using windows debugger to view a memory location written to in C?

Question

I have Turbo C and windows debug running in dosbox

I have this C program, it has two main lines, as you can see. int a=5 and then a line to show the address of a, printf("address of a=%x",&a)

I run it

It seems to tell me that a has been allocated the address of fff4

Now I want to use debug to hopefully see the value of 5 at that memory address

But it is not showing

How can I see it in debug?

Once your program exits then any temporary changes to memory are effectively gone. You need to debug while your program is running - set a breakpoint in the program and then examine memory while it's halted at the breakpoint. — Paul R
– Paul R, Commented Oct 6, 2015 at 6:01
@PaulR , rohan, So is it possible to write a C program that writes to RAM in a non temporary way? — barlop
– barlop, Commented Oct 6, 2015 at 6:05
@barlop: in some embedded environments you might be able to, but in general no - RAM is temporary, and in most modern operating systems it's virtual memory space anyway, so once your process is gone then the addresses and memory contents have no meaning. — Paul R
– Paul R, Commented Oct 6, 2015 at 6:08
Can you give us a little bit more details why you want to do this ? What is your goal ? In a real dos 16 bit environment - which you are targetting with turbo c - you could write to memory outside of your process and have this change survive the exit of your program. — Marged
– Marged, Commented Oct 6, 2015 at 6:27

rkhb · Accepted Answer · 2015-10-06 08:22:28Z

1

This is my DEBUG's output of the compiled main function:

16E1:01FA 55            PUSH    BP                                 
16E1:01FB 8BEC          MOV BP,SP                              
16E1:01FD 83EC02        SUB SP,+02                             
16E1:0200 C746FE0500    MOV WORD PTR [BP-02],0005              
16E1:0205 8D46FE        LEA AX,[BP-02]                         
16E1:0208 50            PUSH    AX                                 
16E1:0209 B89401        MOV AX,0194                            
16E1:020C 50            PUSH    AX                                 
16E1:020D E8AA06        CALL    08BA                               
16E1:0210 59            POP CX                                 
16E1:0211 59            POP CX                                 
16E1:0212 8BE5          MOV SP,BP                              
16E1:0214 5D            POP BP                                 
16E1:0215 C3            RET

int a=5; is a local variable inside the function main which is stored on the stack (MOV WORD PTR [BP-02],0005). A value on the stack is lost, when you leave the function (RET). You cannot see it outside the running program.

Your plan can go well, if you

Initialize a global variable and
produce a tiny .com program.

simplepr.c:

#include <stdio.h>

int a=5;

void main()
{
    printf ("address of a=%x",&a);
}

Compile:

TCC.EXE -mt -lt simplepr.c

DEBUG session:

n simplepr.com
l
g         -> address of a=125c (example)
d 125c    -> or what the address is

edited Oct 6, 2015 at 8:22

answered Oct 6, 2015 at 7:51

rkhb

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7 Comments

barlop Over a year ago

thanks, that looks fascinating, i'd like to try it.. I have DOS 6.22 in a VM in virtualbox, and I have debug and my EXE.. How did you get the assembly instructions of the EXE(that you show at the beginning of your answer)?

rkhb Over a year ago

@barlop. In DEBUG the command 'u'. I used Turbo Debugger to get the address of "_main".

barlop Over a year ago

I can't quite get there i.sstatic.net/Q0jDA.png I can't get it outputting a memory location, and if you can include the turbo debugger command to get the address of _main then even better.

rkhb Over a year ago

@barlop: 'd 1562' like "dump"! 'g' is "go" and '?' is "help" ;-) The explanation with Turbo Debugger is a little bit complicated. Have you installed Turbo Debugger and which version?

barlop Over a year ago

I see it , when int a=5, the 5 is there i.imgur.com/X0bdtxh.png and i.imgur.com/PL66kGc.png thanks

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