custom_bitstream.c

/*
 * © 2020 Aaron Taylor <ataylor at subgeniuskitty dot com>
 * See LICENSE.txt file for copyright and license details.
 */

/*
 * Outputs a file compatible with PDP-11 paper tape loader.
 * File will load memory with integer counting (ex: 0, 1, 2, ..., NUMWORDS-2, NUMWORDS-1).
 * Load starts at 01000 and ends at 01000 + NUMWORDS - 1.
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#define NUMWORDS 20000

void main( int argc, char ** argv ) {
    uint16_t * packet = malloc((2*NUMWORDS) + 16);
    /*
     *  +16 comes from:
     *     3 for start of first packet header
     *     1 for first checksum plus padding byte
     *     4 for second packet header + checksum on empty packet
     *     Then double because PDP-11 word is 2 byte.
     */
    packet[0] = 1;                  /* Standard header */
    packet[1] = (2*NUMWORDS) + 6;   /* Size of packet, including header but excluding checksum */
    packet[2] = 01000;              /* Address to load packet */
    packet[NUMWORDS+3] = 0;         /* Empty checksum. SIMH complains but still loads the image. */
    packet[NUMWORDS+4] = 1;         /* Start of next packet, standard header */
    packet[NUMWORDS+5] = 6;         /* Size of packet (just the header since blank body */
    packet[NUMWORDS+6] = 01000;     /* Blank body means start execution at this address */
    packet[NUMWORDS+7] = 0;         /* Empty checksum. SIMH complains but still loads the image. */

    uint16_t i;
    for(i=0; i < NUMWORDS; i++) packet[i+3] = i; /* Start at 0 so first instruction halts machine */

    FILE * fp;
    fp = fopen("testimg.pdp11","w+");
    fwrite(packet,(2*NUMWORDS)+16,1,fp);
    fclose(fp);
}