WITHOUT BIOS: Get Cursor X and Y

[matias_beretta]

How to Get Cursor X and Y without B. I. O. S.

[SpooK]

You have to be specific. Your question could mean one of 4 different things that include entirely different approaches off the top of my own head.

So go ahead and ask what you specifically want (e.g. the word "get" is often misused, instead use READ or WRITE.)

[frank]

Do you mean the blinking line that usually indicates where the next character will go?

[matias_beretta]

Do you mean the blinking line that usually indicates where the next character will go?

Yes yes...

[SpooK]

Do you mean the blinking line that usually indicates where the next character will go?

Yes yes...

That is a two-stroke approach. That "blinking line" does not indicate where the next character will go, but instead represents the current/next position in a properly designed CLI.

The commands 0x0E and 0x0F of the VGA Index Register Port (0x03D4) allow access (at least write access to my knowledge) so go ahead and investigate that route.

All-in-all, you are dealing with direct memory access and I/O ports... not the BIOS.

[XCHG]

You have to work with CRTC Address Register and the CRTC Data Register. The former is located at the port 0x03D4 and the latter at 0x03D5.

You can put an index into the Address Port and then read the result back from the Data Port. The location of the blinking cursor on the screen has a 16-bit value. Since you can read 8-bits at a time from each of the CRTC registers named above, you have to read the Low Order Byte and the High Order Byte of this value in two steps.

The 16-bit value that you read is the 16-bit offset of the blinking cursor from the top left corner of the screen. For example, if the cursor is at the top left corner, the value that you will get is 0. If it is at the second character on the first row, you will get 1 (0..1 = 2).

I have coded this little Assembly routine that I think will give you the idea what to do:

Code: Select all

  MOV     EDX , 0x00003D4         ; CRTC Address Register
  MOV     EAX , 0x000000E         ; (Index = Cursor Location High Register)
  OUT     DX , AL                 ; Tell the CRTC about what we need
  INC     EDX                     ; EDX = CRTC Data Register (0x03D5)
  IN      AL , DX                 ; Read the High Order Byte of the cursor
  MOV     EBX , EAX               ; Store this value in EBX
  SHL     EBX , 0x00000008        ; BH = High Order Byte of the cursor
  MOV     EAX , 0x000000F         ; (Index = Cursor Location Low Register)
  DEC     EDX                     ; EDX = CRTC Address Register
  OUT     DX , AL                 ; Tell the CRTC what value we need
  INC     EDX                     ; EDX = CRTC Data Register
  IN      AL , DX                 ; Read the Low Order Byte of the cursor
  OR      EAX , EBX               ; AH = High Order Byte, AL = Low Order Byte
  XOR     EDX , EDX               ; Do a simple division (EDX = 0, 32-bits Div)
  MOV     EBX , 80                ; Divide EDX:EAX by 80 (Number of columns per row)
  DIV     EBX                     ; Divide now
  ; EDX = Cursor.X
  ; EAX = Cursor.Y