CR8

• 8 bit data width
• 16-bit address bus (64KB) + 8 bit memory bank
• Little endian
• Designed to be implemented with 74HC logic gates
• RISC architecture
• Custom Assembly language inspired by Rust and AT&T Assembly
• Supports communications with other devices by sending/recieving a byte
• Minimal native instruction set. Macro-supported development.
• 0.20054 Instructions per clock cycle (4.9865 cycles/instruction)
• Designed to run at 1-4 Mhz
• Not pipelined (Instructions are executed synchronously)

Registers

Registers are written as %x where x is the id of the register to access. Registers include:

Id	Usage	Description
A	GP	General purpose
B	GP	General purpose
C	GP	General purpose
D	GP	General purpose
F	GP	Flags regarding the last mathematical operation.
X	GP	Low index byte (X + Y are used to read/write to memory).
Y	GP	High index byte (X + Y are used to read/write to memory).
Z	GP	General purpose
K	Internal	Memory Bank (the selected bank to use when reading/writing).

K register can only be modified with the bank instruction.

Flags

LSB - MSB

• Equal (EF): cmp resulted in arg1 == arg2
• Lessthan (LF): cmp resulted in arg1 < arg2
• Carry (CF): Previous addition operation carried.
• Borrow (BF): Previous subtraction operation borrowed.

Instructions

Id	Mnemonic	Args	Result
0	MOV	reg, reg/imm8	reg = reg/imm8
1	JNZ	reg, XY/imm16	if reg/imm8 != 0; PC = [XY/imm16]; else: NOP
2	JMP	XY/imm16	PC = [XY/imm16]
3	LW	reg, XY/imm16	reg = [XY/imm16]
4	SW	XY/imm16, reg,	[XY/imm16] = reg
5	PUSH	reg/imm8	[SP++] = reg/imm8
6	POP	reg	reg = [SP--]
7	IN	reg, reg/imm8	reg = PORT[reg/imm8]
8	OUT	reg/imm8, reg	PORT[reg/imm8] = reg
9	ADC*	reg, reg/imm8	reg = reg + reg/imm8 + CF
A	SBB*	reg, reg/imm8	reg = reg + ~(reg/imm8 + BF)
B	CMP*	reg, reg/imm8	reg - reg/imm8
C	AND	reg, reg/imm8	reg = reg & reg/imm8
D	OR	reg, reg/imm8	reg = reg | reg/imm8
E	NOR	reg, reg/imm8	reg = ~(reg | reg/imm8)
F	BANK	reg/imm8	%k = reg/imm8

*: Updates FLAG register

Memory

• 16-bit address bus (64KB) + 8 bit Memory Bank
• Little endian

Memory Layout

Start Address	End Address	Size	Purpose
0x0000	0x7FFF	32Kb	ROM
0x8000	0xBFFF	16Kb	Banked RAM
0xC000	0xFBFF	~14Kb	GP RAM
0xFC00	0xFEFF	~1Kb	Stack
0xFF00	0xFFFF	~256b	Misc

Memory Banks

Each bank is 16Kb long. Banks are accessed through the Memory Bank Register. If the K register is set to 0, builtin ram is used.

• 0x00: Builtin-memory
• 0x01: VRAM
• ...: Extensible

Devices

Up to 256 devices can be connected at once. A device can either send a byte to the system or receive a byte from the system.

0x00: SysCtrl - BUILTIN

• 0x00: PING - Ping the controller.
• 0x01: HALT - Stop the clock.
• 0x02: DBG - Debug the system's register state.
• 0x03: BRKPT - Pause the runner until a line is sent to stdin. Only enabled if --dbg flag is true.

0x01: Keyboard

• Stores state for 8 keys. Stores keys' pressed-state in 1 byte.
• Currently does not support receiving information.
• If it is asked to send it will flush its state to the receiver.
• When a key is pressed, it sets its corresponding bit into the state, which is stored until it is flushed.
• Key bit number (Can be modified):
  • 0: ↑
  • 1: ↓
  • 2: ←
  • 3: →
  • 4: spacebar
  • 5: r
  • 6: +
  • 7: -

0x02: Rng

• send: Sends a random byte
• receive: Does nothing

Assembler

asm - Compiler for the custom Assembly language.