Tutorial: How to put inline assembly functions into ZX Basic

[LCD]

Code:

function mirror(dowedoit as ubyte, number as ubyte) as ubyte
    dim result as ubyte
    if dowedoit then
        result=(number&1)<<7|(number&2)<<5|(number&4)<<3|(number&8)<<1|(number&16)>>1|(number&32)>>3|(number&64)>>5|(number&128)>>7
    Else
        result=number
    end if
    return result
end Function

Though if I was going to, I'd say why bother with a new variable? :mrgreen:

Code:

function mirror(dowedoit as ubyte, number as ubyte) as ubyte
   if dowedoit then
      number=(number&1)<<7|(number&2)<<5|(number&4)<<3|(number&8)<<1|(number&16)>>1|(number&32)>>3|(number&64)>>5|(number&128)>>7
   end if
   return number
end Function

That said, if you read the behemoth assembly that makes. Ouch. Hopping on and off the stack like crazy! Not to mention a lot of very expensive reads and writes to (IX+7) - which cost 19 T states each. I wonder if my original beginners algorithm is faster!

:oops:
I overseen it... It was my mirroring function from my unfinished fractal picture decompressor which I adopted to match yours, mine had no dowedoit, so it was:

Code:

function MirrorByte(num as ubyte) as ubyte
    return (num&1)<<7|(num&2)<<5|(num&4)<<3|(num&8)<<1|(num&16)>>1|(num&32)>>3|(num&64)>>5|(num&128)>>7
end function

Your original beginners function was not faster, it was in fact slower because multiplication and division is allways slower than bitshifting. I'm happy that boriel included it as it is a big help in optimizing programs, so you could use SHR and SHL in place of >> and <<. With this your code will match the ASM version much better, and work faster.

Anyway, I think my suggested optimized version is pretty tight:

Code:

function fastcall mirror (dowedoit as uByte, number as uByte) as uByte
asm
   pop hl
   pop bc
   AND A
   LD A,B
   RET Z
   ld b,8
   ld c,a
   XOR A
        
   mirrorLoop:
      RR C
      RLA  
   DJNZ mirrorLoop
   jp (hl)        
end asm
END FUNCTION

It is!!!
Much faster than both compiled codes.

The books looks very good by the way!!! My compliments!