Copper SKIP instruction

[sparhawk]

While the copper instructions are easy to understand in itslef, it's really cool what kind of stuff you can do with it. However, I was wondering what an actual use case would be where the SKIP instructions can be used. I don't see how much benefit it would be to skip the next move instructions, because you can only skip a single line which hast to be a MOVE, right?


I also don't see when the SKIP instruction would actually trigger, or is this only the case when the copper starts the blitter and waits for it? In that case I could see that the SKIP can trigger, but I don't see how a single MOVE can make much of a difference then.

[ross]

Originally Posted by sparhawk

While the copper instructions are easy to understand in itslef, it's really cool what kind of stuff you can do with it. However, I was wondering what an actual use case would be where the SKIP instructions can be used. I don't see how much benefit it would be to skip the next move instructions, because you can only skip a single line which hast to be a MOVE, right?


I also don't see when the SKIP instruction would actually trigger, or is this only the case when the copper starts the blitter and waits for it? In that case I could see that the SKIP can trigger, but I don't see how a single MOVE can make much of a difference then.

A hint: there are two copper pointers

[mc6809e]

Skip allows for conditional execution.

The logic of it can be confusing.

Often the instruction being skipped is a strobe to change the flow of execution. In effect this causes execution of the current list to continue if the condition is TRUE.

The current list may be a loop so the skip can be seen as skipping a break statement as in C.

[sparhawk]

Now the piucture becomes clearer. I was not aware when I started to with the copper, that the copper can wait for the blitter to finish which is probably the only condition that can cause the SKIP to trigger, right?

[Kalms]

Originally Posted by sparhawk

Now the piucture becomes clearer. I was not aware when I started to with the copper, that the copper can wait for the blitter to finish which is probably the only condition that can cause the SKIP to trigger, right?

Quoting from http://amigadev.elowar.com/read/ADCD.../node0049.html:

SKIP the next instruction if the video beam has already reached a specified screen position.

[sparhawk]

Originally Posted by Kalms

Quoting from http://amigadev.elowar.com/read/ADCD.../node0049.html:

I know what it does, but it was not clear to me, under which circumstances this condition occurres.

[ross]

Originally Posted by sparhawk

I know what it does, but it was not clear to me, under which circumstances this condition occurres.

It is a very sporadic use and in any case for complex situations.
Suppose you wrote a copper subroutine (which is not very common already).
You want to execute it only if you are in or after the 100th line of the screen, otherwise immediately returns to the main flow.

The subroutine could start with:

SKIP if yPOS >= 100
MOVE to COPJMP1 ;return
copper_subroutine
MOVE to COPJMP1 ;return

(I implied the prior setting of the 2 COPxLC pointers)

Off course durig subroutine execution you can completely change copper flux because you can 'jump around' with the two copper pointers.

[roondar]

I always wondered how close you can get to a 'full program' using the Copper. Using skip and move to COPJMPx you should be able to at least conditionally jump. It seems to me you should also be able to create loops. Perhaps other common programming constructs are also possible using skip/move?

Probably breaks down if you want to start doing math, though.

[ross]

Originally Posted by roondar

I always wondered how close you can get to a 'full program' using the Copper. Using skip and move to COPJMPx you should be able to at least conditionally jump. It seems to me you should also be able to create loops. Perhaps other common programming constructs are also possible using skip/move?

Probably breaks down if you want to start doing math, though.

Blitter can do some basic math (with big effort just to do a sum ).
And can modify copper list, so probably a full simple math program can be done.
But I certainly don't try it

[DanScott]

Originally Posted by roondar

I always wondered how close you can get to a 'full program' using the Copper. Using skip and move to COPJMPx you should be able to at least conditionally jump. It seems to me you should also be able to create loops. Perhaps other common programming constructs are also possible using skip/move?

Probably breaks down if you want to start doing math, though.

In "De Profundis" on the roto-zoom,it uses combinations of SKIPs and COPJMP's to create loops... it repeats each line of moves, 4 times

[Kalms]

Originally Posted by sparhawk

I know what it does, but it was not clear to me, under which circumstances this condition occurres.

Alright. Think about it like this: when the Copper reaches the SKIP instruction, it will test the blitter busy flag (if requested), and perform a masked comparison against the current VHPOSR register value. It will then increment the current copper read pointer by 4 or 8, depending on the comparison results. It will not wait for anything to happen.

[roondar]

Originally Posted by ross

Blitter can do some basic math (with big effort just to do a sum ).
And can modify copper list, so probably a full simple math program can be done.
But I certainly don't try it

Originally Posted by DanScott

In "De Profundis" on the roto-zoom,it uses combinations of SKIPs and COPJMP's to create loops... it repeats each line of moves, 4 times

Ah, I love the Amiga chipset sometime. This kind of stuff is very cool

I'm gonna go and think about Copper loops, Copper subroutines and Blitters doing basic math a bit now. Might come up with something cool. Though knowing me, it'll probably take a year

[ross]

Originally Posted by roondar

Though knowing me, it'll probably take a year

Fully acceptable

[NorthWay]

I had this rough idea that you could do a complete blitter+copper basic logic building blocks where you run a simple demo purely by them with the cpu halted with a STOP instruction.
"You can now remove your cpu."

[StingRay]

Originally Posted by NorthWay

I had this rough idea that you could do a complete blitter+copper basic logic building blocks where you run a simple demo purely by them with the cpu halted with a STOP instruction.

Something like this has been already done back in the early 90s, I think it was Amicom/Spreadpoint who made a demo that would be able to run without CPU once the init code was run.

[Toni Wilen]

Just a quick reminder that SKIP isn't actually skipping much:

"Skipped" instruction is still processed normally (DMA slot usage won't change, no DMA slots saved). Only difference is that if following instruction is MOVE, MOVE's register write becomes dummy write. If following instruction is WAIT or SKIP: it is executed normally.

[nogginthenog]

Stupid question: When the vertical beam reaches the bottom of the frame the copperlist restarts right?

[sparhawk]

AFAIK it restarts with the VBI.

[mc6809e]

Originally Posted by ross

Blitter can do some basic math (with big effort just to do a sum ).
And can modify copper list, so probably a full simple math program can be done.
But I certainly don't try it

The blitter can actually compute any computable function since it can implement cellular automata and there exist universal CA that can compute anything a Turing machine can.

An interesting example of a CA using the blitter to implement Conway's Game of Life is here.

[roondar]

Originally Posted by mc6809e

The blitter can actually compute any computable function since it can implement cellular automata and there exist universal CA that can compute anything a Turing machine can.

An interesting example of a CA using the blitter to implement Conway's Game of Life is here.

Fascinating stuff... Though probably not all that efficient for solving some problems I guess.