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The bit type defines a single-bit variable. It is used as follows:

bit name <[>= value<]>;


name is the name of the bit variable.
value is the value to assign to the bit.

The bit type may be used for variable declarations, argument lists, and function-return values. A bit variable is declared like other C data types. For example:

static bit done_flag = 0;    /* bit variable */

bit testfunc (               /* bit function */
  bit flag1,                 /* bit arguments */
  bit flag2)
return (0);                  /* bit return value */

All bit variables are stored in a bit segment located in the 8051-compatible internal memory area of the 251. Because this area is only 16 bytes long, a maximum of 128 bit variables may be declared within any one scope.


  • Accesses to the 251's extended bits generate more code than accesses to the standard 8051-compatible bit area. Therefore, you should use the standard 8051-compatible bit area as much as possible to maximize performance.

The 251 supports an extended bit-addressable area that is located in the data memory address range 0x20-0x7F. This extended bit area supports a maximum of 768 bit variables. To locate a bit in the extended bit area, specify bit ebdata as shown in the example below.

bit ebdata state1;          /* bit in 251 extended space         */
bit function (bit flag)  {  /* parameters are only in 8051 space */
  bit ebdata  local_bit;    /* local bit in 251 space            */

  local_bit = flag;

  return (local_bit);       /* return 251 extended bit           */


  • It is not possible to locate bit function parameters in the extended 251 bit space.

The following restrictions apply to bit variables and bit declarations:

  • A bit cannot be declared as a pointer. For example:
    bit *ptr;         /* invalid */
  • An array of type bit is invalid. For example:
    bit ware [5];     /* invalid */
  • Functions that disable interrupts (#pragma disable) and functions that are declared using an explicit register bank (using n) cannot return a bit value. The C251 Compiler generates an error message for functions of this type that attempt to return a bit type.
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