ELET2100 - Microprocessors I

The 8085 Microprocessor

Well, finally, this is what all the previous lessons have been getting you ready for. To start looking at the micro we will be using in this course. All the previous lessons have been laying the ground work and basic concepts common to most micros. The one we will be using is the INTEL 8085 microprocessor. This chip was the last 8-bit general purpose CPU made by INTEL and has 40 pins.

The address bus requires 16 pins and the data bus requires 8 pins but INTEL cleverly decided to share or multiplex these two busses so the data bus share the lower (A0-A7) 8 pins of the address bus. This caused no problem since address and data are never on the bus at the same time.

Two pins are for serial communications with the 8085. Through these pins, serial data can be sent or received with another computer. This is how we will load a program into the 8085 kit used in the lab, from a PC.

Five more pins are for a different kind of input called interrupts. In our example in lesson 1 of the program where we are standing at the street corner, watching the light and the traffic, if a person walked up and tapped us on the shoulder and asked what time it is, this would be an example of an interrupt. It doesn't alter the program we are doing, it just temporarily stops us while we tell the time to the person. As soon as we tell the time, we go back to watching the lights and traffic as before. This describes the action of an interrupt.

The interrupt has a program associated with it to guide the micro through a problem. In the case of the above example, this program would be to look at our watch, read the time, and then tell it to the person. This is called an interrupt service routine (ISR). Each time an interrupt occurs, the current program is temporarily stopped and the service routine is executed and when complete, returns to the current program. We will spend a lot more time later describing interrupts and how we'll use them.

Inside the 8085 there are 10 seperate registers. They are called A, B, C, D, E, H, L, PSW, PC, and SP. All but the PSW, PC, and SP registers are used for temporary storage of whatever is needed by the program. The accumulator called A is also different from the other registers. It is used to accumulate the results of various instructions like add or sub (subtract). The Program Counter (PC) we have already mentioned while the Stack Pointer (SP) actually holds addresses and is 16 bits wide. All the others are 8 bits wide.

There are other features to be covered later, as they come up. In the next lesson we will start looking at assembly language, the method we will use to write a program.

On to lesson 6.

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