Stellar Fortress v0.03

Stellar Fortress

Entry posted by Guest December 26, 2005

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cdk1.zipNew Features:- 5 rotating shield rows implemented.To Do:- Add the player ship and enemy cannon.The shield code is complete. This was challenging and quite fun to work on. The shield consists of 14 rows, with 4 pixels per row. There are 14 columns per row. There are now 5 rings to the shield, and each ring rotates in the opposite direction of its neighbors. The rotation requires a lot of bit shifting. At first I tried to brute-force it, but the code was getting ugly. In the end I used a series of bit masks, and the code is much more managable. It requires about 1800 cycles per pass, but this only needs to run every 16th frame, so there's plenty of compute time left.

RotateShield	LDA Frame	AND #%00001111; Rotate the shield only on this multiple.	BNE VBlankWait;aaaaaaa0 13 <^;bbbbbba0 12 >v;cccccba0 11 <^;ddddcba0 10 >v;eeedcba0 9  <^;  edcba0 8;  edcba0 7;  edcba0 6;  edcba0 5;eeedcba0 4  >^;ddddcba0 3  <v;cccccba0 2  >^;bbbbbba0 1  <v;aaaaaaa0 0  >^; Shift all bitfields, ignoring carry.; This will also shift the interior bits, but that will be cleared; by the clean up mask.	LDX #13ShiftSideways	LDA LShield,X	ASL	STA LShTemp,X	LDA RShield,X	ROR	STA RShTemp,X	DEX	LDA RShield,X	ASL	STA RShTemp,X	LDA LShield,X	ROR	STA LShTemp,X		DEX	BPL ShiftSideways;47 x 7 = 329 cycles; Clean up the horizontal and vertical side-effects of the bit shifting.	LDX #13CleanShiftedBits	LDA LCleanMask,X;4	AND LShTemp,X;3	STA LShTemp,X;4	LDA RCleanMask,X;4	AND RShTemp,X;3	STA RShTemp,X;4	DEX  ;2	BPL CleanShiftedBits;3    27 x 14 = 378 cycles	LDX #12ShiftUp	LDA LShiftOddUp,X;4	AND LShield,X;3	ORA LShTemp+1,X;3	STA LShTemp+1,X;4	LDA RShiftEvenUp,X	AND RShield,X	ORA RShTemp+1,X	STA RShTemp+1,X	DEX	BPL ShiftUp;33 x 13 = 429 cycles		LDX #1ShiftDown	LDA LShiftEvenDown,X	AND LShield,X	ORA LShTemp-1,X	STA LShTemp-1,X	LDA RShiftOddDown,X	AND RShield,X	ORA RShTemp-1,X	STA RShTemp-1,X	INX	CPX #14	BNE ShiftDown;35 x 13 = 455 cycles; Copy the temp buffer into the main shield memory.	LDX #13CopyShield1	LDA LShTemp,X	STA LShield,X	LDA RShTemp,X	STA RShield,X	DEX	BPL CopyShield1;------------------------------------------------------------	align 256;Left side, shift odd bits up.  Note, these are in reverse order of RAM.LShiftOddUp	dc.b #%00000010;0	dc.b #%00000010	dc.b #%00001010	dc.b #%00001010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00001010	dc.b #%00001010	dc.b #%00000010	dc.b #%00000010;	dc.b #%00000000;13 Placeholder.  Not actually used.;Right side, shift odd bits down.RShiftOddDown	dc.b #%00000000;0	dc.b #%00000010	dc.b #%00000010	dc.b #%00001010	dc.b #%00001010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00101010	dc.b #%00001010	dc.b #%00001010	dc.b #%00000010	dc.b #%00000010;13;Left side, shift even bits down.LShiftEvenDown	dc.b #%00000000;0	dc.b #%00000000	dc.b #%00000100	dc.b #%00000100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00000100	dc.b #%00000100;	dc.b #%00000000;13 Shares byte with next mask.;Right side, shift even bits up.RShiftEvenUp	dc.b #%00000000	dc.b #%00000100	dc.b #%00000100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00010100	dc.b #%00000100	dc.b #%00000100	dc.b #%00000000	dc.b #%00000000;Left side post-shift clean mask.LCleanMask	dc.b #%11111110	dc.b #%11111000	dc.b #%11111000	dc.b #%11100000	dc.b #%11100000	dc.b #%00000000	dc.b #%00000000	dc.b #%00000000	dc.b #%00000000	dc.b #%11000000	dc.b #%11110000	dc.b #%11110000;	dc.b #%11111100; Shares 2 bytes with next mask.;	dc.b #%11111100;Right side post-shift clean mask.RCleanMask	dc.b #%11111100	dc.b #%11111100	dc.b #%11110000	dc.b #%11110000	dc.b #%11000000	dc.b #%00000000	dc.b #%00000000	dc.b #%00000000	dc.b #%00000000	dc.b #%11100000	dc.b #%11100000	dc.b #%11111000	dc.b #%11111000	dc.b #%11111110

This implementation will be useful for assigning difficulty levels to the game. I can now easily define how many rings will be in the shield, by leaving unwanted shield sections blank. In the demo, ring #3 is blank. On the hardest level, all 5 rings will be enabled.