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[/] [zipcpu/] [trunk/] [sw/] [zasm/] [twoc.cpp] - Blame information for rev 69

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1 2 dgisselq
////////////////////////////////////////////////////////////////////////////
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//
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// Filename:    twoc.cpp
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//
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// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
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//
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// Purpose:     Some various two's complement related C++ helper routines.
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//              Specifically, these help extract signed numbers from
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//              packed bitfields, while guaranteeing that the upper bits
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//              are properly sign extended (or not) as desired.
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//
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// Creator:     Dan Gisselquist, Ph.D.
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//              Gisselquist Technology, LLC
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//
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///////////////////////////////////////////////////////////////////////////
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//
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// Copyright (C) 2015, Gisselquist Technology, LLC
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//
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// This program is free software (firmware): you can redistribute it and/or
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// modify it under the terms of  the GNU General Public License as published
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// by the Free Software Foundation, either version 3 of the License, or (at
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// your option) any later version.
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//
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// This program is distributed in the hope that it will be useful, but WITHOUT
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// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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// for more details.
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//
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// You should have received a copy of the GNU General Public License along
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// with this program.  (It's in the $(ROOT)/doc directory, run make with no
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// target there if the PDF file isn't present.)  If not, see
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// <http://www.gnu.org/licenses/> for a copy.
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//
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// License:     GPL, v3, as defined and found on www.gnu.org,
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//              http://www.gnu.org/licenses/gpl.html
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//
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//
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///////////////////////////////////////////////////////////////////////////
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#include <stdio.h>
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#include <assert.h>
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#include "twoc.h"
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long    sbits(const long val, const int bits) {
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        long    r;
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        r = val & ((1l<<bits)-1);
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        if (r & (1l << (bits-1)))
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                r |= (-1l << bits);
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        return r;
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}
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bool    sfits(const long val, const int bits) {
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        long    alt = sbits(val, bits);
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        return (sbits(val, bits) == bits);
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}
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unsigned long   ubits(const long val, const int bits) {
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        unsigned long r = val & ((1l<<bits)-1);
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        return r;
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}
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unsigned long   rndbits(const long val, const int bits_in, const int bits_out) {
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        long    s = sbits(val, bits_in); // Signed input value
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        long    t = s;  // Truncated input value
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        long    r;      // Result
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        // printf("RNDBITS(%08lx, %d, %d)\n", val, bits_in, bits_out);
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        // printf("S = %lx\n", s);
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        // printf("T = %lx\n", t);
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        // assert(bits_in > bits_out);
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        if (bits_in == bits_out)
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                r = s;
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        else if (bits_in-1 == bits_out) {
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                t = sbits(val>>1, bits_out);
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                // printf("TEST! S = %ld, T = %ld\n", s, t);
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                if (3 == (s&3))
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                        t = t+1;
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                r = t;
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        } else {
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                // A. 0XXXX.0xxxxx -> 0XXXX
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                // B. 0XXX0.100000 -> 0XXX0;
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                // C. 0XXX1.100000 -> 0XXX1+1;
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                // D. 0XXXX.1zzzzz -> 0XXXX+1;
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                // E. 1XXXX.0xxxxx -> 1XXXX
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                // F. 1XXX0.100000 ->  ??? XXX0;
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                // G. 1XXX1.100000 ->  ??? XXX1+1;
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                // H. 1XXXX.1zzzzz -> 1XXXX+1;
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                t = sbits(val>>(bits_in-bits_out), bits_out); // Truncated value
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                if (0 == ((s >> (bits_in-bits_out-1))&1)) {
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                        // printf("A\n");
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                        r = t;
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                } else if (0 != (s & ((1<<(bits_in-bits_out-1))-1))) {
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                        // printf("D\n");
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                        r = t+1;
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                } else if (t&1) {
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                        // printf("C\n");
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                        r = t+1;
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                } else { // 3 ..?11
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                        // printf("B\n");
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                        r = t;
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                }
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        } return r;
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}
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