupts

Interrupt provisions are included on many central processors, asa means of improving

the processor'sefficiency. Consider the case of a computer that is processing a large volume of data, portions of which are to be output to a printer. The CPU can output a byte of data within asingle machine cycle but it may take the printer the equivalent of many machine cycles to actually print the char- acter specified by the data byte. The CPU could then remain idle waiting until the printer can accept the next data byte. If an interrupt capability is implemented on the computer, the CPU can output a data byte then return to data processing. When the printer is ready to accept the next data byte, it can request an interrupt. When the CPU acknowledges the interrupt, it suspends main program execution and automatically branches to a routine that will output the next data byte. After the byte is output, the CPU continues with main program execution. Note that this is, in principle, quitesimilarto a subroutine call, except that the jump is initiated externally rather than by the program.

More complex interrupt structures are possible, in which several interrupting devices share the same processor but have different priority levels. Interruptive proces- sing is an important feature that enables maximum utilization of a processor'scapa- city for high system throughput.

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