To check the current video display mode from a program, the video BIOS function call 15 to get the CRT mode can be used. To invoke the function, have register AH set to 15 and execute interrupt 10H. On exit from the interrupt, register AL has the current CRT mode, register AH has the number of columns on the screen, and register BH has the current active page number. This call can be used after a set CRT mode call to verify that the system is using the correct mode.

To write a new pixel value to the display, on entry have register AH set to 0CH, register DX set with the row number, register CX set with the column number, register AL set with the color value data, and register BH set to display page number (this value is normally set to 0 for graphics). Execute an interrupt 10H to invoke the function call.

To read the current value of a pixel, on entry have register AH set to 0DH, register DX set to the row number, register CX set to the column number, and register BH set to display page number (normally a value of 0). On return from the interrupt 10H function, register AL has the color value of the pixel.

The following code example shows how to draw a horizontal line in graphics mode.

.CODE
;draw line code
draw_line proc near c, line_color:BYTE, line_length:WORD,
     line_row_start:WORD, line_col_start:WORD
     mov  AL,line_color
     mov  DX,line_row_start
     mov  CX,line_col_start
     mov  BX,0      ;display page
     mov  SI,line_length
     mov  AH,12
draw_line_loop:
     push AX
     push BX
     push CX
     push DX
     push SI
     int  10H       ;video BIOS call
     pop  SI
     pop  DX
     pop  CX
     pop  BX
     pop  AX
     inc  CX        ;point to next line pixel position
     dec  SI        ;adjust line length counter
     jnz  draw_line_loop
     ret
draw_line endp