[Oberon] FPGA - Screen.Mod

[Tomas Kral]

Hi Paul,

This made the trick in the end, but I fear is not much effective as the
`ReverseByte' is called four times per display word. The masking code
of yours seems four times faster, if could be updated for big endian
somehow?

  PROCEDURE ReverseByte(wd: INTEGER): INTEGER;
    VAR i: INTEGER; rwd, t: SET;
  BEGIN
    rwd := {}; FOR i := 0 TO 7  DO
      t := SYSTEM.VAL(SET, ASR(wd, i))*{0};
      t := SYSTEM.VAL(SET, LSL(SYSTEM.VAL(INTEGER, t), 7-i));
      rwd := rwd+t;
    END    
  RETURN SYSTEM.VAL(INTEGER, rwd) END ReverseByte;

  ...

  (*little endian: high, low words*)
  hwd := bits MOD 10000H; lwd := bits DIV 10000H;

  (*big endian: high, low bytes, bit reversed*)
  bits := ReverseByte(lwd DIV 100H)*1000000H +
          ReverseByte(lwd MOD 100H)*10000H +
          ReverseByte(hwd DIV 100H)*100H +
          ReverseByte(hwd MOD 100H);

  ...

Why do we need bit reversal at all?
Why the bitmap is stored as inverse video?

Many thanks

-- 
Tomas Kral <thomas.kral at email.cz>