EttinOS

EttinOS is a minimalist 16-bit hobbyist disk operating system for the IBM Personal Computer and compatible machines. Its git repository can be found at https://ahti.space/git/crazyettin/EttinOS and that of EttinOS-extra, a collection of programs for the system, at https://ahti.space/git/crazyettin/EttinOS-extra.

System requirements

• An Intel 8086 or compatible CPU
• BIOS, or UEFI in legacy mode
• 64 KiB of RAM
• 1-4 floppy disk drives (hard disk drives are not supported)

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 1440 KiB 3.5" images instead use the argument -1440. If you want to use another format you will have to build the system manually.

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 assigns the drives letters from A to D and uses a version of the MS-DOS 3.0 FAT12 file system (in actual use since 2.1) without support for file attributes and thus disk labels and subdirectories. Drive letters and file names are case-insensitive and the latter follow the 8.3 format. Text files use CRLF line endings.

Drives and files are specified as ([A-D]:) and ([A-D]:)FILENAME.EXT respectively. Specifying the current drive, indicated in the prompt, is optional.

A command can be followed by arguments separated from eachother and the command with spaces. Extra spaces are ignored. Commands other than changing the current drive are stored as external programs: the command for a program is its file specification without the extension.

Commands included in EttinOS:

• [A-D]:: Change the current drive.
• ECHO: Print a message. Syntax: ECHO Message to be printed
• HELLO: Print "Hello world!".
• LIST: Print a list of the files on a drive. Syntax: LIST DRIVE
• PRINT: Print a text file. Syntax: PRINT FILE

Both LIST and PRINT page their output if all of it does not fit on the screen at the same time. Press any key other than escape to continue printing after each screen or escape to quit the program.

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 number of the current drive 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: String operations:
  • 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.
  • AH = 0x4: (Under construction) Convert a decimal string ending in a null at SI to a value in AL.
  • AH = 0x5: (Under construction) Convert a value in AL to a decimal string ending in a null at DI.
  • AH = 0x6: (Under construction) Convert a hexadecimal string ending in a null at SI to a value in AL.
  • AH = 0x7: (Under construction) Convert a value in AL to a hexadecimal string ending in a null at DI.
• Interrupt 0x22: Disk operations:
  • AH = 0x0: (Under construction) Load the directory of a drive named at SI as a string ending in a null to the offset BX and store the error codes in AL: * AL = 0x0: Succesful load * AL = 0x1: Drive not found * AL = 0x2: Unable to read disk
  • AH = 0x1: (Under construction) Store a directory at the offset BX to a drive named at SI as a string ending in a null.
  • AH = 0x2: Load a file named at SI as a string ending in a null to the offset BX and store the file size in CX and the error codes in AL: * AL = 0x0: Succesful load * AL = 0x1: Drive not found * AL = 0x2: Unable to read disk * AL = 0x4: File or command not found * AL = 0x8: Not enough memory
  • AH = 0x3: (Under construction) Save a file.

Known bugs

• Trying to access a disk of different format than the current one can crash the system.
• Files beyond the ~40th one on the disk might not load properly.