NodeCPU

NodeCPU is a "virtual computer" built in NodeJS. While this isn't exactly a proof-of-concept, it is still a bit of an experiment.

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Application Binary Interface

This is NodeCPU ABI v0.0.3. This version includes the new stack.

Instruction set

Opcode	Name	Description	Notes	Link
01 /rn imm16	mov r,imm16	Put an immediate into a register		mov
02 imm16	push imm16	Push an immediate onto the stack		push
03 /rn	inc r	Increment a register by 1		inc
04 /rn	pop r	Pop from the stack to a register		pop
05 /rn imm16	add imm16,r	Add immediate to a register		add
07 addr16	jmp addr16	Jump to an address in memory		jmp
08 /r	mov r,r	Put the contents of a register into a register		mov

Low Level System Information

Memory Map

Addr	Desc
0x8065	End of memory
0x8000	Stack
0x4000	Randomly Accessible Memory (RAM)
0x0000	Read-Only Memory (ROM)

The entry point for programs is 0x0000. ROM can only be modified in "entry mode".

Stack

At memory address 0x8000, there are 100 bytes allocated to the stack. You can use push and pop to interact with the stack. The stack itself operates on a FILO¹ (First In; Last Out) system and the stack pointer is kept in register SP. When the stack pointer reaches 0x64 it goes back to 0x00 until it reaches existing elements, when it will throw a Segmentation Fault. Be careful!

Modes

Mode	Byte	Features	When to use
Entry	0x00	Full access to processor, including ROM	While initialising
Boot	0x01	Access to bootloader and RAM	When booting an operating system
Protected	0x02	Access to RAM	When in an operating system

To switch modes, put the byte into Register MR.

Calling Convention

General-purpose registers

These registers can be used for any operation, but the recommened uses are as follows:

Name	Recommended Use	RM Nibble
RA	Temporary storage/Parameter 1	0x1
RB	Parameter 2	0x2
RC	Counter/Parameter 3	0x4
RD	Result/Parameter 4	0x5

Special registers

These registers can only be used for the operation specified.

Name	Use	RM Nibble
MR	Mode Register	0x3
SP	Stack Pointer	0x6

Definitions

Immediate: A value or address in memory ²
RAM: Randomly Accessible Memory
ROM: Read-Only Memory
RM Byte: A byte with two registers (nibbles)³

Footnotes

1. See https://techterms.com/definition/filo for more details. ↩

2. Uses big endian ↩

3. 01: Input = RA, Target = RB. Used for transferring data between registers. In instruction set docs is shown with /r. If the opcode has /rn, the target should be left blank. ↩