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read char from screen

Posted on 2000-04-03
Medium Priority
Last Modified: 2010-04-02
how do i read a character from any given location on the screen
Question by:FeVeR
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Expert Comment

ID: 2682204
My OS is _____

the compiler is _________

LVL 22

Expert Comment

ID: 2682334
And don't forget:

my CPU is ______

Assuming DOS/Intel, video memory starts at either b800: or A000:, depending on video card and mode.  Further assuming you're in text mode, characters are stored in two bytes - the first for the character, the second for its' display attribute (color, blink, etc).

If you're in graphics mode, all you can get is pixels - there are no characters per se stored.

If you're under Windows, same thing - it's all graphics data.
LVL 22

Expert Comment

ID: 2682348
BTW, you really ought to update your other two questions in this topic.  They've been languishing for almost two weeks.

There are lots of people here who won't take the time to respond to folks they think have a spotty grading record.
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Accepted Solution

kmullin earned 100 total points
ID: 2684857
The ANSI C standard answer is, you can't.  That screen is hardware, and as any C programmer knows, C doesn't know anything about the hardware.  It knows standard-in and standard-out.  But, in a sense, it doesn't know what those really are.  They are byte streams, as far as C is concerned.  Getting to a specific pont on the screen and interpreting what is there as a character (as opposed to some sort of graphic image) is hardware realated.  You can probably do it via some sort of API function that is provided to you by the hardware manufacturer.  But C doesn't know how.  C doesn't know anything about the hardware of your particular machine.  It has getchar and scanf (which uses getchar, by the way).  There are other derivations of these basics.  But they all simply get the next byte from the standard-in stream, which is NOT a terminal, but a byte stream.
LVL 22

Expert Comment

ID: 2685001
kmullin - I see you're new here, so you probably aren't aware of the convention of posting only comments.  Posting an answer locks the question, and many people don't even bother looking at locked questions, thereby denying the questioner the added audience.

But don't worry about it - lots of folks do it.

And welcome to EE...

Here's some sample MSC 16-bit code:
unsigned char _far *Addr;
unsigned int        Addr_Seg;
unsigned int        Addr_Off;
unsigned char       ScreenChar;
unsigned char       AttrByte;
// Get top left hand screen character

Other compilers use different ways of specifying address segments and offsets.  Your mileage may vary.

Expert Comment

ID: 2693013
This will work in text mode on standart PC:

#include <dos.h>

int get_char(int x,int y) {
    return peekb(0xb800,y*80*2+x*2);

Author Comment

ID: 2729114
no pc for a few weeks

ummm ok i didn't know that it was not possible. i thought C was pretty much capable of anything anyway, ill try out a few of these ideas and get back....

thanks all

Expert Comment

ID: 2730931
I may be splitting hairs here, but C can do anything.  Because of the way it deals with addresses and how it deals with bit manipulation, you can code it to do anything.  BUT, C doesn't know HOW to do everything.  You have to provide the HOW TO DO IT, and then you can code C to do it.  For example, on your particular hardware, perhaps setting a certain video register bit to 1 will cause your display to clear itself on the next refresh cycle.  So, you have to do the investigation to find out 1) what address is the video register stored in, 2) code a C program to access that address (via a pointer set to that address), then set that particular bit with the appropriate bit operations.  Does the above mean that C can clear the screen?  No, but you knew how to do it for your hardware, and using that knowledge, you coded a C routine to do it for you.

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