Windows Console API Functions and Console Buffer

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Nyatta

Joined: 23 Feb 2019
Posts: 3

Nyatta 01 May 2019, 13:04

Hello all,

I'm fairly new to assembly programming and have only dabbled here and there over the past 5-6 years. I understand the basic-basics, so please bear with me.

That aside, I've been attempting to invoke console API functions the past few days with limited success - specifically SetConsoleWindowInfo and SetConsoleScreenBufferSize in an attempt to remove the scroll bar from the console window in order to display and repeatedly update a neat block of text.
I'd also ran into unexpected behavior attempting to move the cursor with SetConsoleCursorPosition where I'd had to assign COORD as a quad value instead of the documented word value. The only way I'd been able to pass the value correctly has been as a hex quad whereas two decimals would be optimal.

These are the pages I'd been using for reference:
https://docs.microsoft.com/en-us/windows/console/setconsolewindowinfo
https://docs.microsoft.com/en-us/windows/console/setconsolescreenbuffersize
https://docs.microsoft.com/en-us/windows/console/setconsolecursorposition

My code is as follows:

Code:

format PE64 GUI 5.0
include 'win64ax.inc'

.code
  start:
        invoke  AllocConsole
                invoke  GetStdHandle, STD_OUTPUT_HANDLE                                 ;Get Console Output Handle
                mov             [console],      EAX                                                                     ;Store Handle
                invoke  GetConsoleWindow, 0                                                             ;Get Window Handle
                mov             [window], EAX                                                                   ;Store
        invoke  WriteConsole, [console], buffer, 9600, 0                ;Write Stored Message to Output Buffer
                invoke  SetConsoleCursorPosition, [console], 0                  ;Set Cursor to Top to Scroll Up
                invoke  SetConsoleCursorPosition, [console], 0x0028003C ;Center Cursor in Console
                invoke  SetConsoleScreenBufferSize, [console], [scale]  ;Adjust buffer size?
                invoke  SetConsoleWindowInfo, [console], 1, scale               ;Adjustment 2?
        invoke  Sleep,-1                                                                                ;Remain Open
.end start

.data
        console dd 0
        window  dd 0
        scale   dd 0x00007D50
        buffer  db 9600 dup (" ")                                                                       ;Placeholder

Any suggestions or obvious errors?

01 May 2019, 13:04

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 01 May 2019, 13:50

I assume your problem is in creating the dword value from two word values?

There are a number of ways it can be done.Here is one way.

Code:

x = 40
y = 60
mov eax,x shl 16 + y

01 May 2019, 13:50

Nyatta

Joined: 23 Feb 2019
Posts: 3

Nyatta 01 May 2019, 20:42

Code:

mov             ax, [curposy]
shl             eax, 16
add             ax, [curposx]
invoke  SetConsoleCursorPosition, [console], eax

Awesome, thank you!

A questions regarding that: is there any way to calculate the result of that algorithm upon compilation in order to bake it in as an immediate value and a stored value? Terminology/Vocabulary has been a killer for my Google queries.
Something like:

Code:

x dw 40
y dw 60
xy dd y shl 16 + x

Edit: I was under the impression that by defining two values side-by-side, say two word values, I could simply call the first value as a double to retrieve both. Upon compilation are the sections in my code block that attempt to access that data being set to only access a word as part of the opcode itself due to the value being defined as a word within my data section? Is there a way to define I am retrieving a double from a word stored in the data block?

01 May 2019, 20:42

Ali.Z

Joined: 08 Jan 2018
Posts: 727

Ali.Z 02 May 2019, 00:11

to simplify things, and an aim for a better readability use structures.

Code:

.code
; ...
mov [crd.x],28h
mov [crd.y],3Ch

invoke SetConsoleCursorPosition,[console],[crd]
; ...

.data
crd COORD

define coord struct anywhere, like so:

Code:

struct COORD
x dw ?
y dw ?
ends

_________________
Asm For Wise Humans

02 May 2019, 00:11

Nyatta

Joined: 23 Feb 2019
Posts: 3

Nyatta 03 May 2019, 20:44

Ah, that's great to know!
I'm employed that as follows:

Code:

format PE64 GUI 5.0
include 'win64ax.inc'

.code
  start:
        invoke  AllocConsole
        invoke  GetStdHandle, STD_OUTPUT_HANDLE
        mov     [console],      EAX
        invoke  GetConsoleWindow, 0
        mov     [window], EAX
        invoke  WriteConsole, [console], buffer, 9600, 0
        invoke  SetConsoleCursorPosition, [console], 0
        mov     EAX, [curs]
        invoke  SetConsoleCursorPosition, [console], EAX
        invoke  Sleep,-1
.end start

.data
        struct  COORD
                y       dw 3Ch
                x       dw 28h
        ends
        curs    COORD
        
        console dd 0
        window  dd 0
        buffer  db 9600 dup (" ")

My next question, I read the structure above as defining two words, whereas before I defined a single word containing the data of the two doubles... What is the structure actually doing between defining it an moving it into a register? Assigning x and y as doubles results in only the x coordinate being entered when the coordinate is stored this way.

03 May 2019, 20:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20430
Location: In your JS exploiting you and your system

revolution 04 May 2019, 02:47

To make your COORD structure more flexible you can try this:

Code:

        struct  COORD
                y       dw ?
                x       dw ?
        ends
        curs    COORD 40, 60
        curs2   COORD 30, 70

This is functionally the same as doing this:

Code:

curs rd 0
curs.y dw 60
curs.x dw 40
curs2 rd 0
curs2.y dw 70
curs2.x dw 30

Then when you want to access the data do this:

Code:

mov eax,[curs]
invoke function, [curs2]
mov bx,[curs2.y]

04 May 2019, 02:47

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