6502 BIT instruction confusing

[Marius]

There is something strange going on with the BIT instruction.

 

I was wondering: is this instruction only giving a usable result when you check the status of one bit at the time?

 

Please take a look at this code:

 

On memory location $0600 I saved value #$14 (%00010100)

         .OR $5000
START     LDA #$04 
         BIT $0600
         BNE TRUE
FALSE     LDA #$00
         STA COLPF2
         RTS
TRUE      LDA #$C0
         STA COLPF2
         RTS

 

OK this works great. It tests for bit 2 (#$04) and indeed this bit is SET so the screen turns green (the TRUE routine)

 

But now the same code, except for the START line...

 

On memory location $0600 I saved value #$14 (%00010100)

         .OR $5000
START     LDA #$24 
         BIT $0600
         BNE TRUE
FALSE     LDA #$00
         STA COLPF2
         RTS
TRUE      LDA #$C0
         STA COLPF2
         RTS

 

 

And AGAIN the screen turns Green (the TRUE routine!) ... this is strange in my opinion since bit 5 isn't true on $0600. I thought: this should result in a black screen.

 

This routine gives a black screen:

 

On memory location $0600 I saved value #$14 (%00010100)

         .OR $5000
START     LDA #$20 
         BIT $0600
         BNE TRUE
FALSE     LDA #$00
         STA COLPF2
         RTS
TRUE      LDA #$C0
         STA COLPF2
         RTS

 

So I only can conlude: the BIT routine is only usable for BIT checking, one bit at a time.

 

Is this a right conclusion?

 

thanks

Marius

[GroovyBee]

On memory location $0600 I saved value #$14 (%00010100)

         .OR $5000
START     LDA #$04 
         BIT $0600

 

BIT performs a logical AND between the accumulator and the memory location (as well as updating the N and V flags with bits 7 and 6 respectively from the memory location) so

 

%00000100 AND

%00010100 =

%00000100

 

Which is a result not equal to zero.

 

On memory location $0600 I saved value #$14 (%00010100)

         .OR $5000
START     LDA #$24 
         BIT $0600

 

%00100100 AND

%00010100 =

%00000100

 

Which is a result not equal to zero.

 

If you want to check multiple bits you'll have to AND #MASK and CMP #MASK unless you can get the bits of interest into bits 6 and 7.