Editing Mode 13h (section)

== Notation ==
<code>13h</code> refers to the number 19 written in the hexadecimal notation in a format typical of assembly code for Intel processors. In [[C (programming language)|the C programming language]], hexadecimal 13 is written <code>0x13</code>.

In [[x86 assembly language]] the video mode is entered by setting the ''AL'' register to 13h, the ''AH'' register to 0, and then calling [[INT 10h|BIOS interrupt 10h]].<ref name="Brown">{{cite web |url=http://www.ctyme.com/intr/rb-0069.htm |title=Ralf Brown's Interrupt List |access-date=2018-08-24 |author-first=Ralf D. |author-last=Brown |author-link=Ralf D. Brown}}</ref> The two 8-bit registers ''AL'' and ''AH'' comprise the 16-bit ''AX'' register.

Although 13<sub>[[Hexadecimal|hex]]</sub> is 19 in decimal, in [[QuickBASIC]]'s screen mode numbering scheme it is simply screen mode 13 (not hexadecimal).<ref name="SCREEN_13">{{cite web |url=http://www.qbasicnews.com/qboho/qckadvr.m111213.shtml |title=Microsoft QuickBasic 4.5 Advisor |access-date=2007-07-19 |date=1990 |publisher=[[Microsoft]] |archive-date=2009-01-05 |archive-url=https://web.archive.org/web/20090105235430/http://www.qbasicnews.com/qboho/qckadvr.m111213.shtml |url-status=dead }}</ref> Many QuickBASIC screen modes have numbers which differ from the BIOS modes on which they are based.<ref>A major example is QuickBASIC screen mode 0, which covers all of the BIOS text modes 00h..03h.  Also, QuickBASIC screen modes 1 and 2 correspond to the CGA BIOS modes 04h through 06h. Further note that QuickBASIC has modes for the [[Hercules Graphics Card|Hercules graphics mode]], which has no standard BIOS support at all, and for [[Olivetti M24#CGA compatible video card|Olivetti/AT&T extended-CGA graphics modes]].</ref>

=== Source code examples ===

Assembly code to enter Mode 13h:
<syntaxhighlight lang="asm">
mov ax, 0013h
int 10h
</syntaxhighlight>

As an illustration of mode 13h in a video game source code, [[Wolfenstein 3D]] includes this function in a file named "ID_VL.C". It enters mode 13h by using two assembly instructions (prefixed with <code>asm</code>) embedded in the C source code.<ref>https://github.com/id-Software/wolf3d/blob/master/WOLFSRC/ID_VL.C#L108</ref> This function also executes three other functions.
<syntaxhighlight lang="c">
void    VL_SetVGAPlaneMode (void)
{
asm     mov     ax,0x13
asm     int     0x10
        VL_DePlaneVGA ();
        VGAMAPMASK(15);
        VL_SetLineWidth (40);
}
</syntaxhighlight>
''[[Wolfenstein 3D]] source code (circa 1992) which calls the BIOS to change to mode 13h.''