| |
Krill
Registered: Apr 2002
Posts: 2980 |
Shortest code for stable raster timer setup
While working on my ICC 2019 4K entry (now postponed to ICC 2020, but i hope it'll be worth the wait), i came up with this (14 bytes):initstabilise lda $d012
ldx #10 ; 2
- dex ; (10 * 5) + 4
bpl - ; 54
nop ; 2
eor $d012 - $ff,x; 5 = 63
bne initstabilise; 7 = 70
[...]; timer setupThe idea is to loop until the same current raster line is read at the very beginning (first cycle) and at the very end (last cycle) of a raster line, implying 0 cycles jitter.
With 63 cycles per line on PAL, the delay between the reads must be 63 cycles (and not 62), reading $d012 at cycle 0 and cycle 63 of a video frame's last line (311), which is one cycle longer due to the vertical retrace.
The downside is that effectively only one line per video frame is attempted, so the loop may take a few frames to terminate, and the worst case is somewhere just beyond 1 second.
The upside is that it always comes out at the same X raster position AND raster line (0), plus it leaves with accu = 0 and X = $ff, which can be economically re-used for further init code.
Now, is there an even shorter approach, or at least a same-size solution without the possibly-long wait drawback? |
|
... 177 posts hidden. Click here to view all posts....
|
| |
Krill
Registered: Apr 2002
Posts: 2980 |
No worries, i actually took it for a mistake.
Though i wonder how that could happen, did you drunkenly type in some notes scribbled on a napkin, mistaking an A for a 4? =) |
| |
Raistlin
Registered: Mar 2007
Posts: 680 |
So you're suggesting there was a fault in Copyfault's copy? |
| |
Copyfault
Registered: Dec 2001
Posts: 478 |
Quoting RaistlinSo you're suggesting there was a fault in Copyfault's copy? Yeah, must be the handle, obviously ;) |
| |
Oswald
Registered: Apr 2002
Posts: 5094 |
0843 A2 9E LDX #$9E
0845 A0 08 LDY #$08
0847 E0 00 CPX #$00
0849 D0 F9 BNE $0844
could someone explain how this works ? :D |
| |
Krill
Registered: Apr 2002
Posts: 2980 |
Quoting Oswald0843 A2 9E LDX #$9E
0845 A0 08 LDY #$08
0847 E0 00 CPX #$00
0849 D0 F9 BNE $0844
could someone explain how this works ? :D Corrected version of the code and Copyfault's explanation annotated:
08A3 A2 9E LDX #$9E
08A5 A0 08 LDY #$08
08A7 E0 00 CPX #$00
08A9 D0 F9 BNE $08A4
Branching to $08A4 leads to SHX $08A0,Y with Y = 8, so the operand byte of the CPX #imm at $08A8 is altered continuously.
As long as the "&(hi+1)" plays in, the CPX #imm operand will be $9E & $09 = $08, which is not equal X = $9E, so branching back.
When "&(hi+1)" disappears, the full $9E is written to the CPX's operand byte and the loop will end (X = $9E with CPX #$9E will yield Z=1). This happens if and only if the critical SHX-cycle appears on a badline. |
| |
Oswald
Registered: Apr 2002
Posts: 5094 |
why does the &hi disappear on a badline? |
| |
chatGPZ
Registered: Dec 2001
Posts: 11386 |
thats what the opcode does - why exactly it happens is unknown, but its likely some analog effect |
| |
Krill
Registered: Apr 2002
Posts: 2980 |
Yes, something to do with DMA interference* disturbing the inner workings of that opcode. And it's a non-intended (illegal) opcode anyways, so short-circuiting inner logic to begin with. :)
* It has been observed that other 6502-based platforms without DMA (1541, e.g.) do not exhibit the &H-dropoff behaviour. |
| |
Copyfault
Registered: Dec 2001
Posts: 478 |
Quoting Oswaldwhy does the &hi disappear on a badline? In my text "badline" was short for "the 1st DMA-overtake-cycle @cycle#12 in every badline".
If the 4th cycle of the SHX abs,Y coincides with a DMA-overtake cycle, the &(hi+1) drops off, see e.g. post#88 or the latest No More Secrets V0.95.
Ofcourse all sprites must be turned off while the routine is running (they'd throw in other DMA-overtake-cycles and break the whole thing). So the basic trick (courtesy of Quiss;)) is to uniquely mark badlines with the DMA-overtake-cycles and loop until the beforementioned 4th cycle of the SHX lands on the correct cycle (or short: "on a badline";)). |
| |
Devia
Registered: Oct 2004
Posts: 401 |
What an awful thread.. fell down the rabbit hole, turned utterly insane, lost several days of sleep...
Quiss' original approach is short and elegant, but opcode rewriting confuses my old brain.
Adding a single byte to it's size, the flexibility can be increased somewhat (from 64 to 112 possible pages) and no opcode rewrites, just operand rewrite.
* = $xy00, where x=$0-$f and y=$0-$6
loop: ldy #$08
ldx #$0a
shx loop,y
jmp loop
$xx0A:
Loop times kept at either 10 or 12 cycles, depending on address choice.
As an added bonus, A is not touched - which may or may not matter in your overall timer setup.
So, it's obvious my priority is not size, but readability - and I find this approach a tad more readable ;-) |
| Previous - 1 | ... | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 - Next |
