Subversion Repositories zipcpu

////////////////////////////////////////////////////////////////////////////

//

// Filename:    twoc.cpp

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     Some various two's complement related C++ helper routines.

//              Specifically, these help extract signed numbers from

//              packed bitfields, while guaranteeing that the upper bits

//              are properly sign extended (or not) as desired.

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

///////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015-2016, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// You should have received a copy of the GNU General Public License along

// with this program.  (It's in the $(ROOT)/doc directory, run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

///////////////////////////////////////////////////////////////////////////

#include <stdio.h>

#include <assert.h>

#include "twoc.h"

long    sbits(const long val, const int bits) {

        long    r;

        r = val & ((1l<<bits)-1);

        if (r & (1l << (bits-1)))

                r |= (-1l << bits);

        return r;

}

bool    sfits(const long val, const int bits) {

        return (sbits(val, bits) == bits);

}

unsigned long   ubits(const long val, const int bits) {

        unsigned long r = val & ((1l<<bits)-1);

        return r;

}

unsigned long   rndbits(const long val, const int bits_in, const int bits_out) {

        long    s = sbits(val, bits_in); // Signed input value

        long    t = s;  // Truncated input value

        long    r;      // Result

        // printf("RNDBITS(%08lx, %d, %d)\n", val, bits_in, bits_out);

        // printf("S = %lx\n", s);

        // printf("T = %lx\n", t);

        // assert(bits_in > bits_out);

        if (bits_in == bits_out)

                r = s;

        else if (bits_in-1 == bits_out) {

                t = sbits(val>>1, bits_out);

                // printf("TEST! S = %ld, T = %ld\n", s, t);

                if (3 == (s&3))

                        t = t+1;

                r = t;

        } else {

                // A. 0XXXX.0xxxxx -> 0XXXX

                // B. 0XXX0.100000 -> 0XXX0;

                // C. 0XXX1.100000 -> 0XXX1+1;

                // D. 0XXXX.1zzzzz -> 0XXXX+1;

                // E. 1XXXX.0xxxxx -> 1XXXX

                // F. 1XXX0.100000 ->  ??? XXX0;

                // G. 1XXX1.100000 ->  ??? XXX1+1;

                // H. 1XXXX.1zzzzz -> 1XXXX+1;

                t = sbits(val>>(bits_in-bits_out), bits_out); // Truncated value

                if (0 == ((s >> (bits_in-bits_out-1))&1)) {

                        // printf("A\n");

                        r = t;

                } else if (0 != (s & ((1<<(bits_in-bits_out-1))-1))) {

                        // printf("D\n");

                        r = t+1;

                } else if (t&1) {

                        // printf("C\n");

                        r = t+1;

                } else { // 3 ..?11

                        // printf("B\n");

                        r = t;

                }

        } return r;

}