Varvara

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Varvara is a fantasy computer system that uses runs a stack-based virtual machine called Uxn. Its main programming language is a Forth-like language called Uxntal.

Uxn

Memory

The Uxn virtual CPU can address 64kb of memory; programs can move stacks into addressable range to allow reading and writing directly into stack memory. In this way, peripherals such as a controller, mouse, and keyboard can be addressed.

Memory	RAM	Data	64kb
Stacks	Working Stack	Data	254 bytes
		Error	1 byte
		Pointer	1 byte
	Return Stack	Data	254 bytes
		Error	1 byte
		Pointer	1 byte
I/O	Devices	Data	256 bytes

Uxn is a single-page virtual machine with 32 opcodes. This system has no registers, and instead uses a 127-level stack.

Devices

Devices are external systems connected to the Uxn CPU, such as the screen, the mouse and the keyboard. Each device has 16 bytes, also called ports, of I/O memory. Vectors are ports holding an address in memory to evaluate when a device event is triggered, such as when the mouse is moved, or a key is pressed.

• 00 - System device
• 10 - Console device
• 20 - Screen device
• 30, 40, 50, 60 - Audio device
• 70 - MIDI device
• 80 - Controller
• 90 - Mouse
• a0, b0 - File
• c0 - Datetime
• d0 - Unused
• e0, f0 - Reserved

The two reserved devices can be used for implementation specific features that do not need to be part of the specs, or other Uxn/Varvara instances.