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feat: update project tt_um_urish_spell from urish/tt07-spell
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7 changes: 7 additions & 0 deletions projects/tt_um_urish_spell/commit_id.json
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{
"app": "Tiny Tapeout tt06 03711492",
"repo": "https://github.com/urish/tt07-spell",
"commit": "1aa1740ca4b21cf5c10d935dee1864fced67f9f2",
"workflow_url": "https://github.com/urish/tt07-spell/actions/runs/9274545565",
"sort_id": 1716922256901
}
152 changes: 152 additions & 0 deletions projects/tt_um_urish_spell/docs/info.md
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## How it works

SPELL is a minimal, stack-based programming language created for [The Skull CTF](https://skullctf.com).

The language is defined by the following [cryptic piece of Arduino code](https://skullctf.com/spell):

```c
void spell() {

uint8_t*a,pc=16,sp=0,
s[32]={0},op;while(!0){op=
EEPROM.read(pc);switch(+op){case
',':delay(s[sp-1]);sp--;break;case'>':
s[sp-1]>>=1|1;break;case'<':s[sp-1]<<=1;
break;case'=':pc=s[sp-1]-1;sp--;break;case
'@':if(s[sp-2]){s[sp-2]--;pc=s[sp-1]-1;sp+=
1;}sp-=2;break;case'&':s[sp-2]&=s[sp-1];sp-=1;
break;case'|':s[sp-2]|=s[sp-1];sp-=1;break;case
'^':s[sp-2]^=s[sp-1];sp--;break;case'+':s[sp-2]+=
s[sp-1];sp=sp-1;break;case'-':s[sp-2]-=s[sp-1];sp--;
break;case'2':s[sp]=s[sp-1];sp=sp+1;break;case'?':s[
sp-1]=EEPROM. read(s[sp-1]|0 );break;case
"!!!"[0]: 666,EEPROM .write(s
[sp-1] ,s[sp-2] );sp=+
sp-02; ;break; 1;case
"Arr"[ 1]: s[+ sp-1]=
*(char*) (s[+ sp-1 ]);break
;case'w':* (char*)( s[+sp-1]) =s[sp-+2];
sp-=2;break;case+ 'x':s[sp] =s[sp-1
];s[sp-1]=s[sp + -2];s[sp-2]=s[
0|sp];break; ;; case"zzz"[0
]:sleep();" Arrr ";break;case
255 :return;; default:s [sp]
=+ op;sp+= 1,1 ;}pc=
+ pc + 1; %>

}
```

This design is an hardware implementation of SPELL with the following features:

- 256 bytes of program memory (volatile, simulates EEPROM)
- 32 bytes of stack memory
- 32 bytes of data memory
- 8 bidirectional pins and up to 8 output-only pins

Initially, all the program memory is filled with `0xFF`, and the stack and data memory are filled with `0x00`.
The program counter is set to `0x00`, and the stack pointer is set to `0x00`.

To load a program or inspect the internal state, the design provides access to the following registers via a simple serial interface:

| Address | Register name | Description |
|---------|---------------|----------------------------------------------------|
| 0x00 | PC | Program counter |
| 0x01 | SP | Stack pointer |
| 0x02 | EXEC | Execute-in-place (write-only) |
| 0x03 | STACK | Stack access (read the top value, or push a value) |

The serial interface is implemented using a shift register, which is controlled by the following signals:

| Pin | Type | Description |
|-------------|--------|-------------------------------------------------------------------|
| `reg_sel` | input | Select the register to read/write |
| `load` | input | Load the selected register with the value from the shift register |
| `dump` | input | Dump the selected register value to the shift register |
| `shift_in` | input | Serial data input |
| `shift_out` | output | Serial data output (when `porta[3]` is disabled) |

When `load` is high, the value from the shift register is loaded into the selected register. When `dump` is high, the value of the selected register is dumped into the shift register, and can be read after two clock cycles by reading `shift_out` (MSB first).

For example, if you want to read the value of the program counter (PC), you would:

1. Set `reg_sel` to 0x00 and set `dump` to 1
3. Wait for two clock cycles for the first bit (MSB) to appear on `shift_out`.
4. Read the remaining bits from `shift_out` on each clock cycle.

To write a value to the program counter, you would:

1. Write the value to the shift register, one bit at a time, starting with the **MSB**.
2. Set `reg_sel` to 0x00 and set `load` to 1.
3. Wait for a single clock cycle for the value to be loaded.

Writing an opcode to the `EXEC` register will execute the opcode in place, without modifying the program counter (unless the opcode is a jump instruction).

The `STACK` register is used to push a value onto the stack or read the top value from the stack (for debugging purposes).

### Data memory and registers

The data memory space is divided into two regions:

| Address range | Description |
|---------------|--------------------------------------------|
| 0x00 - 0x1F | General-purpose data storage (data memory) |
| 0x20 - 0x5F | I/O and control registers |

Other addresses are reserved for future use, and should not be accessed.

The following registers are available in the data memory space:

| Address | Name | Description |
|---------|-------|--------------------------------------------------------------------------|
| 0x36 | PINB | Read the value of the `portb` pins, or toggle the output when written to |
| 0x37 | DDRB | Set the direction of the `portb` pins (0 = input, 1 = output) |
| 0x38 | PORTB | Write to the `portb` pins |
| 0x39 | PINA | Toggle the output on `porta` pins (write only; read returns 0x00) |
| 0x3A | DDRA | Enables of the `porta` pins (0 = disabled, 1 = output) |
| 0x3B | PORTA | Write to the `porta` (output only) pins |

For example, to toggle the value of the `portb[2]` (`uio[2]`) pin, you would write `0x04` to the `PINB` register.

The `porta[3:0]` pins are also used for debug output, and their function is determined by the `DDRA` register:

| Output pin | DDRA[n] == 0 | DDRA[n] == 1 |
|------------|--------------|--------------|
| 0 | `sleep` | `porta[0]` |
| 1 | `stop` | `porta[1]` |
| 2 | `wait_delay` | `porta[2]` |
| 3 | `shift_out` | `porta[3]` |
| 4 | 0 | `porta[4]` |
| 5 | 0 | `porta[5]` |
| 6 | 0 | `porta[6]` |
| 7 | 0 | `porta[7]` |

## How to test

To test SPELL, you need to load a program into the program memory and execute it. You can load the program by repeatedly executing the following steps for each byte of the program:

1. Write the byte to the top of the stack (using the `STACK` register)
2. Write the address of the byte in the program memory to top of the stack
3. Write the opcode `!` to the `EXEC` register

After loading the program, you can execute it by writing the address of the first byte in the program memory to the `PC` register, and then pulsing the `run` signal.

### Test programs

The following program will spell "SPELL" on the Tiny Tapeout demo board's 7-segment display: (see what we did there?)

```python
[127, 58, 119, 0, 129, 57, 57, 244, 62, 116, 109, 50, 0, 38, 94, 59, 119, 250, 44, 0, 59, 119, 25, 44, 11, 64, 3, 61]
```

The program bytes should be loaded into the program memory starting at address 0.

And of course, the obligatory blink, rapidly blinking an LED connected to the `uio[0]` pin:

```python
[1, 55, 119, 1, 54, 119, 250, 44, 3, 61]
```

## External hardware

None
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