SPELL is a minimal, stack-based programming language created for The Skull CTF.
The language is defined by the following cryptic piece of Arduino code:
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:
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 |
output | Dump the selected register value to the shift register |
shift_in |
input | Serial data input |
shift_out |
output | Serial data output |
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, you would:
reg_sel to 0x00 and set dump to 1shift_out.shift_out on each clock cycle.To write a value to the program counter, you would:
reg_sel to 0x00 and set load to 1.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).
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:
STACK register)! to the EXEC registerAfter 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.
The following program which will rapidly blink an LED connected to the uio[0] pin. The program bytes should be loaded into the program memory starting at address 0:
[1, 56, 119, 250, 44, 1, 54, 119, 250, 44, 3, 61]
None
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | run | sleep | gpio[0] |
| 1 | step | stop | gpio[1] |
| 2 | load | wait_delay | gpio[2] |
| 3 | dump | shift_out | gpio[3] |
| 4 | shift_in | gpio[4] | |
| 5 | reg_sel[0] | gpio[5] | |
| 6 | reg_sel[1] | gpio[6] | |
| 7 | gpio[7] |