Z80 things that weren't obvious that you wish you'd known earlier

[Morkin]

I still can't get my head around:

Code: Select all

LD H,(HL)

Just hurts my brain for some reason.

Why? It’s no different than, say, IN A,(*) (where the full port address is formed by the contents of A placed in the top eight bits).

I'm not sure - it feels like you're trying to scoop water out of a bucket, using the same bucket that's holding the water...

[Ast A. Moore]

Why? It’s no different than, say, IN A,(*) (where the full port address is formed by the contents of A placed in the top eight bits).

I'm not sure - it feels like you're trying to scoop water out of a bucket, using the same bucket that's holding the water...

I see what you mean, but in fact the instruction takes several machine cycles to execute, so by the time H is loaded with the value of the address pointed to by HL, the HL has already been used up, as it were. There are two internal registers in the Z80—W and Z—that are used to temporarily hold some values. They are invisible to the programmer, but they’re there.

[Ast A. Moore]

Not strictly Z80, but it took me a very long time to realise the benefits of fixed point arithmetic in writing a 2D game. I was always thinking in terms of pixels and could never really get the kind of smoothness I wanted.

Having read how Super Mario does it, the answer is obvious. Use 16-bit math but only pay attention to the top 8-bits as an on screen position (effectively the lower 8-bits being the decimal portion). Now you can manage much smoother speeds etc without sacrificing performance.

Yes, that’s the approach I use in Yankee. Sometimes, though, there’s an issue with rounding, but overall it works great. I use it to calculate the position of almost all the sprites.

[presh]

fixed point arithmetic in writing a 2D game ... Use 16-bit math but only pay attention to the top 8-bits as an on screen position (effectively the lower 8-bits being the decimal portion). Now you can manage much smoother speeds etc without sacrificing performance.

That's how I'm doing things in my overhead driving engine. The decimal part is only considered for movement, literally everything else uses the integer part only (as if all values had been floor()ed first)

For anyone interested, [mention]ZedaZ80[/mention]'s got a wonderful collection of fast routines for 8.8 Fixed Point maths, you can find them here - they are the filenames with a _88 suffix

[flatduckrecords]

This thread is a goldmine, thank you all for sharing.

Having read how Super Mario does it, the answer is obvious. Use 16-bit math but only pay attention to the top 8-bits as an on screen position (effectively the lower 8-bits being the decimal portion). Now you can manage much smoother speeds etc without sacrificing performance.

And so, indeed, does Sonic! By coincidence Jon Burton posted a new Coding Secrets video today which demonstrates the same technique in 68000 assembly (assuming I’ve understood and recognised it correctly!)

I’ve been working on some Spectrum pixel-plotting and rudimentary sprite-drawing routines recently so I’ll certainly give this approach a try. Thanks!

[Dr beep]

Z80 things that weren't obvious that you wish you'd known earlier:

When I started programming with Z80, I thought that using IM2 would be a symbol of professional developing... but I never found a real use or utility to implement interrupts: it makes code run so slow, avoids to have sounds meanwhile playing... Never understood the reason for using IM2. My Anteater game doesn't use it at all ( well, only for detecting the coin at the beginning )

So far I have used an IM 2 for two reasons

1) in my emulators (ZX81 and videopac) the intrupt is used to read the keyboard and update the screen
2) in a game I set a 20 seconds timer. During the time the computer does calculations but after this time the best result counts. The IM-2 stops the calculation.

A third option is to play music

[Ralf]

Never understood the reason for using IM2

You may use IM2 just not to use other modes. And actually have an empty interrupt routine

This way you get free from Basic interrupts which, as was said here before, change registers, use system variables (so you can't overwrite them) and do a lot of other stuff that you may don't want in your program.

Some may say - then disable interrupts. But sometimes it's not possible. You may need interrupts to use HALT instruction and synchronise
your sprite drawing with the screen refresh and at the same time don't need to do anything useful in interrupt code.