INMC 80 News

Lastly we come the title of this piece. Op-codes and operands. How does the processor know what it is about ? Now we’ve talked about the ‘PC’ register pointing at each byte of the program in turn. Provided the ‘PC’ is pointed at the first byte of a program, and provided that program is correct, it can’t go wrong. It works like this: each instruction may be between one and four bytes long. The first byte contains important information for the processor, this byte is called the ‘operation code’ or op-code for short. Firstly, it tells the processor how many bytes there are in the instruction. Then back to our railway analogy, it sets the points. This first byte operates the switches which route the data round the processor. In some instructions (the 4 byte ones), the op-code is two bytes long as there is too much for the op-code (or instruction) decoder to handle at once.

The bytes that follow are known as the operand, that is the bytes that need to be worked on. In some instances the operand may be data, like the LD A,10H instruction (above), the 10H is data. In other instances the operand might be an address offset like the JR NZ instruction. In many instances there is no operand at all, as all the information is implied like the DEC A instruction, where ‘A’ is implicit within the instruction, and a decrement simply means subtract 1 from it. The processor is clever enough to handle all that in a 1 byte instruction.

It can be seen that if instructions are given to the processor properly, then by the time the processor has finished with one instruction it is already pointing at the next (or has been pointed at another by a jump or call instruction). More often than not, with novice programmers, things start going wrong when a jump or call goes to the wrong place, or the program is allowed to run on into a data or unprogrammed area. The poor little processor doesn’t know something is wrong, it faithfully carries on and tries to interpret the instructions it finds. Suppose (because you programmed it wrong) a jump instruction sent the processor to the middle byte of a LD HL,0123H instruction. That looks like this:

21 23 01

Instead of loading ‘HL’ with 0123, it will first increment what is already in ‘HL’ because 23H is the instrtuction to increment ‘HL’, then, seeing 01H, it will think that that is the first byte of a LD BC instruction so the next two bytes will interpreted as the operand for the LD BC instruction, which they are not. We needn’t go on, the program is blown already. From then on the processor will run wild, actually, ‘wild’ is the wrong word. What the processor does is quite logical, if you could only find where it had gone. The processor was only doing what it was told, it can’t know that that is not what was intended. Really it’s the program which ran wild. The usual result of all this (determined by Sod’s Law) is that if the program was not saved on tape, it will write rubbish all over it, and then try to execute the rubbish, and so on, and so on. This is what is known as a ‘program crash’, and one thing will be impressive, that is the speed at which it can all happen. After hours of painstaking typing putting a program in, there is nothing so humbling as to conduct a postmortem on the program, that, because of a single typing error the processor managed to completely ‘scramble’ in a few thousandths of a second. You wouldn’t have had time to think about hitting the reset button, yet alone do anything about it.

So what have we learned. Object code listings. A glimpse at the insides of the Z80. How the registers are arranged, and op-codes and operands. In the next episode we will go on to discuss the workings of the other registers, and take a closer and more detailed look at some of the instructions.

One final bit of encouragement. This stuff is not easy, it’s rather like walking through a fog, and not even knowing where you are supposed to be going. Even now, if you are still totally lost, just keep ‘plugging away at it’. In my experience, and I am certain I am not unique, at some point, after one or two false starts, there will come a small glimmer of understanding. Suddenly everything becomes clear. All the facts fall into place, and although you may be wrong about some of the details, the ‘core’ or ‘heart’ of the problem is exposed and to your surprise, you will find that you had already learned most of what there is to know. You will kick yourself for not having grasped the concepts earlier.