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    BltClear - Clear a block of memory words to zero.

    BltClear( memBlock, bytecount, flags )
                a1         d0       d1

    void BltClear( void *, ULONG, ULONG );

    For memory that is local and blitter accessable, the most
    efficient way to clear a range of memory locations is
    to use the system's most efficient data mover, the blitter.
    This command accepts the starting location and count and clears
    that block to zeros.

    memBloc - pointer to local memory to be cleared
              memBlock is assumed to be even.
    flags   - set bit 0 to force function to wait until
              the blit is done.
              set bit 1 to use row/bytesperrow.

    bytecount - if (flags & 2) == 0 then
                            even number of bytes to clear.
                            low 16 bits is taken as number of bytes
                            per row and upper 16 bits taken as
                            number of rows.

    This function is somewhat hardware dependant. In the rows/bytesperrow
    mode (with the pre-ECS blitter) rows must be <- 1024. In bytecount mode
    multiple runs of the blitter may be used to clear all the memory.

    Set bit 2 to use the upper 16 bits of the Flags as the data to fill
    memory with instead of 0 (V36).

    The block of memory is initialized.



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