### Undefined Behavior Examples in LISP

Source: https://github.com/jart/sectorlisp/wiki/Eval

Illustrates cases where the LISP interpreter's behavior is undefined, as specified in McCarthy's paper. Users are responsible for providing valid input.

```LISP
(CAR NIL)
... the result is undefined ...

(COND (NIL NIL))
... may never decide what to answer ...
```

--------------------------------

### Run sectorlisp.bin with QEMU Emulator

Source: https://github.com/jart/sectorlisp/blob/main/README.md

Use QEMU to emulate an i386 system and boot sectorlisp.bin directly. This provides a convenient way to test the boot sector without physical hardware.

```sh
qemu-system-i386 -nographic -fda sectorlisp.bin
```

--------------------------------

### Build and Run sectorlisp C Implementation

Source: https://github.com/jart/sectorlisp/blob/main/README.md

Compile the C reference implementation using make and run it. This process also generates the sectorlisp.bin master boot record.

```sh
make
./lisp
```

--------------------------------

### Run sectorlisp.bin with Blinkenlights Emulator

Source: https://github.com/jart/sectorlisp/blob/main/README.md

Download and execute the blinkenlights emulator, then load sectorlisp.bin to run it. This is useful for testing the master boot record in an emulated environment.

```sh
curl --compressed https://justine.lol/blinkenlights/blinkenlights-latest.com >blinkenlights.com
chmod +x blinkenlights.com
./blinkenlights.com -rt sectorlisp.bin
```

--------------------------------

### Optimized Zero Comparison in Assembly

Source: https://github.com/jart/sectorlisp/wiki/Eval

Demonstrates assembly code for testing a zero value, showing a shorter machine code sequence compared to explicit comparison with zero.

```assembly
  test %ax,%ax
  jnz ...
```

```assembly
  cmp $0, %ax
  jne ...
```

--------------------------------

### GetToken() - Optimized Version 2

Source: https://github.com/jart/sectorlisp/wiki/Read

An optimized version of GetToken() that reduces assembler jumps by leveraging assumptions about GetChar().

```C
...
  for (;;) {
    x = XlatSyntax(b);
    if (x == ' ') {
      b = GetChar();
      continue;
    }

    do {
      STOS(t, b);
      b = GetChar();
      if (x) break;
      x = XlatSyntax(b);
    } while (!x);

    STOS(t, 0);
    q->look = b;
    return;
  }
...
```

--------------------------------

### GetToken() - First Version

Source: https://github.com/jart/sectorlisp/wiki/Read

The initial implementation of the GetToken function for parsing tokens.

```C
void GetToken(void) {
  char *t;
  unsigned char b, x;
  b = q->look;
  t = q->token;
  for (;;) {
    x = XlatSyntax(b);
    if (x != ' ') break;
    b = GetChar();
  }
  if (x) {
    STOS(t, b);
    b = GetChar();
  } else {
    while (b && !x) {
      STOS(t, b);
      b = GetChar();
      x = XlatSyntax(b);
    }
  }
  STOS(t, 0);
  q->look = b;
}
```

--------------------------------

### GetToken() - Further Optimized Version

Source: https://github.com/jart/sectorlisp/wiki/Read

A further optimized version of GetToken() that minimizes assembler calls by using goto statements.

```C
void GetToken(void) {
  char *t;
  unsigned char b, x;
  b = q->look;
  t = q->token;
L0:
    x = XlatSyntax(b);
    if (x == ' ') goto L2;
L1: STOS(t, b);
L2: b = GetChar();
    if (x == ' ') goto L0;
    if (x) goto L3;
    x = XlatSyntax(b);
    if (!x) goto L1;
L3:
  STOS(t, 0);
  q->look = b;
  return;
}
```

--------------------------------

### Bind() Function Implementation in C

Source: https://github.com/jart/sectorlisp/wiki/Eval

The C implementation of the Bind() function, which handles argument evaluation and pairing with dot notation. It's noted that C/C++ argument evaluation order is unspecified.

```C
WORD Bind(WORD v, WORD a, WORD e) { // evlis + pair w/ dot notation
  if (v == NIL)
     return e;
  WORD t1 = Eval(Car(a), e);
  WORD t2 = Cons(Car(v), t1);
  WORD t3 = Bind(Cdr(v), Cdr(a), e);
  return Cons(t2, t3);
}
```

--------------------------------

### Bind() Function Optimized Assembly

Source: https://github.com/jart/sectorlisp/wiki/Eval

An optimized assembly transcription of the Bind() function, reordering statement evaluation for size efficiency. This version manually manages the stack for function calls and returns.

```assembly
Bind:                   # Bind(v:di,a:si,e:dx):ax
  cmp     $NIL,%di
  je      1f
  push    2(%di)        # 1 save Cdr(v)
  push    2(%si)        # 2 save Cdr(a)
  push    %dx           # 3 save e
  push    (%di)         # 4 save Car(v)
  mov     (%si),%ax     # ax = Car(a)
  xchg    %dx,%si
  call    Eval
  xchg    %ax,%si
  pop     %di           # 4 restore di = Car(v)
  call    Cons
  pop     %dx           # 3 restore e
  pop     %si           # 2 restore Cdr(a)
  pop     %di           # 1 restore Cdr(v)
  push    %ax           # 5 save
  call    Bind
  xchg    %ax,%si
  pop     %di           # 5 restore
  jmp     Cons
1:xchg    %dx,%ax
  ret
```

--------------------------------

### ATOM and CONS Tagging Definitions (Tag 0)

Source: https://github.com/jart/sectorlisp/wiki/Eval

Defines macros for ATOM and CONS types when ATOM tag is 0. Includes macros for creating tagged values and extracting their values.

```C
#define ATOM 0
#define CONS 1
#define OBJECT(t, v)    (((v)<<1)|(t))
#define VALUE(x) ((x) >> 1)
#define NIL  0
```

--------------------------------

### Cadr() Implementation (ATOM Tag 1)

Source: https://github.com/jart/sectorlisp/wiki/Eval

Assembly code for the Cadr() function when ATOM tag is 1. This implementation requires comparing against the NIL value.

```assembly
Cadr:
  mov 2(%di),%di
  mov (%di),%ax
  ret
```

--------------------------------

### Car() Implementation (ATOM Tag 0)

Source: https://github.com/jart/sectorlisp/wiki/Eval

Assembly code for the Car() function when ATOM tag is 0. This version assumes non-ATOM input and is simplified.

```assembly
Car:
  and $-2,%di
  mov (%di),%ax
  ret
```

```assembly
Car:
  dec %di
  mov (%di),%ax
  ret
```

--------------------------------

### ATOM and CONS Tagging Definitions (Tag 1)

Source: https://github.com/jart/sectorlisp/wiki/Eval

Defines macros for ATOM and CONS types when ATOM tag is 1. In this case, CONS cells are addresses, and NIL requires explicit comparison.

```C
#define ATOM 1
#define CONS 0
...
#define NIL  (ATOM | 0)
```

--------------------------------

### Test ATOM condition in Assembly

Source: https://github.com/jart/sectorlisp/wiki/Print

This assembly code tests if the object in the AX register is an ATOM. It's a common optimization for x86 instructions.

```asm
test $1,%al		# 2 bytes
```

```asm
test $1,%di		# more bytes
```

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