no_bios_hello_world

Author: cirosantilli

README.adoc

@@ -691,6 +691,34 @@ Open source x86 BIOS implementation.
 
 Default BIOS for QEMU and KVM.
 
+== No BIOS
+
+Here we will collect some examples that do stuff without using the BIOS!
+
+These tend to be less portable, not sure they will work on real hardware. But they were verified in QEMU.
+
+=== No BIOS hello world
+
+....
+./run no_bios_hello_world
+....
+
+Source: link:no_bios_hello_world.S[]
+
+Outcome:
+
+....
+hello world
+....
+
+with red foreground and blue background shows on the top left of the cleared screen.
+
+This example uses the fact that BIOS maps video memory to address 0xB8000.
+
+We can then move 0xB800 to a segment register and use segment:offset addressing to access this memory.
+
+Then we can show characters by treating `0xB800:0000` as a `uint16_t` array, where low 8 bytes is the ASCII character, and the high 8 bytes is the color attribute of this character.
+
 == Modes of operation
 
 The x86 processor has a few modes, which have huge impact on how the processor works.

baremetal_hello_world.S

@@ -0,0 +1,33 @@
+/* https://github.com/cirosantilli/x86-bare-metal-examples#no-bios-hello-world */
+
+#include "common.h"
+
+BEGIN
+    mov $0xB800, %di
+    mov %di, %es
+    xor %di, %di
+    lea msg, %si
+    /* clear screen from SeaBIOS messages */
+    xor  %ax, %ax
+    movw $2000, %cx
+    repz stosw
+    xor  %di, %di
+    /* write a string on the screen */
+.loop:
+    lodsb
+    test %al, %al
+    jz .halt
+    /* write the character */
+    movb %al, %es:(%di)
+    /* write color attribute of this character
+     * 20d = 0x14 = 10100b = color attributes (red on blue)
+     * background color = 1b    = blue
+     * foreground color = 100b  = red
+     */
+    movb $20, %es:1(%di)
+    add $0x2, %di
+    jmp .loop
+.halt:
+    hlt
+msg:
+    .asciz "hello world"