SIC Assembler PASS 2

This is the code i did for pass2 of a SIC assembler. Please go through it and feedback on how to improve it.

#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<ctype.h>
#include<math.h>
int newrec(FILE *fp,int *lim,int l,int pos,int c,char add[]){
    if(l>=30){
        *lim=30;
        }
    else{
        *lim=l;
        }
    fseek(fp,pos,SEEK_SET);
    fprintf(fp,"%02X",c);
    fseek(fp,0,SEEK_END);
    fprintf(fp,"\nT^00%s^",add);
    pos=ftell(fp);
    fprintf(fp,"--");
    c=0;
    return pos;
    }
int convhex(char str[]){
    int hex=0,l,d,i=0;
    l=strlen(str)-1;
    while(str[i]!='\0'){
        switch(str[i]){
            case '0':case '1':case '2':case '3':
            case '4':case '5':case '6':case '7':
            case '8':case '9':
            d=str[i]-48;
            break;
            case 'A':case'B':case'C':case'D':case'E':case'F':
            d=str[i]-55;
            break;
            }
        hex=hex+(d*pow(16,l));
        l--;
        i++;
        }
    return hex;
    }
int check(char op[]){
    int i;
    for(i=0;op[i]!='\0';i++){
        if(op[i]==','){
            op[i]='\0';
            return 1;
            }
        }
    return 0;
    }

int main(){
    FILE *fint,*ftab,*flen,*fsym,*fobj;
    int i,len,l,pos,flag,lim=30,c=0,chk,f=0,sym;
    char add[5],symadd[5],op[5],start[10];
    char label[20],mne[10],operand[10],symtab[10],opmne[10];
    clrscr();
    fint=fopen("intermed.txt","r");
    flen=fopen("length.txt","r");
    ftab=fopen("optab.txt","r");
    fsym=fopen("symtab.txt","r");
    fobj=fopen("objcode.txt","w");
    fscanf(fint,"%s%s%s%s",add,label,mne,operand);
    if(strcmp(mne,"START")==0){
        strcpy(start,operand);
        fscanf(flen,"%X",&len);
        }
    fprintf(fobj,"H^%s^%s^%X\n",label,start,len);
    l=len;
    fscanf(fint,"%s%s%s%s",add,label,mne,operand);
    fprintf(fobj,"T^00%s^",add);
    pos=ftell(fobj);
    fprintf(fobj,"--");
    while(strcmp(mne,"END")!=0){
        fscanf(ftab,"%s%s",opmne,op);
        while(!feof(ftab)){
            if(strcmp(mne,opmne)==0){
                f=0;
                fclose(ftab);
                fscanf(fsym,"%s%s",symadd,symtab);
                chk=check(operand);
                while(!feof(fsym)){
                    if(strcmp(operand,symtab)==0){
                        f=1;
                        sym=convhex(symadd);
                        if(lim<3){
                            pos=newrec(fobj,&lim,l,pos,c,add);
                            c=0;
                            }
                        lim-=3;
                        l-=3;
                        fprintf(fobj,"^%s%04X",op,(sym|(chk*32768)));
                        c=c+3;
                        break;
                        }
                    else
                    fscanf(fsym,"%s%s",symadd,symtab);
                    }
                if(f==0){
                    if(lim<3){
                        pos=newrec(fobj,&lim,l,pos,c,add);
                        c=0;
                    }
                    lim-=3;
                    l-=3;
                    c+=3;
                    fprintf(fobj,"^%s0000",op);
                }
                break;
            }
            else
            fscanf(ftab,"%s%s",opmne,op);
        }
        if((strcmp(mne,"BYTE")==0)||(strcmp(mne,"WORD")==0)){
            if(strcmp(mne,"WORD")==0){
                int b;
                b=atoi(operand);
                if(lim<3){
                    pos=newrec(fobj,&lim,l,pos,c,add);
                    c=0;
                }
                lim-=3;
                l-=3;
                c+=3;
                fprintf(fobj,"^%06X",b);
            }
            else{
                int j=0,k=0;
                char str[4];
                len=strlen(operand);
                if((operand[j]=='X')){
                    j=2;
                    while(j<len-1){
                        str[k++]=operand[j];
                        j++;
                    }
                    str[k]='\0';
                    if(lim<1){
                        pos=newrec(fobj,&lim,l,pos,c,add);
                        c=0;
                    }
                    lim-=1;
                    l-=1;
                    c+=1;
                    fprintf(fobj,"^%s",str);
                }
                else{
                    if(lim<3){
                        pos=newrec(fobj,&lim,l,pos,c,add);
                        c=0;
                    }
                    lim-=3;
                    l-=3;
                    c+=3;
                    fprintf(fobj,"^");
                    for(i=2;i<len-1;i++){
                        fprintf(fobj,"%X",operand[i]);
                    }
                }
            }
            fscanf(fint,"%s%s%s%s",add,label,mne,operand);
        }
        else if(strcmp(mne,"RESW")==0||strcmp(mne,"RESB")==0){
            while(strcmp(mne,"RESW")==0||strcmp(mne,"RESB")==0){
                if(strcmp(mne,"RESW")==0){
                    l=l-(3*atoi(operand));
                }
                else
                l=l-(atoi(operand));
                fscanf(fint,"%s%s%s%s",add,label,mne,operand);
            }
            flag=0;
            if(strcmp(mne,"END")==0){
                flag=1;
            }
            if(flag==0){
                pos=newrec(fobj,&lim,l,pos,c,add);
                c=0;
            }
        }
        else{
            fscanf(fint,"%s%s%s%s",add,label,mne,operand);
        }
        ftab=fopen("optab.txt","r");
        fseek(ftab,SEEK_SET,0);
        fseek(fsym,SEEK_SET,0);
    }
    fseek(fobj,pos,SEEK_SET);
    fprintf(fobj,"%02X",c);
    fseek(fobj,0,SEEK_END);
    fprintf(fobj,"\nE^00%s",start);
    fcloseall();
    getch();
    return 0;
}