Tutorial: How to put inline assembly functions into ZX Basic

[britlion]

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!

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