/* tstTVG 
 * Test the test-vector-generator software for Mark5 
 * Revised:  2001 December 18, JAB */ 
#include <stdio.h> 
unsigned long tvg(int init); /* Below */ 

int main (int argc, char * argv[]) { /* tstTVG */ 
  unsigned long u; 
  int i; 

  for (i = 1; i <= 20; i++) { 
    u = tvg(i == 1); 
    (void) printf(" %.3d %#.8lx \n", i, u); 
    } /* End of for i */ 
  return(0); 
  } /* End of main = tstTVG */ 


unsigned long tvg(int init) { 
  /* Calculate the test-vector-generator (TVG) pattern. 
   * Reference:  VSI-H Specification, Rev 1.0, 7 August 2000, available 
   * at http://web.haystack.mit.edu/vsi/.  See especially Figure 6, 
   * page 29 and Table 13, page 20.  Referring to Figure 6, q (below) 
   * represents the Q outputs of the flip-flops, r the upper inputs of 
   * the exors, and u the lower 16 bits of the answer (output). 
   * Revised:  2001 December 18, JAB */ 
  static unsigned short q; 
  unsigned short r, u; 

  if (init) /* Initialize? */ 
    q = 0xffff; /* Yes */ 
  /* Set r to a copy of q right shifted by 1 bit */ 
  r = q >> 1; 
  /* And set r bit 15 to a copy of q bit 2 */ 
  if (q & 4) /* Bit 2 set? */ 
    r |= 0x8000; /* Yes */ 
  /* We need to overwrite r bit 14 with a copy of u bit 0, 
   * so we need to do q exor r temporarily to get this bit */ 
  if ((q ^ r) & 1) /* Bit 0 set? */ 
    r |= 0x4000; /* Yes */ 
  else /* Not set */ 
    r &= 0xbfff; 
  /* Now set u to q exor r */ 
  u = q ^ r; /* Lower 16 bits of the answer */ 
  /* For next time, set q to a copy of u right shifted by 1 bit */ 
  q = u >> 1; 
  /* And set q bit 15 (invisible) to a copy of q bit 0 */ 
  if (q & 1) /* Bit 0 set? */ 
    q |= 0x8000; /* Yes */ 
  /* u has the lower 16 bits of the answer; the upper 16 bits 
   * are the lower 16 bits inverted and shifted up there */ 
  return(((unsigned long) ~u << 16) + u); 
  } /* End of tvg() */