EttinOS
=======

EttinOS is a minimalist 16-bit DOS-like hobbyist operating system for
the IBM PC and compatible computers. Its git repository can be found at
https://ahti.space/git/crazyettin/EttinOS.

System requirements
-------------------

  * An Intel 8086 or compatible CPU
  * BIOS, or UEFI in legacy mode
  * 64 KiB of RAM
  * A floppy disk drive

Building
--------

Build dependencies:
  * A Unix-like operating system
  * bash
  * coreutils
  * dosfstools
  * mtools
  * nasm

Running make.sh will build EttinOS and create a bootable 360 KiB 5.25"
floppy disk image named EttinOS.img. To get a 1.44 MB 3.5" one instead
use the argument -1440. If you want to use another floppy disk format
you will have to adjust the bootloader disk description tables and
install the system manually. Hard disk drives are not supported.

Input
-----

The EttinOS input system is inspired by typewriters. Typing a character
overwrites the cursor location and the erase (=tab) key erases it. The
space and backspace keys move the cursor.

Programming
-----------

EttinOS has a flat address space of 64 KiB with the data, stack, and
extra segments set at the beginning of the RAM. Programs are loaded at
address 0x2000.

System calls:
  * Interrupt 0x20: Return to the shell.
  * Interrupt 0x21: Input and output:
      * AH = 0x0: Print a string ending in a null from SI.
      * AH = 0x1: Read a string ending in a null of at most AL
                  characters to DI until a return.
      * AH = 0x2: Print a string ending in a null from SI followed by a
                  CRLF.
      * AH = 0x3: Read a string ending in a null of at most AL
                  characters to DI until a return and print a CRLF.
  * Interrupt 0x22: Disk operations:
      * AH = 0x0: Load a file named in SI as a string ending in a null
                  to the offset BX and set AL to 0x0 if the load was
                  succesfull and 0x1 if there was an error.
      * AH = 0x1: Save a file (under construction).