The startup address is read by the address decoder. The decoder then applies a chip select signal on the Control bus. This signal then selects a ROM chip. All other chips ignore the signal since they have not been selected. The address enters the ROM chip and the row and address decoders access a memory cell. The contents of that cell are put on the data bus to be read by the microprocessor.
The second read
The microprocessor sends out the next address onto the address bus. The address decoder will activate the ROM chip again. The ROM chip will put some more data on the data bus, which will again be read by the microprocessor.
What does the microprocessor do with all this data?
If the first read of the ROM resulted in the data C0H and the second read produced 86H, the microprocessor combines the two to produce a 16-bit address, C086H. Remember that all this will be in binary – the use of hex is just to help us see what is happening. The microprocessor places this new address C086H on the address bus. The choice of C086H is not made by the microprocessor designer.
This number is chosen by the system designer – that is, the person that incorporates the microprocessor into a microsystem.
Then what?
The address, C086H in our example, accesses a ROM chip, which sends a binary number back to the microprocessor along the data bus. Inside the microprocessor, this number is interpreted as an instruction. Comparing the incoming number with an internal list of built-in control codes discovers exactly what the instruction is. This is all performed by the micro-program.
What happens next will depend on what the instruction was.
Why?
Why was it an instruction? Because the microprocessor said so!
Microprocessors assume that the first binary number that arrives represents an instruction so if we wanted to give it the instruction ADD 25H then the binary code meaning ADD will go in first, followed by the data to be used, in this case 25H.
What if we made a mistake and put the 25 in first, followed by the binary code for ADD in our program?
The microprocessor would interpret the 25 as an instruction – which could mean anything at all, or nothing. If this mystery instruction needed a number, it would use the binary equivalent of our ADD input – which again could be anything. If the false instruction did not need a number, then ADD instruction would be correctly read as an ADD instruction but it would then take the next available binary input as the number to be added. Our microprocessor has now carried out an incorrect instruction using incorrect data and the program could do almost anything. The whole program has got out of step and may do something quite unexpected. Not much fun in our dynamite factory.
The address being used at any time is known to the microprocessor by referring to the current value in the program counter, which is increased by one every time an instruction or some data is used.
This is the order of operations by a microprocessor and is the cause of all the confusion in the last paragraph.
The microprocessor applies no intelligence at all. It follows the pattern shown in Figure 8.11 regardless of whether it is following the program or it has been fed with a program with an error in it and is now carrying out a totally useless set of random instructions.
Figure 8.11 The fetch–execute cycle
It will follow our instructions so don’t blame the microprocessor – it’s up to us to feed it with something sensible. Always remember GIGO – garbage in, garbage out.
As we have seen, there is nothing inherently different between an instruction and data. They are both binary numbers and the interpretation is only a matter of what the microprocessor is expecting at that particular time.
In each case, choose the best option.
1 The stack is:
(a) an abbreviation for stack pointer.
(b) a series of RAM locations that can be used by the microprocessor to store data.
(c) a collection of programs.
(d) a chimney.
2 A CPU:
(a) is an essential part of any microprocessor.
(b) is an abbreviation for computer processing unit.
(c) can contain more than one MPU.
(d) is a function of a register.
3 Adding the binary numbers 1100 1100 to 0010 1001 within a Z80180 microprocessor would result in C and H flags being in the condition:
(a) C clear and H clear.
(b) C clear and H set.
(c) C set and H clear.
(d) C set and H set.
4 The interrupt vector register (I) in the Z80180 microprocessor is used to store the:
(a) high byte of the interrupt address.
(b) start up address for the microprocessor.
(c) low byte of the interrupt address.
(d) stack pointer address.
5 The fetch–execute cycle:
(a) works in hexadecimal.
(b) assumes the second, fourth, sixth etc. inputs are data.
(c) assumes the first input is an instruction.
(d) is a system used by the Instruction register to channel information to the correct part of the microprocessor.
9. Programming – using machine code and assembly
We can use a microprocessor as part of a computer or as part of a dynamite process controller and the only essential difference is in the instructions that it is given.
Since the microprocessor has no intelligence at all, it relies entirely on following a sequence of instructions as we discussed in the fetch-execute cycle.
If you were sitting in a lecture theatre and the speaker said, ‘We’ll finish now’, you would know that it was time to pick up your bits and pieces and leave the room. Those words were not the only possibility, other instructions would have served the same purpose like, ‘Right, that’s it, thank you’; ‘We’ll break here’; ‘It seems a good time for a break’; ‘We’ll stop at this point and continue at the next session’, and many other variations. Teachers can clear a whole classroom instantly just by looking at their watch and saying, ‘Well, …’. There are literally dozens of ways of telling people that it is time to leave a room even before resorting to the less polite possibilities.
Microprocessors, on the other hand, have nothing like this degree of flexibility, in fact they have almost no flexibility at all. It is very frustrating, but we do not share any common language with a microprocessor (Figure 9.1). Each microprocessor has a built-in list of instructions that it can understand. This list is called its ‘instruction set’ and may consist of about a hundred or so instructions which must be put together in the right order to carry out the function required. This is the job of the programmer and is similar to a builder, constructing a house by putting a lot of simple pieces like wood, tiles, bricks in the correct order. When the microprocessor is designed, the instruction decoder recognizes these inputs and starts an internal process that allows the microprocessor to carry out the instruction. This presupposes that the microprocessor is familiar with the instruction or, to put it another way, the instruction is in a language that is understood by that particular microprocessor.
Figure 9.1 A small communication problem
As we have seen, we cannot talk directly to a microprocessor and, even worse, microprocessors often cannot talk directly to each other (Figure 9.2). This is particularly the case when the microprocessors have been developed by rival organizations.
Figure 9.2 They are all saying ‘ADD’