INMC 80 News


February–April 1981, Issue 3

Page 34 of 55

Actually, all this ‘groups’, ‘immediate’, ‘indirect’, etc, lark is not all that relevant. It’s only when it comes to trying to describe the functions of the instructions to someone else that it really matters at all. For instance, I use these terms so rarely that I had to get the book out to look them up when writing the previous paragraph. The important part to learn is the fact that instruction mnemonics are always written destination first, and that brackets indicate that we are interested in the CONTENTS of the location ‘pointed at’ by the address (immediate or indirect) supplied and not the address itself. No brackets means we are shunting data about inside the processor.

End of the theory bit, and back to the story. First I must locate the middle of the screen. Well there was a memory map that gave the screen area as being from 0800H to 0BFFH. Good, the middle must be 09FFH (and a half). Well I’ll be happy if it’s anywhere on the screen at this stage. Now how to get an asterisk there. Fumble through the Z80 technical book, and there is an instruction which is LD (nn),A. Now if I interpet the syntax of that correctly, it means, load nn with the contents of A. But (thinks) I don’t know what A has in it in the first place. Fumble, fumble, ah-ah!! LD A,n. That must mean stick n in A. Well whats the code for an asterisk. Easy, look it up in the character set table. Good, I’m getting somewhere. If L write the following code, I should get an asterisk in the middle of the screen.

3E 2A        LD A,2A
32 09 FF     LD (09FF),A

So using the ‘M’ command I enter 3E 2A 32 09 FF, starting at memory location 0D00H, (having already ‘sussed’ that that was a RAM area I could play with). Exit from the ‘M’ command and execute the program by typing E 0D00. Well, not quite what I expected. The screen filled with any old rubbish, and having pressed RESET, even the program had disappeared. So back to the drawing board. Oh dear, what a stupid mistake, I forgot to tell the thing to stop. So I type it in again and tag 76 (a HALT instruction) on the end. Re-execute it, – and – , nothing!!! Hitting RESET, and re-examining the program (at least it’s still there this time), what’s wrong now? Back to the technical manual; what’s this about two byte operands having to be entered low byte first? (What ever that means.) Well I think a ‘two byte operand’ might be the 09FF bit of the LD (nn),A instruction. Worth a try. Retype it, 3E 2A 32 FF 09 76. Re-execute it, Oh heck (or similar words) still no asterisk on the screen. What can I have done wrong now. Looking closely at the video map in the construction manual, I notice that the video addresses are not continuous. That is, they start at 080AH to 0839H and then the next line starts at 084AH. What happened to addresses 083AH to 0849H? A little mind boggling this, so I work out 16 lines times 48 characters across equals 768. Now even I know that there are 1,024 bytes in 1K of memory (and the book says it has a 1K video RAM) what happened to the other 256 characters? Well after more puzzling T decide that there must be a hidden margin in the screen, and if thats so, then my asterisk must have been put there. So I set to, using the video map this time, and recalulate where the middle of the screen is. I end up with 09E2H, re-enter the program and try it. Ye Gods!!!! There’s an asterisk in the middle of the screen. It only took all evening to put it there.

Fantastic, I’ve written a program. It did what I wanted (at last). The most important lesson learned was that two byte operands are entered low byte first. Now to make it flash on and off. Simple methinks, put a space back at the same address I put the asterisk. The code for a space is 20H, so now my program looks like this.

Well having typed it in, I execute it. Nothing again!!! What the ..... Oh yes, it did it so fast I didn’t see it happen. What I must do is replace the HALT instruction with a jump back to the start. Ok, so I look up a jump. Yipes, I’ve got a choice of ‘jumps absolute’, ‘jumps relative’, jumps on zero, jumps on no zero, carries, no carries, and goodness only knows what else. Well JP on looks easiest. ‘nn’ must be where to jump

This is an OCR’d version of the scanned page and likely contains recognition errors.

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