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			<h2 class="western" style="font-variant: normal; font-style: normal">
			<font size="5" style="font-size: 18pt">Writing applications for
			ELKS with the GNU as assembler</font></h2>
			<p style="margin-bottom: 0.5cm"><font face="Arial, sans-serif">If
			you want to compile an assembler program with gnu as to use it
			with DLKS, here is a hello world example:</font></p>
			<pre class="western">        .code16
        .text
        .global entry
entry:
        movw    $len,%dx
        movw    $msg,%cx
        movw    $1,%bx
        movw    $4,%ax
        int     $0x80

        movw    $0,%bx
        movw    $1,%ax
        int     $0x80
        .data
msg:
        .ascii  &quot;Hello, world\n&quot;
        len = . - msg</pre><p style="margin-bottom: 0.5cm">
			<code class="western"><font face="Arial, sans-serif">Make a new
			&bdquo;testing&ldquo; directory and copy and save this </font></code><code class="western"><font face="Arial, sans-serif">code</font></code><code class="western"><font face="Arial, sans-serif">
			as 'hello.S' with a text editor in the elks/testing directory.
			Then get into this directory and compile it with these commands: </font></code>
			</p>
			<pre class="western"><code class="western">$ ia16-elf-gcc -melks-libc -mcmodel=small -c hello.S -o hello.o</code>
<code class="western">$ ia16-elf-gcc -melks-libc -mcmodel=small -nostdlib hello.o -o hello</code></pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif"><font size="3" style="font-size: 12pt">To
			see if this program will work with ELKS enter: &bdquo;../elksemu/elksemu
			hello&ldquo; on the command line. The elksemu emulator will run
			the hello program then. Further alternatives to run the program
			are described in the &bdquo;Writing applications for ELKS in C&ldquo;
			document.</font></font></p>
			<p style="margin-bottom: 0.5cm"><font face="Arial, sans-serif"><font size="3" style="font-size: 12pt">The
			entry points can be defined as entry or _start, as handled in
			ELKS's C library linker scripts.</font></font></p>
			<p style="margin-bottom: 0.5cm"><font face="Arial, sans-serif">The
			system calls implemented by ELKS are listed in the file</font>
			<font face="Arial, sans-serif">Documentation/function.lst. The
			system call numbers are identical to Linux. So you will find these
			numbers on this page: <a href="http://asm.sourceforge.net/syscall.html">http://asm.sourceforge.net/syscall.html</a>
			and further details here:
			<a href="https://www.tutorialspoint.com/assembly_programming/assembly_system_calls.htm">https://www.tutorialspoint.com/assembly_programming/assembly_system_calls.htm</a></font></p>
			<p style="margin-bottom: 0.5cm; font-variant: normal; font-style: normal">
			<font face="Arial, sans-serif">If an error number is returned you
			can look that up in the file 'elks\include\arch\errno.h'.</font></p>
			<h2 class="western" style="margin-top: 0cm; margin-bottom: 0.5cm; font-variant: normal; font-style: normal">
			<font face="Arial, sans-serif"><font size="5" style="font-size: 18pt">Writing
			applications for ELKS with the NASM assembler</font></font></h2>
			<p style="margin-bottom: 0.5cm"><font face="Arial, sans-serif">It
			is also possible to use NASM with the -f elf switch to generate an
			object file for ELKS. Here is an example:</font></p>
			<pre class="western">BITS 16
section .text                   ;section declaration
global _start                   
_start:                         ;write our string to stdout

    mov     dx,len              ;3rd arg: message length
    mov     cx,msg              ;2nd arg: pointer to message to write
    mov     bx,1                ;1st arg: file handle (stdout)
    mov     ax,4                ;system call number (sys_write)
    int     0x80                ;call kernel

                                ;exit now
    mov     bx,0                ;1st syscall arg: exit code
    mov     ax,1                ;system call number (sys_exit)
    int     0x80                ;call kernel

section .data                   ;section declaration

msg db      &quot;Hello, world!&quot;,0xa ;the message string
len equ     $ - msg             ;length of string
</pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif">Save this code as the file
			hello.</font><font face="Arial, sans-serif">asm</font><font face="Arial, sans-serif">.
			</font><font face="Arial, sans-serif">Make sure the paths are set
			to find ia16-elf-gcc by running the script &bdquo;. env.sh&ldquo;
			in an elks directory.</font><font face="Arial, sans-serif"> Then
			you can compile t</font><font face="Arial, sans-serif">he code</font><font face="Arial, sans-serif">
			with these commands:</font></p>
			<pre class="western" style="margin-bottom: 0.5cm">nasm -f elf -o hello.o hello.asm
ia16-elf-gcc -melks-libc -mcmodel=small -nostdlib hello.o -o hello</pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif"><font size="3" style="font-size: 12pt">Then
			enter: &bdquo;../elksemu/elksemu hello&ldquo; on the command line</font></font><font face="Arial, sans-serif">
			and see the message on the screen.</font></p>
			<h2 class="western"><span style="font-variant: normal"><font size="5" style="font-size: 18pt"><span style="font-style: normal">Writing
			applications for ELKS with the as86 assembler</span></font></span></h2>
			<p style="margin-bottom: 0.5cm"><font face="Arial, sans-serif">Unlike
			GNU as the as86 assembler has Intel syntax like masm or tasm. If
			you want to compile an assembler program with as86, here is an
			example:</font></p>
			<pre class="western">.text
.global _main
_main:
push es
push ax
!write ABC at the left border of line 20
      mov ax,#0xb800
      mov es,ax
      seg es
      mov [3200],#0x41
      seg es
      mov [3201],#0x4B
      seg es
      mov [3202],#0x42
      seg es
      mov [3203],#0x2E
      seg es
      mov [3204],#0x43
      seg es
      mov [3205],#0x1f
pop ax      
pop es   

xor bx,bx
ret
.data
.bss</pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif">Save this code in the file abc.S.
			Then you can compile it with these commands:</font></p>
			<pre class="western">as86 -o abc.o abc.S
ld86 -i -o abc abc.o</pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif">You can also call the ld86 linker
			via bcc like this instead:</font></p>
			<pre class="western" style="margin-bottom: 0.5cm">bcc -o abc abc.o</pre><p style="margin-bottom: 0.5cm">
			<font face="Arial, sans-serif">You can reduce the memory
			requirements of a program if you modify the total program size
			(chmem) with the provided ld86 options. The abc.s program above is
			linked to require 32.752 bytes by bcc/ld86. If you reduce the size
			to 1.024 bytes instead it will run just as well. ELKS considers a
			memory requirement below 1.024 bytes invalid and will refuse to
			load the program with the message &bdquo;out of space&ldquo;. See
			the check in fs/exec.c. Add the code, data and bss sections and
			add e.g. 256 bytes for the stack. </font>
			</p>
			<p style="margin-bottom: 0.5cm; font-variant: normal; font-style: normal">
			<font face="Arial, sans-serif">14</font><font face="Arial, sans-serif">th</font>
			<font face="Arial, sans-serif">of </font><font face="Arial, sans-serif">April</font><font face="Arial, sans-serif">
			2020 Georg Potthast with help from tkchia</font></p>
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