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mhindsbo
Registered: Dec 2014
Posts: 51 |
Code optimization
I thought I would tap into the creativity here to see if you have optimization suggestions for the following.
For my game I have a list of objects. When an object spawns I set bit 7 in the first byte of the table so I dont spawn it again while it is active (or if it is destroyed). If the object leaves the screen without being destroyed I flip the bit back again.
Each active objects stores the address of its location in the spawn table and uses the following code to 'reactivate' itself if/when it leaves the screen.
lda object_d6,x ; lo byte of address
sta tempz+0 ; location in zero page
lda object_d7,x ; hi byte of address
sta tempz+1
ldy #0
lda (tempz),y
and #%01111111 ; clear bit 7
sta (tempz),y
That is 29 cycles +3 if page boundary is crossed. Any optimization ideas?
I could of course just add another byte in the object table so I dont have to set/clear a bit, but that adds potentially hundreds of bytes ekstra for a given level. |
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Icon
Registered: Jul 2012
Posts: 2 |
You could preset tempz + 0 to zero (perhaps during initiation of the game) and do the following:
ldy object_d6,x ; lo byte of address
lda object_d7,x ; hi byte of address
sta tempz+1
lda (tempz),y
and #%01111111 ; clear bit 7
sta (tempz),y
Then you are down to 24 cycles and the code is 4 bytes shorter. |
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Frantic
Registered: Mar 2003
Posts: 1648 |
...or you could move that byte out of the object and keep it in a table, and just do:
lda objectflagtable,x
and #%01111111 ; clear bit 7
sta objectflagtable,x
Not sure if that would work in your case, as it may break your approach of using "objects", but generally speaking it seems like a possibility. |
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ChristopherJam
Registered: Aug 2004
Posts: 1409 |
What is the range of values stored in that first byte? (clearly it's at most 0..127 or you couldn't safely trash bit 7)
If you're also not using bit 6 you could store (0x40+value) for an inactive object, (0x80+value*2) for an active one.
Then you can just use ASL and LSR to activate/deactivate. Combining with Frantic's suggestion, your code then becomes
lsr objectflagtable,y ; 7 cycles
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mhindsbo
Registered: Dec 2014
Posts: 51 |
Thanks. Some good suggestions here. I should perhaps have mentioned that XR refers to the active objects of which there are ~30.
All objects sit in a list linked to the map so they can be spawned when the player is at a specific location. The address I store relate to this table that contains hundreds of objects.
If the total number of objects ends up less than 256 I can use the object flag table idea. But could end up exceeding it. |
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Raistlin
Registered: Mar 2007
Posts: 684 |
Could the information be stored in a bit field? With 30 objects you could squeeze this into just 4 bytes and use something similar to the following code (17 cycles, 64 bytes of data and 12 bytes of code). Untested - but I -think- it should work..?
.byte Div8table:
.fill 32, i/8
.byte NegBitMask:
.fill 32, 0xFFFFFFFF - (1<<(i&7))
x is index 0-31
ldy Div8table, x //; 4 cycles
lda AddrTable, y //; 4
and NegBitMask, x //; 4
sta AddrTable, y //; 5 |
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Raistlin
Registered: Mar 2007
Posts: 684 |
Worth also saying ... you’d need an additional table with my suggestion of the regular bitmasks:-
BitMask:
.fill 32, 1<<(i&7)
And any other code for setting the bit and testing it becomes much simpler too. |
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Hoogo
Registered: Jun 2002
Posts: 105 |
The code looks nice, but all the tables take so much memory, 2 bytes of table for every used bit. In that case it would be better to spend a whole byte for the flag. Such a flag could also be easily reset by lsr, set by sec:ror and tested by bit. |
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Raistlin
Registered: Mar 2007
Posts: 684 |
True... it would only make sense if it helped in other parts of the code as well... if there were more flags other than Activated/Deactivated... |
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Oswald
Registered: Apr 2002
Posts: 5095 |
"If the total number of objects ends up less than 256 I can use the object flag table idea. But could end up exceeding it."
if exceeds it then use 2 object flag tables :)
Also it would be better to separate all object attributes into such tables, and just use ,x to reach any of your objects.
ldx #objectnr
lda xcoordinatetable,x
lda ycoordinatetable,x
lda isobjactivetable,x
etc. |
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Raistlin
Registered: Mar 2007
Posts: 684 |
Yep, Oswald’s right. Having 30 current objects makes this perfect - increase that to 32 (or pad by the extra 2) and align the data so that you never cross pages - that way your reads are always 4 cycles, writes are 5. Try to avoid indirect read/writes where you can. |
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