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[/] [xulalx25soc/] [trunk/] [sw/] [twoc.cpp] - Rev 113
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//////////////////////////////////////////////////////////////////////////// // // 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; }
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