The SPU Mark II Project

A project that focuses on the development and improvement of the SPU Mark II instruction set architecture.

Another focus is development and creation of a concrete implementation of the CPU in VHDL as well as building a small "home computer" around an FPGA board similar to other computers from the 80ies.

SPU Mark II

The SPU Mark II is a 16 bit RISCish cpu that uses the stack machine approach instead of a register machine approach.

The instrution set is documented in documentation/isa.md.

Short feature list:

• Highly flexible instruction set
• Conditional instructions instead of special conditional jumps or movs
• Optional hardware multiplication/division units (WIP)
• Optional interrupt handling (WIP)

To get a feel for the instruction set, here's a small example for a void puts(char*str) function:

puts:
	bpget ; function prologue
	spget
	bpset
	
	get 2 ; fetch arg 1
puts_loop:
	ld8 [i0:peek] [f:yes]
	[ex:nonzero] st8 0x4000     ; Use MMIO for text output
	[ex:nonzero] add 1
	[ex:nonzero] jmp puts_loop
	pop

	bpget ; function epilogue
	spset
	bpset
	ret

Ashet Home Computer

The Ashet Home Computer is a computer built on top of the SPU Mark II cpu and provides a small environment to use the cpu.

Planned Features

• MMU
• Video Output (either FBAS or VGA)
• Audio Output (signed 16 bit PCM)
• SD Card Storage
• Keyboard Interface
• Joystick Port (C64 style)
• UART interface

Current Memory Map

Note that this memory map right now does not utilize the MMU, so bus width is 16 bit.

Range	Function
0x0000 … 0x3FFF	Builtin ROM
0x4000 … 0x4FFF	UART Interface
0x6000 … 0x60FF	256 byte fast RAM
0x6100 … 0x7FFF	Unmapped
0x8000 … 0xFFFF	32k byte slow RAM