old-DasOS/documentation/virtual-device.md

DasOS Virtual Device

DasOS emulates a virtual device that is shared between all platform implementations. This device provides a uniform environment with fixed specifications of hardware and software as well as syscalls.

Hardware

This section describes the virtual hardware components.

CPU

The virtual device uses an SPU Mark I as the main processor.

RAM

The CPU has a 32 MB RAM connected. In this RAM, both data and code resides and the RAM has no memory protection.

Screen

The screen connected to the vGPU has a resolution of 640x480 pixels and a color depth of 24 bits.

vGPU

The vGPU provides access to the Screen and allows the manipulation of its contents.

The gpu uses a double buffering system with a front and a back buffer where as the front buffer is displayed on the screen and the back buffer is modified by the CPU.

The gpu provides functions for printing text to the screen as well as drawing visual primitives and clearing or swapping the back buffer.

In addition to the features mentioned above, the back buffer is accessible by a memory mapped area in the RAM.

Persistent Storage

Two persistent storage devices are connected to the virtual device. Both storage devices behave like IDE block devices where as the first device is a fixed device and the second device is hot-pluggable.

The size and file format of the storage device is not specified.

Software

This section describes the basic software components and interfaces of the virtual device.

BIOS

Each device implementation provides a BIOS that initialized the device and provides host-dependant configuration.

Boot Sequence

The boot sequence is initiated by the BIOS of the device and can only be changed with a BIOS configuration.

• Determine existence of persistant storage devices
  • Fixed device
  • Swappable device
• Bootloading
  • Searching for a fitting partition
    • FAT32 formatted partition
    • /system.exp must exist
  • Exploading /system.exp into RAM
• Clearing vGPU text screen.
• Jump to Entry Point

Memory Areas

Memory Mapped IO

The memory between 0x200000‬ and 0x‭400000‬ is reserved for hardware use.

Entry Point

After successfully initialization, the virtual device jumps to the address 0x‭400000‬ which is equal to the instruction index 0x‭100000‬.

vGPU Interface

Back Buffer

The back buffer of the vGPU is organized in n RGB24 format where pixels are stored in a 24 bpp format with R as the first byte, G as the second and B as the last. It is 900 kB large and maps the full screen area of 640x480 pixels as a linear frame buffer from 0x200000‬ to 0x2E1000.

MMIO Registers

The vGPU also uses some registers accessible via memory mapped io.

Register	Address	Type	Description
COLOR	0x2E1000	u32	The color used for any draw commands.
CURSORX	0x2E1004	u16	The x position of the draw cursor
CURSORY	0x2E1006	u16	The y position of the draw cursor
SPRITE	0x2E1008	ptr	The address of the sprite to copy
WIDTH	0x2E100C	u16	The width of the sprite.
HEIGHT	0x2E101E	u16	The height of the sprite.
PITCH	0x2E1010	u32	The pitch (memory width) of the sprite.

HWIO Interface

The vGPU is controllable via the HWIO command of the SPU with the command info >= 0x10000.

Command	CmdInfo	Input0	Input1	Description
Clear	0x10000	y	x	Clears the screen and moves the cursor to (x,y)
Line	0x10001	y	x	Draws a line from the cursor to (x,y).
Rect	0x10002	y	x	Draws a rectangle between the cursor and (x,y).
Fill	0x10003	y	x	Fills a rectangle between the cursor and (x,y).
Copy	0x10004	y	x	Renders the current sprite at (x,y) and moves the cursor.
Pixel	0x10005	y	x	Sets the pixel at (x,y).
Move	0x10006	y	x	Moves the cursor to (x,y).

TODO: Text Commands