Hi everybody
i am writing simple emulator in (QBasic) for educational purpose (I don't care about speed)
I am now work in 6502 , it should reads an atari 2600 rom ,, yet every thing is ok
but I just have a problem with branches , for example BEQ I know it means branch if zero flag set
and the next byte is a signed address from $00 - $F7 forward branching and $80 - $FF backwards branching
all that ok , but how Implement backwards branching using QBasic ?
how to make my program branches backwards if values $80 - $FF ?

Something like this should work:


Edit: could also do: instead of the XOR.

I'm thinking something like:

This example assumes that pc is your program counter, and that it's already pointing to the op-code of the instruction immediately following the branch instruction, which is where it should point if the branch is not taken. It also assumes that the conditional expression (operand > 127) is 0 if false and -1 if true.

The branch = true part prevents updating of pc if the branch is not taken.

The pc = pc + operand part adjusts the pc correctly for positive displacements.

The + 256 * (operand > 127) part corrects the adjustment for negative displacements only. It could be rewritten as - 2 * (operand AND 128) if that floats your boat.

Mike B.

OK I believe all your posted solutions are great , I have worked on barrym post :
in QBasic of course


now if the address less than 128 , then it will not be changed and the branch is forward
else , the address will be negative and the result is branching backwards
*if the address given is 128 and the pc points to 1000 then the result is 872 (new pc content)
*if the address given is 255 the branch will be 1 step back
is this right results ?

That seems to be correct. In practice, it would be very unusual to branch with a value of 255, since this would branch back to the middle of your branch instruction and try to use the value of 255 as the next op-code!

I'm glad you got it working. Please consider sharing your code with us when you're ready.

Mike B.

Thanx Mike B
yes it works nicely ,, and the most funny part for me is 256*(operand>127)
that was really genius , and about sharing , of course I would be happy when sharing my work with you
however I have long way to finish .