New standard procedure BITS

[cfbsoftware]

Release v4.2 of Astrobe for Cortex-M3 has a new Oberon language extension - the built-in procedure BITS:

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PROCEDURE BITS(i: INTEGER): SET;

BITS is a feature of the language Component Pascal (i.e. Oberon-2+) which we use to develop the Astrobe Oberon compiler. BITS takes an INTEGER parameter and returns a SET with the same bit pattern. It is a type transfer procedure like ORD rather than a type conversion procedure. By definition, the following expressions, where i is an INTEGER and s is a SET, are TRUE:

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  BITS(i) = SYSTEM.VAL(SET, i)
  ORD(BITS(i)) = i
  BITS(ORD(s)) = s

BITS is convenient to use in expressions which are a mixture of INTEGERs, masks and bit fields.

For example, in the Astrobe library procedure Serial.SetFormat, to modify the wordlength in the ULCR register we have to clear the six least significant bits of ULCR and replace them with the integer value: (wordlength - 5). In earlier versions of Astrobe, one way to do this would be to write:

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PROCEDURE SetFormat*(wordLength, stopBits: INTEGER; parity: SET);
VAR
  ULCR: SET;
BEGIN
  SYSTEM.GET(MCU.U0LCR, ULCR);
  ULCR := ULCR - {0..5} + parity + SYSTEM.VAL(SET, (wordLength - 5));
 

In v4.2 this last statement simplifies to:

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  ULCR := ULCR - {0..5} + parity + BITS(wordLength - 5);

Note that SYSTEM.VAL can still be used if you prefer.

The following examples show the use of BITS with a constant value and the equivalent SET constants:

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  BITS(0) = {}
  BITS(1) = {0}
  BITS(3) = {0, 1}
  BITS(0FFFFFFFFH) = {0..31}