Re: 16-Bit Math on 8-Bit Numbers (HPLOT in Machine Language, Part II)

[russotto@wanda.vf.pond.com (Matthew T. Russotto)]

In article <jonINTERNETrelayCHAT-1303002116420001@nn-157-22-181-90.napanet.net>,
Jon Bettencourt <jonINTERNETrelayCHAT@napaVALLEYnet.CLARInet> wrote:
}OK, remember my question about what variables to set up for Applesoft's
}HPLOT routine in machine language? Forget it! It's solved, and now I'm
}changing my question.
}
}I need a machine language routine that will:
}
}* take a number from 0 to 34 at zero page address $EB, multiply it by 8,
}and put the 16-bit result in $FA/$FB, low byte first
      LDA #00
      STA $FB
      CLC
      LDX #2
      LDA $EB
loop: ASL $FB
      ASL      
      ADC #0, $FB
      DEX
      BNE loop
      STA $FA

Hopefully I haven't used any illegal addressing modes there.

}* take a number from 0 to 23 at zero page address $EC, multiply it by 8,
}and put the 8-bit result in $FC

      LDA $EC
      ASL
      ASL
      ASL
      STA $FC


}* take a number from 0 to 511 at zero page addresses $ED/$EE (low byte
}first), multiply it by 8, add 20480 to the product, and put the 16-bit
}result in $FD/$FE, low byte first

The multiply part is the same as above, only pre-load $FE with the
contents of $EE.  Then just ADC #$50 to $FE.
-- 
Matthew T. Russotto                                russotto@pond.com
"Extremism in defense of liberty is no vice, and moderation in pursuit
of justice is no virtue."