How do we check if two bits are different?

[+Random Terrain]

I know we can't use this:

 

if a{0} <> a{1} then blah blah blah

 

If I knew how to compare two bits, I forgot and I don't see it on the bB page.

 

The only thing I can think of is this:

 

if a{0} then if !a{1} then goto __Pause_Game

 

if !a{0} then if a{1} then goto __Pause_Game

 

Is that the best way to do it?

 

 

 

Thanks.

Edited September 28, 2011 by Random Terrain

[RevEng]

You can xor them together. If they differ, the result is 1. If they are the same, the result is 0.

 

As far as efficiency, if the bits are neighbors you come out ahead 9 bytes of rom space, for your example...

 

temp1=a^(a/2)
if temp1{0} then goto __Pause_Game

[+Random Terrain]

You can xor them together. If they differ, the result is 1. If they are the same, the result is 0.

 

As far as efficiency, if the bits are neighbors you come out ahead 9 bytes of rom space, for your example...

 

temp1=a^(a/2)
if temp1{0} then goto __Pause_Game

Thanks. Since it uses division, does that mean it's a little slower?

[RevEng]

Yours is a bit faster because the comparisons avoid some work that mine does in constant time. (a division by 2 in this case only adds 2 cycles) I think your worst case scenario is 24 cycles, and mine is 33 cycles.

[Mr SQL]

You can xor them together. If they differ, the result is 1. If they are the same, the result is 0.

 

As far as efficiency, if the bits are neighbors you come out ahead 9 bytes of rom space, for your example...

 

temp1=a^(a/2)
if temp1{0} then goto __Pause_Game

Thanks. Since it uses division, does that mean it's a little slower?

RT,

Bitwise operations are fast but I think it would be easier to just evaluate the bitwise expression if it is handled:

if (a{0} ^ !a{1}) then goto __Pause_Game

[+Random Terrain]

Yours is a bit faster because the comparisons avoid some work that mine does in constant time. (a division by 2 in this case only adds 2 cycles) I think your worst case scenario is 24 cycles, and mine is 33 cycles.

OK, thanks. I better put this on the bB page too.

 

 

 

RT,

Bitwise operations are fast but I think it would be easier to just evaluate the bitwise expression if it is handled:

if (a{0} ^ !a{1}) then goto __Pause_Game

Thanks, but I don't think we can do that with batari basic:

 

http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#bit

[Emehr]

You can xor them together. If they differ, the result is 1. If they are the same, the result is 0.

 

As far as efficiency, if the bits are neighbors you come out ahead 9 bytes of rom space, for your example...

 

temp1=a^(a/2)
if temp1{0} then goto __Pause_Game

Can you just bit-shift right once instead of dividing by 2? Or would that use the same number of cycles? (does bBasic support bit shifting?)

[+Random Terrain]

Can you just bit-shift right once instead of dividing by 2? Or would that use the same number of cycles? (does bBasic support bit shifting?)

We can use assembly language:

 

http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#assembly

 

But I usually try to keep things in bB mode for the bB page to avoid confusing new users. Unless it's something cool like this:

 

http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#points_roll_up

[Mr SQL]

Yours is a bit faster because the comparisons avoid some work that mine does in constant time. (a division by 2 in this case only adds 2 cycles) I think your worst case scenario is 24 cycles, and mine is 33 cycles.

OK, thanks. I better put this on the bB page too.

 

 

 

RT,

Bitwise operations are fast but I think it would be easier to just evaluate the bitwise expression if it is handled:

if (a{0} ^ !a{1}) then goto __Pause_Game

Thanks, but I don't think we can do that with batari basic:

 

http://www.randomter...mmands.html#bit

Sorry RT,

I meant:

if(a{0}^a{1}) then ...

must have left the other operator in there; should work without the parenthesis if the compiler doesn't like them since only a single term is in the expression.

[+Random Terrain]

Sorry RT,

I meant:

if(a{0}^a{1}) then ...

must have left the other operator in there; should work without the parenthesis if the compiler doesn't like them since only a single term is in the expression.

Check out the "Did You Know?" box here:

 

http://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#bit

 

Unless we were never told about it or something has changed, I think we are very limited when it comes to bit operations and if-thens.

Edited September 28, 2011 by Random Terrain

[Mr SQL]

Sorry RT,

I meant:

if(a{0}^a{1}) then ...

must have left the other operator in there; should work without the parenthesis if the compiler doesn't like them since only a single term is in the expression.

Check out the "Did You Know?" box here:

 

http://www.randomter...mmands.html#bit

 

Unless we were never told about it or something has changed, I think we are very limited when it comes to bit operations and if-thens.

You're right, you can't use ^ that way, it flips two arguments I thought it was a comparison operator

I think your first solution has the least overhead.

[RevEng]

Can you just bit-shift right once instead of dividing by 2? Or would that use the same number of cycles? (does bBasic support bit shifting?)

bB automatically turns a divide with a power-of-2 into a number of LSR bit-shifts.

[ScumSoft]

[edit]Very nice bogax!

My solution was way off

Edited October 1, 2011 by ScumSoft

[bogax]

Not sure I remember the conventions rightly, but assuming bit 0 is the

least significant bit ...

 

If you add a 1 to a bit you can, in effect, propagate it to a more significant

position via carry

 

If you add two bits together the sum is the XORing of the two

 

You can do both at once in this case by adding 1 to a

 

Then use & to isolate bit 1 to test it (ie a bitwise AND with 2)

 

if (a + 1)& 2 then goto __pause_game

__pause_game

 

produces

 

.L00 ; if ( a + 1 ) & 2 then goto __pause_game

; complex statement detected
LDA a
CLC
ADC #1
AND #2
BEQ .skipL00
.condpart0
jmp .__pause_game

.skipL00
.
;

.__pause_game
; __pause_game

 

That ought to be something like what you want

Edited September 30, 2011 by bogax

[bogax]

if a/2 ^ a & 1 then goto __pause_game

__pause_game

 

.L00 ; if a / 2 ^ a & 1 then goto __pause_game

; complex condition detected
; complex statement detected
LDA a
lsr
EOR a
AND #1
 BEQ .skipL00
.condpart0
jmp .__pause_game

.skipL00
.
;

.__pause_game
; __pause_game