Problems of one pass assembler

Answer 1

1. The first problem is that the program in one pass involves forward references.

Some operands are used before they are defined and hence the assembler does not know what address to insert in the translated instruction.

2. We can list syntax errors like missing commas or parenthesis and semantic errors like duplicate definitions of symbols , other subtle errors can't be detected till pass2.

Answer 2

The translation performed by an assembler is essentially a collection of substitutions:

* machine operation code for mnemonic * machine address for symbolic * machine encoding of a number for its character representation, etc. Except for one factor these substitutions could all be performed in one sequential pass over source text.The factor is the forward reference(reference to an instruction which has not yet been scanned by assembler).

Now it's that the separate passes of two pass assemblers are required to handle forward references without restriction.

Now if we impose certain restriction that means handling forward references without making two passes. These different sets of restrictions lead to one pass assembler.

And these one-pass assembler are particularly attractive when secondary storage is either slow or missing entirely, as on many small machines.

Answer 3

Question - Implement a single pass assembler.

Program

#include "stdio.h"

#include "stdlib.h"

#include "string.h"

int count[20];

void main()

{

FILE *f1,*f2,*f3,*f4;

int linenumber,locctr,f;

char lbl[10],mne[10],opd[10],ch,mneu1[10],sval[10];

char sadr[10],slbl[10],op1[10],label[10];

void wordcount();

printf("Word count for Input Program:");

wordcount();

printf("\n Output \n");

printf("\n Source Code \t Object code \n\n");

f1=fopen("INPUT.C","r");

f2=fopen("SYMTAB.C","w+");

f4=fopen("INTER.C","w");

fscanf(f1,"%s %s %x\n",lbl,mne,&locctr);

linenumber=2;

while(!feof(f1))

{

if(count[linenumber]==1)

{

fscanf(f1,"%s\n",mne);

fprintf(f4,"%x\t%s",locctr,mne);

}

if(count[linenumber]==2)

{

fscanf(f1,"%s%s\n",mne,opd);

fscanf(f4,"%X \t %s \t %s \n",locctr,mne,opd);

printf("%s\t%s\t",mne,opd);

f3=fopen("OPCODE.C","r");

while(!feof(f3))

{

fscanf(f3,"%s %s \n",mneu1,op1);

if(strcmp(mne,mneu1)==0)

printf("%s\t",op1);

}

fclose(f3);

f=0;

rewind(f2);

while(!feof(f2))

{

fscanf(f2,"%s %s %s \n",sadr,slbl,sval);

if(strcmp(opd,slbl)==0)

{

printf("%s\n\n",sadr);

f=1;

}

}

if(f==0)

printf("0000\n");

}

if(count[linenumber]==3)

{

fscanf(f1,"%s %s %s \n",lbl,mne,opd);

fprintf(f4,"%X \t %s \t %s \t %s \n",locctr,lbl,mne,opd);

fprintf(f2,"%X \t %s \t %s \n", locctr, lbl,opd);

if((strcmp(mne,"RESW")==0) (strcmp(mne,"RESB")==0))

printf("%s \t %s \t 00 \t 000 %s \n\n",lbl,mne,opd);

else

printf("%s \t %s \t 00 \t 000 %s \n\n",lbl,mne,opd);

}

linenumber+=1;

if(strcmp(mne,"WORD")==0)

locctr+=3;

else if(strcmp(mne,"BYTE")==0)

locctr+=strlen(opd);

else if(strcmp(mne,"RESW)==0)

locctr+=3 * (atoi(opd));

else if(strcmp(mne,"RESB")==0)

locctr+=atoi(opd);

else

locctr+=3;

}

fclose(f1);

fclose(f2);

fclose(f4);

getch();

}

void wordcount()

{

FILE *f3;

char c;

int word=0,i=1;

printf("\n Word Count");

f3=fopen("INPUT.C","r");

c=fgetc(f3);

while(c!=EOF)

{

if(c==' ')

word+=1;

if(c=='\n')

{

word+=1;

count[i]=word;

printf("\n No.of words in %d:%d",r,word);

i+=1;

word=0;

}

c=fgetc(f3);

}

fclose(f3);

}

INPUT.c

WC START 1000

FIRST WORD 5

SECOND WORD 6

THIRD RESW 1

LDA FIRST

ADD SECOND

STA THIRD

END

OPCODE.c

LDA 00

ADD 18

SUB 1C

STA 0C

SYMTAB.c

FIRST 1009

SECOND 100C

THIRD 100F

INTER.c

1000 LDA FIRST

1003 ADD SECOND

1006 STA THIRD

1009 FIRST WORD

100C SECOND WORD

100F THIRD RESW

1012 END

1015 ENDL

Answer 4

A C++ program will be able to be used for a single pass assembler program in C. This program can be used to write many different types of C++ programming.

Answer 5

Nothing. They are the same thing.