Page 192 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes Pressed or when the mouse is moved. Software interrupts are generated by a program requiring
disk input or output.
An internal timer may continually interrupt the computer several times per second to keep the

Time of day current or for time sharing purposes. When an interrupt occurs, control is transferred
to the operating system, transferred to the operating system, which determines the action to be
taken. Interrupts are prioritized; the higher the priority, the faster the interrupt will be serviced
Basically, a single computer can perform only one computer instruction at a time. But, because it
can be interrupted, it can take turns in which programs or sets of instructions that it performs.
This is instructions that it performs. This is known as multitasking.
An operating system usually has some code that is called an interrupt handler. The interrupt
handler prioritizes the interrupts and saves them in a queue if more than one is waiting to be
handled.

The operating system has another little program, sometimes called a scheduler. The operating
system has another little program, sometimes called a scheduler.

14.1 8085 Interrupts

Interrupt is a process where an external device can get the attention of the microprocessor. The
process starts from the I/O device and is asynchronous.

Interrupts in 8085 microprocessor are classified into Hardware interrupts and Software interrupts.
1. Hardware interrupt - TRAP, RST7.5, RST6.5, RST5.5, and INTR.
2. Software interrupt - RST0, RST1, RST2, RST3, RST4, RST5, RST6, RST7.

Intel 8085 microprocessor is the next generation of Intel 8080 CPU family. In addition to being
faster than the 8080, the 8085 had the following enhancements:
• Intel 8085 had single 5 Volt power supply.

• Clock oscillator and system controller were integrated on the chip.
• The CPU included serial I/O port.
• Two new instructions were added to 8085 instruction set.
The CPU also included a few undocumented instructions. These instructions were supposed to
be a part of the CPU instruction set, but at the last moment they were left undocumented because
they were not compatible with forthcoming Intel 8086.

Interrupt is a signal send by external device to the processor so as to request
the processor to perform a particular work.

14.2 Type 8085 Interrupts

The 8085 has facilities for servicing interrupts similar to the 8080. The functional items required
are an Interrupt Request (INTR) pin, an Interrupt Acknowledge (INTA) pin, an Interrupt Enable
(INTE) pin; eight interrupt vectors in low RAM, and the Restart instruction. These perform in the
same way as the 8080 interrupt system. Here is a brief review:
1. A program is running normally in the system. The 8214 Priority Interrupt Controller or similar
circuit has its compare mask set to some priority level. The Interrupt Enable bit has been set
on by some previous routine, enabling interrupts.

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