[Simh] PDP7 DECtapes

[Bob Supnik]

Yeah, PDP-7 (and earlier) DECtapes are different. They use different 
checksum algorithms, for one thing, and some early PDP-4 and -7 DECtape 
controllers used four-word headers instead of five. The checksum is 
probably key; only five "four word header" DECtape controllers escaped 
the factory.

/* Checksum routine */

int32 dt_csum (UNIT *uptr, int32 blk)
{
int32 *fbuf = (int32 *) uptr->filebuf;
int32 ba = blk * DTU_BSIZE (uptr);
int32 i, csum, wrd;

#if defined (TC02)                                      /* TC02/TC15 */
csum = 077;                                             /* init csum */
for (i = 0; i < DTU_BSIZE (uptr); i++) {                /* loop thru buf */
     wrd = fbuf[ba + i] ^ DMASK;                         /* get ~word */
     csum = csum ^ (wrd >> 12) ^ (wrd >> 6) ^ wrd;
     }
return (csum & 077);
#else                                                   /* Type 550 */
csum = 0777777;
for (i = 0; i < DTU_BSIZE (uptr); i++) {                /* loop thru buf */
     wrd = fbuf[ba + i];                                 /* get word */
     csum = csum + wrd;                                  /* 1's comp add */
     if (csum > DMASK)
         csum = (csum + 1) & DMASK;
     }
return (csum ^ DMASK);                                  /* 1's comp res */
#endif
}

This is documented in the 18b PDP DECtape controller, based on sources I 
have for PDP-7 DECtape code from the late 60s; but for some reason, I 
never published a paper on it.

This is one reason why recovering DECsys required reading the DECtape on 
the last working PDP-7 in the world.

/Bob