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 (<prefix>0)
```
1 -- push 1
251 -- add
244 -- dup
224 -- push 224
252 -- write result to 224
1 -- push 1
249 -- jump to 1
```