relocate.c

#include <stdio.h>
#include <stdbool.h>
#include "cpmimage.h"

unsigned int cksum;
const unsigned char *dataptr;
unsigned char databit;
#define TOPBIT 128

bool relocate_nextbit(void);
unsigned char relocate_nextbyte(void);

/*
 * relocate2.s contains faster assembler implementations of these two functions.
 *
 *  bool relocate_nextbit(void)
 *  {
 *      bool r;
 *  
 *      r = *dataptr & databit;
 *      databit = databit >> 1;
 *      if(databit == 0){
 *          databit=TOPBIT;
 *          dataptr++;
 *      }
 *  
 *      return r;
 *  }
 *  
 *  unsigned char relocate_nextbyte(void)
 *  {
 *      unsigned char out = 0;
 *      unsigned char bits = 8;
 *  
 *      while(1){
 *          if(relocate_nextbit())
 *              out |= 1;
 *          if(--bits == 0){
 *              cksum += out;
 *              return out;
 *          }
 *          out = out << 1;
 *      }
 *  }
 */

unsigned int relocate_read_int(unsigned char bits)
{
    unsigned int out = 0;

    while(1){
        if(relocate_nextbit())
            out |= 1;
        if(--bits == 0)
            return out;
        out = out << 1;
    }
}

unsigned int relocate_read_length(void)
{
    unsigned char i;
    unsigned int l;

    i = 0;
    while(true){
        l = relocate_read_int(cpm_image_encoding[i]);
        if(l){
            return (l + cpm_image_offsets[i]);
        }
        i++;
        // check for end of stream
        if(cpm_image_encoding[i] == 0)
            return 0;
    }
}

bool relocate_cpm(unsigned char *dest)
{
    unsigned char *target;
    unsigned int length;
    unsigned int reloc_offset;
    unsigned int *r;
    bool first;

    reloc_offset = (unsigned int)dest;
    target = dest;
    dataptr = cpm_image_data;
    databit = TOPBIT;
    cksum = 0;
    first = true;

    while(length = relocate_read_length()){
        if(first){
            // do not relocate the first run
            first = false;
        }else{
            // we wrote the low byte in the previous pass
            // now we write the high byte;
            *(target++) = relocate_nextbyte();
            // then we relocate the value just written;
            r = (unsigned int*)(target - 2);
            *r += reloc_offset;
            length--;
        }

        while(length--)
            *(target++) = relocate_nextbyte();
    }

    if((unsigned int)(target - dest) != cpm_image_length){
        printf("length mismatch\n");
        return false;
    }
    if(cksum != cpm_image_cksum){
        printf("checksum mismatch\n");
        return false;
    }

    // looks OK!
    return true;
}