MICROPROCESSORS

Microprocessors Lecture 2

Revision/summary

The main components of a small computer system are:-

         MEMORY     - Stores the program and data required by the program
         INTERFACES - Communicate with the outside world
         PROCESSOR  - Carries out the program instructions one at a time

All data and instructions in any computer system are stored in binary. If they appear in any other form then a translation must have been carried out.

The meaning of a binary number depends entirely on how it is used, or on its context. E.g. 10000110 is clearly a binary number which can be translated into 86 in hexadecimal notation. If it is part of an 6809 program and is executed as an instruction then it would have the effect of 'Loading accumulator A using immediate addressing'. If it is used as data in unsigned arithmetic then it has the value 134 in decimal. If it is used in twos complement arithmetic, however, it has the value -122. Its negative because the most significant bit is a 1. If it sent to a vdu then it will be interpretted as an ASCII character and the letter N will be displayed. Alternatively it might be in the address part of an instruction and refer to a memory location. It might be sent to an interface connected to a pelican crossing and result in the green traffic lights being turned on, the red 'do not cross, signs being illuminated and the 'WAIT' signs being on.

This is a very important point which many people have difficulty in grasping - that a pattern of 1s and 0s has no intrinsic meaning it all depends on what you do with it.

MEMORY is a mass of registers where binary data can be stored. Some may be read-only; their contents cannot be changed, others may be read-write; what is stored can be changed whenever necessary. Every register has an address, which is also a binary number, by which it can be referenced to distinguish it from other memory registers. Surprisingly this address may not be unique. As we shall see later a memory register may have several different addresses. E.g. the register at address 0000 may also have address 4000 and 8000 and C000. What must not happen, and this is down to the system design, is that two memory registers have the same address otherwise when you read from that address you don't know which data you're getting.

INTERFACES - also appear as registers in memory but they are not simply passive memory locations where you always get back what was stored in them. They are connected to external devices so when you store a binary value in memory location FE00 for example, if that address is an interface to a set of LEDS, then the stored value will cause certain LEDs to be turned on and others to be turned off. When you read data from FE01, if that is an interface connected to a set of switches than the data is not what has been previously stored in that location but the setting on each of the switches. These days most interface devices are programmable which means they can perform a variety of functions. This makes the devices more flexible and means that the same interface device can be used in a wide range of applications. In order to customise the interface for one particular application it will frequently need to be set up by storing values in control registers. These again appear as memory registers.

The PROCESSOR - is obviously the most important part of the system and is where all the instructions are carried out. it takes the instructions one at a time, normally in the order in which they are stored in memory, and executes them. Generally speaking there are three groups of instructions -

Data Movement (or Memory Reference) - These instructions move data from one register to another, usually from memory registers to registers inside the processor and vice versa. E.g 'Load accumulator A with the contents of memory register 002E'.
Arithmetic & Logical - These operate on data usually already in processor registers to perform arithmetic operations such as addition or logical operations such as shifting one bit to the left.
Program Control (Jump and Branch) - These alter the program flow from the normal one instruction after another in the sequence stored in memory. E.g Jump to the instruction in memory register 201A. Often these are conditional so that program loops can be created. E.g. 'Go back 8 instructions if the result of the last arithmetic instruction was not zero'.

Early computers stuck fairly rigidly to these three groups but more modern processors, like the 6809, blur the distinctions by having instructions which combine two operations into a single instruction like 'Add the contents of memory register 201A to accumulator B'. This is both data movement and arithmetic in one instruction.

Registers

In order to achieve this the processor has a number of internal registers -

Data Registers - These are registers where data can be manipulated by the arithmetic and logical instructions. operations which are carried out on these registers are usually fast because they do not require reference to data held in external memory registers. The 6809 has only two of these which are called Accumulator A and Accumulator B. Since the 6809 can only perform arithmetic operations on 8 bits at one time (it has an 8 bit ALU) these registers can only store 8 bits. All memory registers also store 8 bit data. More recent processors have many more data registers, for example the 68000 has 16.
Address Registers - These hold addresses for use in data movement instructions. Addresses in these registers can also be manipulated e.g. one added to them so that a table can be stepped through. The 6809 has two of these called Index Register X and Index Register Y. In order to enable a reasonable amount of memory to be addressed all addresses are 16 bits. It follows that these address registers must be 16 bits. The 68000 has 16 address registers.
Program Counter - This register keeps track of where the processor is in a program. It stores the address of the next instruction which is to be executed. Normally this is stepped up by one automatically as each instruction is carried out. An instruction which causes a jump to a different part of the program obviously modifies this process and operates directly on the program counter. Since this register stores an address it must be a 16 bit register in the 6809.
Instruction Register - This holds the binary instruction word while it is being executed in the processor.' Therefore the first stage in carrying out an instruction is to move the instruction from the appropriate memory register (the PC says which one) into the instruction register. Different parts of this instruction word then indicate different actions. For example in an arithmetic instruction four bits control the ALU to set it into the correct mode for add, subtract, shift etc.
Temporary registers - In addition the processor requires various temporary registers to hold transitory data while an instruction is being carried out. For example an instruction which 'Loads data from memory register 001B into accumulator A' must read the part of the instruction which defines the address which must then be held somewhere so that it can be used to fetch the appropriate data from memory. A temporary register is used for that purpose. An instruction which exchanges the contents of accumulators A and B also needs to store the value in one of the accumulators so that it is not lost in the exchange.
```
                        (A)    ->   (temp)
                        (B)    ->     (A)
                      (temp)   ->     (B)
```
Other registers - We will see in due course that other registers are necessary to make the processor more versatile. We will deal with them as we come to them.

User-programmable registers in the M6809

Buses

These are data paths which enable data to be transferred from one device to another in the system. In a computer system these are normally parallel in nature i.e. to transfer 8 bits of data we need 8 wires (or 8 tracks on a PCB) connected to each device. There are two such buses which are necessary.

Address Bus - Connects the processor to memory devices and interfaces. It is used by the processor to indicate which memory register (or interface register) is the source or destination for data. In 6809 it has to be 16 bits. Since the address is always supplied by the processor (except in DMA) the processor always outputs data on this bus and the other devices always input or receive data.
Data Bus - This is used to transfer data between the processor and memory registers and interface registers. The source of data may be the processor or it may be a memory register or an interface register: To avoid damaging the devices or scrambling data only one device may place data on the bus at a time. This will be either the processor or the device addressed by the binary pattern currently on the address bus. Normal logic gates have only two output states high (1) or low (0). Gates connected to a bus of this type must have a third high-impedence state when they are effectively not outputting anything but allowing another device to output data. The data bus in a 6809 system has 8 bits.

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ACKNOWLEDGMENT: Some of the above was taken from Dr A J Tollyfield's notes.