I'm working on a routine for an Atari 2600 game. The portion of the code is the 192 visible scanlines. I'm attempting to achieve a visual effect by updating the background every other scanline.

The scanline loop simply counts from 192 to 0. In this scheme, #192 represents the top line, #0 the bottom or last line. It seems that there should be a way to determine odd / even numbers to accomplish this. I'm not sure if this is considered "parity" or not, but I haven't found the right flag for odds / evens.

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http://www.thestarrlab.com

Parity is a count of the number of set bits. For oddness/eveness, just test the lowest bit. If it is 0, it's even. if it is 1 it is odd.

Edit: But from the sounds of it you just want to update all the even lines (say) then the odd ones. It is so simple I must be misunderstanding, but can't you simply add 2 on each loop iteration?

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8 bit fun and games: https://www.aslak.net/

It's very simple:

Code:

         lda scanline          ;current scan line number
         lsr a                 ;shift bit 0 to carry
         bcc is_even           ;even number
         bcs is_odd            ;odd number

Alternatively, with the 65C02/65C816:

Code:

         lda scanline          ;current scan line number
         bit #%00000001        ;is bit 0 set?
         beq is_even           ;no, even number
         bne is_odd            ;yes, odd number

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Of course! Shift right, check carry. That's what I get for overthinking it huh?

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http://www.thestarrlab.com

or AND#1, BNE/BEQ. The AND# takes 2 clocks, and if the conditional branch is not taken, it's 2 more, or if the branch is taken, it's usually 3 more instead of 2. You can probably get away with one branch instruction, as the drop-thru is automatically the other condition.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

BIT immediate is the least destructive and produces the same result.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

But you can't do that on a proper 6502!

If all you're interested in is odd/even and not the particular scanline, you can do this on all 65xx-family processors:



It's not as fast as load-and-shift, though.

BTW, 0..192 is 193 lines, not 192. Just sayin'.

Right. I was thinking 'C02 or '816. BIT immediate is one of the many reasons to not use the NMOS 6502 if you have a choice.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

If you store the value 1 to a zero page memory location const1, then you can do "BIT const1" to perform the test without modifying the accumulator on ye olde 6502.

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

I was thinking the same thing, White Flame. Or just find a 1 somewhere in your program or the firmware ... there are bound to be several. Code-as-constants is a tried-and-true tradition in the 65xx world, even more so than self-modifying.

Mike