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EttinOS

EttinOS is a minimalist 16-bit DOS-like hobbyist operating system for the IBM Personal Computer and compatible machines. 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
  • 1-4 floppy disk drives

Building

Build dependencies:

  • A Unix-like operating system
  • coreutils
  • dosfstools
  • mtools
  • nasm
  • rw (optional)

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

Usage

The 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.

EttinOS uses the FAT12 file system, and as a result file names (and thus also commands) are case-insensitive and follow the DOS-like 8.3 format. EBPB and file attributes (and thus also labels and subdirectories) are not supported and are ignored if present. A file specification is of the format ([A-D]:)FILENAME.EXT, where the drive letter is optional in case of the current drive. Text files use DOS-like newlines (CR+LF) and end in a null.

Commands consist of the command itself followed by a tail separated with a space. Extra spaces, be they leading, trailing, or between a command and its tail, are ignored. What the tail contains and how it should be formatted depends on the command. All commands except for changing the drive are stored as external programs: a command for a program is its file specification without the extension.

Commands included in EttinOS:

  • [A-D]:: Change the drive.
  • ECHO: Echo the tail.
  • HELLO: Print "Hello world!".
  • LIST: List the files on the drive specified in the tail.
  • TYPE: Print the contents of a text file specified in the tail.

Programming

EttinOS has a flat address space of 64 KiB. The data, stack, and extra segments are set at the beginning of the RAM and the system stack at the end of the address space. Programs are loaded at address 0x3000, the drive number is loaded in DL, and SI is pointed at the command tail (a string ending in a null) when the program is executed. The stack is reset back to the end of the address space after a program has finished running.

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). <0A>