EricBall's Tech Projects

Good to hear you are continue developing Space War.

Your starfield approach looks interesting I am looking forward to the result.

The biggest thing to watch for when using that style of sine-wave generator is to ensure that numerical roundoff errors don't cause things to go wonky. I'd suggest writing a simple program on the PC to find out what your calculations are going to do before you code them on the Atari.

The biggest thing to watch for when using that style of sine-wave generator is to ensure that numerical roundoff errors don't cause things to go wonky. I'd suggest writing a simple program on the PC to find out what your calculations are going to do before you code them on the Atari.

Yep... I did exactly that. Some spreadsheet testing showed that simply using the MSB for the 1/64K caused the peaks to slowly decay. I thought that fiddling with the Carry bit as a kind of rounding might work, so I made up a small C program to test it out. (More control than the spreadseet & I could let it run longer.) It works perfectly, staying within bounds and not decaying.

 

EP_Y_T	DS	3; working space for EP_Y calculation
EP_Y	DS	3; current value MSB FIRST
EP_Y_P	DS	3; previous value

EP_Y_CALC	; y[n+1] = (2-1/65536)y[n] - y[n-1]
LDX	#2
SEC
1$	LDA	EP_Y,X
SBC	EP_Y_P,X
STA	EP_Y_T,X
DEX
BPL	1$
BIT	EP_Y
BPL
SEC		; EP.Y negative, sign extend
LDA	EP_Y_T+2
SBC	EP_Y+2
STA	EP_Y_T+2
LDA	EP_Y_T+1
SBC	#-1
STA	EP_Y_T+1
LDA	EP_Y_T
SBC	#-1
STA	EP_Y_T
BIT	EP_Y_T+1
BPL	5$	; round down
SEC		; round up
BMI	6$
2$	BIT	EP_Y_T+1
BPL	3$
SEC		; round down
BMI	4$
3$	CLC		; round up
4$	LDA	EP_Y_T+2; EP.Y positive
SBC	EP_Y+2
STA	EP_Y_T+2
LDA	EP_Y_T+1
SBC	#0
STA	EP_Y_T+1
LDA	EP_Y_T
SBC	#0
STA	EP_Y_T
5$	CLC
6$	LDX	#2
7$	LDA	EP_Y,X
STA	EP_Y_P,X
ADC	EP_Y_T,X
STA	EP_Y,X
DEX
BPL	7$

 

The code isn't complex, just a little long to handle both the sign extend and to handle the rounding for both positive and negative values.