MT-test3d/README.md

Test3d

Low-tech computing for Minetest

Memory access

Memory access in this mod occurs over digilines, with the channel as the prefix plus the memory address, and the message as either the new value or 'get' to get it returned, so for example:

Prefix	Address	Value	Channel	Message
A	1	4	A1	4
Default	15	get	DefaultF	get

T408

The T400 is an 8-bit stack machine operating at 1Hz (due to ABMs only running once per second). It has a 16-word stack, 0 registers and a very simple instruction set.

Instructions

Any words encountered that aren't instructions are pushed to the stack. This makes very compact simple program code.

ins	function	mnemonic
255	push program counter	ppc
254	swap TOS	swp
253	read memory from TOS	read
252	write to memory from TOS	write
251	add TOS	add
250	suptract TOS	sub
249	jump to TOS	jmp
248	skip if TOS = zero	sez
247	jump if TOS = zero	jez
246	jump to subroutine, push pc to retstack	jsr
245	return from subroutine, PC = top of retstack	ret
244	duplicate TOS	dup
243	drop TOS	drop
242	halt processor	hlt
241	pre-read address at TOS	prd

I/O

The T408 uses memory-mapped I/O. This may seem like a stupid idea on a machine with 8-bit addressing but port-based I/O isn't hugely sane.

Address 224 to 239 (16 words) are used for access to external devices. In Minetest you need to execute the prd instruction first, though in an emulator that is treated as a no-op.

Examples

Counts upwards from zero, writing the result to address 224 (0)

1 -- push 1
251 -- add
244 -- dup
224 -- push 224
252 -- write result to 224
1 -- push 1
249 -- jump to 1